No A, just CAPS LOCK



What happens when the CapsLock key on your keyboard doesn't have a notch in it?

"This hPPENS."

The goal of this program is to consistently emulate keyboard misses where each A press is replaced with CapsLock. Uppercase 'A's from the source should yield the same effect. When CapsLock is enabled, capitalization is reversed.

Test Cases

"The quick brown fox jumps over the lazy dog."
-> "The quick brown fox jumps over the lZY DOG."

"Compilation finished successfully."

"What happens when the CapsLock key on your keyboard doesn't have a notch in it?"
-> "WhT Hppens when the CPSlOCK KEY ON YOUR KEYBOrd doesn't hVE  notch in it?"

"The end of the institution, maintenance, and administration of government, is to secure the existence of the body politic, to protect it, and to furnish the individuals who compose it with the power of enjoying in safety and tranquillity their natural rights, and the blessings of life: and whenever these great objects are not obtained, the people have a right to alter the government, and to take measures necessary for their safety, prosperity and happiness."
-> "The end of the institution, mINTENnce, ND dministrTION OF GOVERNMENT, IS TO SECURE THE EXISTENCE OF THE BODY POLITIC, TO PROTECT IT, nd to furnish the individuLS WHO COMPOSE IT WITH THE POWER OF ENJOYING IN Sfety ND TRnquillity their nTURl rights, ND THE BLESSINGS OF LIFE: nd whenever these greT OBJECTS re not obtINED, THE PEOPLE Hve  RIGHT TO lter the government, ND TO Tke meSURES NECESSry for their sFETY, PROSPERITY nd hPPINESS."

-> "" (Without the notch, no one can hear you scream)

"CapsLock locks cAPSlOCK"

-> "wHt if CPSlOCK IS lreDY ON?"

The winning criterion is, as usual, the size of the submitted program's source code.


Posted 2018-03-14T18:15:11.627

Reputation: 1 551

110Welcome to the site! This is a nice first challenge, and unfortunately very relatable for me and my fT FINGERS. – James – 2018-03-14T18:29:23.957

What characters will be in the input ? a-zA-Z? – Rod – 2018-03-14T18:42:32.610

5suggested test case : teSTateSTateSTateST – Rod – 2018-03-14T18:44:19.673

@Rod Look at the examples. Printable ASCII seems to be included. – mbomb007 – 2018-03-14T18:44:35.260

@Rod -> teSTTEstteSTTEst. – mbomb007 – 2018-03-14T18:45:18.937

@Rod All characters except 'a' or 'A' are allowed in the output. Non-letters do not get changed. – Broadwell – 2018-03-14T18:50:22.867

1@Broadwell He said "input", not "output" – mbomb007 – 2018-03-14T18:56:48.113

2hey so in the future I would recommend using sandbox :P but very good challenge – Christopher – 2018-03-14T19:00:28.073


@Christopher is referring to the Sandbox, which is where users typically post questions to ask for improvements.

– Giuseppe – 2018-03-14T19:05:16.153

88If only the enter key also had a notch in it so this wouldn' – 12Me21 – 2018-03-14T19:33:18.153

75t happen....... – 12Me21 – 2018-03-14T19:33:28.233

1TIL that @DJMcMayhem was trying to create the nick DJMACMayhem... – Olivier Dulac – 2018-03-15T17:01:37.973

23Literally joined this site to upvote "Without the notch, no one can hear you scream" – lucasvw – 2018-03-15T21:10:11.917

Is the sixth example correct? – Andrea Lazzarotto – 2018-03-16T17:40:48.540

3Why is the title of this question not “No A, just cPS LOCK”? – Janus Bahs Jacquet – 2018-03-16T17:59:29.040

1Wouldn't it be "No , JUST Cps lock"? – 12Me21 – 2018-03-16T20:54:51.550

4I LSO WISH THt my keyboRD Hd a bigger bCKSPce kwt==et=y – Nonny Moose – 2018-03-16T20:58:15.287

@12Me21 I was allowing for not missing the A key when also holding down the shift key at the same time, since shift + caps lock is actually more difficult than shift + A. Also that would make the title completely illegible. But yes, if we went for the purely programmatic approach, it would. – Janus Bahs Jacquet – 2018-03-18T09:58:59.923

Related – Shaggy – 2018-03-19T01:20:54.093

Test case: correct horse bTTERY STple – MilkyWay90 – 2019-06-24T15:46:22.520



AutoHotKey, 7 bytes


// Is this valid? This really do what OP want -- replace a by CapsLock (vk14).

Run this program, and type the input from keyboard..


Posted 2018-03-14T18:15:11.627

Reputation: 13 072

4"Is this valid?" OP did not specify input or output constraints so I´d consider this valid. – Nefrin – 2018-03-15T13:22:10.347

5Don't see too many ahk answers! – HaveSpacesuit – 2018-03-15T13:57:52.363

1"my code runs everywhere" – sudo rm -rf slash – 2018-03-15T15:02:20.060

1I would cLL THIS Vlid! – Grant Garrison – 2018-03-16T00:37:53.547

57ThT'S GREt but how do I disBLE IT? – RobbG – 2018-03-16T13:52:35.650

69@RobbG just type "killLL utohotkey" ... oh wIT – Nefrin – 2018-03-16T14:04:33.133

1@RobbG You could copy/pSTE THE in the title. – jpmc26 – 2018-03-20T02:19:39.270

5@tsh I think you've missed the joke here... – RobbG – 2018-03-20T08:55:00.120


V, 9 bytes


Try it online!


00000000: f22f e361 0a78 677e 24                   ./.a.xg~$


ò       " Recursively:
 /ãa    "   Move forward to the next 'a' (upper or lowercase)
        "   This will break the loop when there are no more 'a's
x       "   Delete the 'a'
 g~$    "   Toggle the case of every character after the cursor's position.


Posted 2018-03-14T18:15:11.627

Reputation: 54 537


Vim, 16 bytes


Assumes the input is on a single line


qq            Start a loop
 /\ca␊         Find the first occurence of an a, end the loop if there are none left
 xg~$          Remove it and invert the case of the rest of the file
@qq@q         End the loop 

Herman L

Posted 2018-03-14T18:15:11.627

Reputation: 3 611

Is this case insensitive finding of 'a'? – Gnudiff – 2018-03-16T21:06:49.943

@Gnudiff \c anywhere in a regex-search enables case-insensitivity – Herman L – 2018-03-16T21:14:24.593

Do you need to set a specific flag for g~$ to work? Because for me it only inverts case until the end of the line, not the whole file, so this doesn't really work for multiline files for me. – Cubic – 2018-03-19T15:34:58.143

1@Cubic As I wrote in the answer, it "assumes the input is on a single line" – Herman L – 2018-03-19T15:58:25.717

@Cubic If you wanted it to go the end of the file and support inputs on multiple lines, you could do g~vG or vG~. – James – 2018-03-19T17:15:01.247

@DJMcMayhem I think using 'n' instead of '$' would work also? – nmjcman101 – 2018-03-19T21:28:09.073


C, 72 bytes

Thanks to @Ton Hospel for helping to save 16 bytes!


Try it online!


Posted 2018-03-14T18:15:11.627

Reputation: 15 798

2You can swap the case of letters using a xor with 32 – Ton Hospel – 2018-03-14T19:19:46.733

You can probably save even more by having t be 0/32 instead of even/odd (xor t with 32 for each a) and then xor letters directly with t – Ton Hospel – 2018-03-14T20:14:31.497

2Nice way to detect a's – Ton Hospel – 2018-03-14T20:17:15.370

And why initialize t ? It starts as 0 anyways :-) (though you will have to set it back to 0 after each f call in the footer) – Ton Hospel – 2018-03-14T21:51:17.637


@TonHospel Functions have to be reusable, and I don't think it counts as reusable if you need external code to make it usable again after each call.

– Steadybox – 2018-03-15T10:49:21.730

16305%c is 0 if c is 13. – Rosie F – 2018-03-16T11:55:12.340


Retina, 33 21 17 bytes


Try it online


i(              i is for case-insensitive, the paren makes it modify both stages
  Tv`           Transliteration, with simple overlaps (v) - 1 match at every start pos
     lL`Ll`     Replace lowercase with uppercase, and vice versa
           a.*  Every 'a' will match, overlapping to the end of the string
                This swaps the case on all letters after each 'a'
a               Replace all 'a's with nothing

-12 bytes thanks to Martin
-4 bytes thanks to Leo


Posted 2018-03-14T18:15:11.627

Reputation: 21 944

Amazingly this is almost as short as the current Pyth solutions – Ton Hospel – 2018-03-14T19:11:39.620

1I think iT`aAlL`__Ll`a[^a]*a? also works for 21 bytes. – Neil – 2018-03-14T20:29:06.253

4 bytes shorter using overlapping matches – Leo – 2018-03-14T21:37:15.243

I'm interested in how this works if you have time to add the explanation. Thanks! – seshoumara – 2018-03-15T07:41:04.417


Husk, 11 bytes


Try it online!


I'm using the somewhat obscure overloading of Γ called listNF, which constructs recursive functions that operate on lists. It corresponds to the following Haskell pattern:

listNF f = g
  where g (x : xs) = f g x xs
        g [] = []

The idea is that listNF takes a helper function f and returns a new function g, which takes a list. The function f takes a function, which will always be g, and the head x and tail xs of the list, and does something with them. In our application, f calls g recursively on xs. The program is interpreted like this:

Γ (· (§ (?m\) : (€"Aa")))
Γ (                     )  Create a function g that takes a list (x:xs) and applies a function on x and xs.
   · (                 )   Compose g with second argument of function in parentheses.
                           Instead of x and xs, the function is called on x and the result of a recursive call of g on xs.
                (€"Aa")    Check if x is 'A' or 'a'.
        (?m\)              If it is, then swap the case of every char in g(xs).
      §       :            Otherwise, prepend x to g(xs).


Posted 2018-03-14T18:15:11.627

Reputation: 39 083

3Wow, good thing I refreshed before I posted my 12 byte solution: Ḟ·+m\ṁx'Ax'a. Could we get an explanation? I can't find any information on what Γ does exactly and this seems like a good chance to learn. – Sophia Lechner – 2018-03-15T00:02:24.023

1@SophiaLechner Done. This version of Γ is a bit hard to explain, I hope you can make sense of it. – Zgarb – 2018-03-15T08:16:56.773

Wow this is slow. Is it just TIO? – FrownyFrog – 2018-03-15T13:00:23.287

1@FrownyFrog It's Husk. Type inference of programs containing Γ seems to be slow in general. If you're not familiar with Husk, a program is interpreted by looping through all possible structures of the program (essentially the possible placements of parentheses) and all overloadings of each built-in, and choosing the first one where the result is well-typed. The interpreter is smart enough to reject some possibilities early, but it seems that the recursive version of Γ can mess with this step and force it to loop through a lot of choices. – Zgarb – 2018-03-15T13:34:00.870

@SophiaLechner I wrote a tip that explains Γ in some detail.

– Zgarb – 2018-03-15T14:42:44.827


C#, 121 bytes


** Update (thanks to @John & @aloisdg) **

C#, 69 bytes



Posted 2018-03-14T18:15:11.627

Reputation: 191

2Welcome to PPCG! Nice first answer! – RedClover – 2018-03-15T07:02:46.777

2you can save 7 bytes changing new[] { 'a', 'A' } to 'a', 'A' – John – 2018-03-15T08:58:34.710


You can do it in 69 bytes with the same logic! Try it online! (use input/ouput instead of console, remove ToList, inverse the ternary and use @John comment) This is a nice first answer. Keep going!

– aloisdg moving to – 2018-03-15T11:02:46.517

3Both of these versions do not swap case (they transform to uppercase only) when CapsLock is enabled. This is a requirement. (See the last test case) – Broadwell – 2018-03-15T23:51:12.577

@Broadwell How would you know if CapsLock is enabled? Are you sure the last test case is correct? It passes all other test cases, as far as I can see. Thanks! – Aalawlx – 2018-03-15T23:55:50.010

@Aalawix If you're not using a Mac, try typing the last case manually with CapsLock enabled first. Instead of a.ToUpper(), my recommendation is to perform something similar to Python's swapcase() method on even indices. – Broadwell – 2018-03-16T02:48:59.427


JavaScript (ES6), 93 88 84 82 bytes

(saved 5 bytes thanks to @Shaggy, 4 bytes thanks to @user81655, and 2 bytes thanks to @l4m2.)

a=>a.replace(A=/./g,c=>c in{a,A}?(A=!A,''):A?c:c[`to${c<{}?'Low':'Upp'}erCase`]())

Test cases:

let f=

a=>a.replace(A=/./g,c=>c in{a,A}?(A=!A,''):A?c:c[`to${c<{}?'Low':'Upp'}erCase`]())

console.log(f("The quick brown fox jumps over the lazy dog."));
console.log(f("Compilation finished successfully."));
console.log(f("What happens when the CapsLock key on your keyboard doesn't have a notch in it?"));
console.log(f("The end of the institution, maintenance, and administration of government, is to secure the existence of the body politic, to protect it, and to furnish the individuals who compose it with the power of enjoying in safety and tranquillity their natural rights, and the blessings of life: and whenever these great objects are not obtained, the people have a right to alter the government, and to take measures necessary for their safety, prosperity and happiness."));
console.log(f("CapsLock locks cAPSlOCK"));
console.log(f("wHAT IF cAPSlOCK IS ALREADY ON?"));

Rick Hitchcock

Posted 2018-03-14T18:15:11.627

Reputation: 2 461

1['to${c<'a'?'Low':'Upp'}erCase'] should save you a few bytes, replacing single quotes with backticks. – Shaggy – 2018-03-15T00:13:06.960

Sure does, @Shaggy. Thanks! – Rick Hitchcock – 2018-03-15T15:51:40.360

Using ^1 to have u be the parity can let you initialise it shorter: s=>s.replace(u=/./g,c=>/a/i.test(c)?(u^=1,''):+u?c[`to${c<'a'?'Low':'Upp'}erCase`]():c) – user81655 – 2018-03-18T07:45:19.653

Also here's another tricky way to test for the letter a which is shorter: a=>a.replace(A=/./g,c=>c in{a,A}?(A^=1,''):+A?c[`to${c<'a'?'Low':'Upp'}erCase`]():c) – user81655 – 2018-03-18T07:53:03.500

Brilliant, @user81655, especially using the in operator like that. Always more to learn! – Rick Hitchcock – 2018-03-18T10:45:12.173

Use {} as "[object Object]" to distingish upper and lower – l4m2 – 2018-04-09T11:18:52.877

Very nice, @l4m2, thanks! – Rick Hitchcock – 2018-04-09T17:47:26.423

@RickHitchcock does a=>a.replace(A=/./g,c=>c in{a,A}?(A=!A,''):A?c:c[`to${c<{}?'Low':'Upp'}erCase`]()) work? – l4m2 – 2018-04-09T17:50:58.803

Ahh, that's what you meant by Use {}. Sorry, yes, that's perfect, thanks again. – Rick Hitchcock – 2018-04-09T18:12:53.650


Perl 5 -p, 31 30 29 bytes

-1 byte thanks to @nwellnhof

-1 byte thanks to @ikegami

#!/usr/bin/perl -p

Try it online!

Ton Hospel

Posted 2018-03-14T18:15:11.627

Reputation: 14 114

Why not simply s/a(.*?)(a|$)/uc$1/egi (22 bytes)? – nwellnhof – 2018-03-16T13:59:51.633

@nwellnhof Because the capslock when active toggles the case, it doesn't just uppercase – Ton Hospel – 2018-03-16T15:18:06.987

1Ah, I see. Then s/a(.*?)(a|$)/$1^uc$1^lc$1/egi is one byte shorter. – nwellnhof – 2018-03-16T18:09:09.847

@nwellnhof Thanks, that is very neat – Ton Hospel – 2018-03-16T20:51:27.040

a([^a]*)a? is shorter than a(.*?)(a|$) – ikegami – 2018-03-18T08:47:52.173


R, 92 bytes


Thank @Giuseppe for fixing the answer.


# Write
  # Replace and return, this is the function that powers
  # the R store at index operations, a[i]<-b
    # First arg - what to replace = extract first list element
    # of a string input after splitting at a or A
    # Second arg - index to replace = abuse vector recycling
    # to create infinite F, T, F, T, F, etc series
    # Third arg - replacement values = replace with case toggled letters
  # Write without separation

Try it online!


Posted 2018-03-14T18:15:11.627

Reputation: 806

Perhaps I didn't make it clear, but this answer does not reverse capitalization when CapsLock is enabled (it only performs toupper), which is a requirement. – Broadwell – 2018-03-14T21:19:24.673

2ooohhhhhhh that's very clever with the c(F,T), although @Broadwell is right; looks like it'll be a chartr("a-zA-Z","A-Za-z",v)[w] rather than toupper – Giuseppe – 2018-03-14T21:22:39.963

@Giuseppe Thanks – Vlo – 2018-03-14T21:35:25.637


PowerShell Core, 105 bytes

"$args"|% t*y|%{if($_-in97,65){$c=!$c}else{Write-Host -n($_,("$_"|%("*per","*wer")[$_-in65..90]))[!!$c]}}

Try it online!

What with no real ternary operator and no default alias for printing to screen, it's not that short.

  • % t*y expands to | ForEach-Object -Method ToCharArray equiv. of "$args".ToCharArray()
  • Write-Host -n is for the parameter -NoNewLine
  • "$_" turns the [char] type back to [string] (chars have no upper/lower case in .Net)
  • |% *per does the same method call shortcut as earlier, but for .ToUpper(), same with .ToLower()
  • ($a,$b)[boolean test] abused as fake-ternary operator
  • !!$c force-casts to [bool] here it starts undefined $null so it gets forced it into existence as "caps lock: $false".


Posted 2018-03-14T18:15:11.627

Reputation: 2 412

1That |% t*y is a neat trick I'll need to remember. Shorter than [char[]], which I use a lot. I'd almost say that should go onto the Tips thread. – AdmBorkBork – 2018-03-19T13:10:14.480

94 bytes: -join($args|% t*y|%{if($_-eq'a'){$c=!$c}else{(("$_"|%("*per","*wer")[$_-in65..90]),$_)[!$c]}}). thanks for a |% *ethod operator! – mazzy – 2018-07-08T11:55:59.810


Java 8, 119 108 98 bytes

s->{int f=0,t;for(int c:s)if((t=c&95)==65)f^=1;else System.out.printf("%c",f<1|t<66|t>90?c:c^32);}

-11 bytes thanks to @OlivierGrégoire.
-10 bytes thanks to @Nevay.


Try it online.

s->{                           // Method with char-array parameter and no return-type
  int f=0,t;                   //  Flag-integer, starting at 0
  for(int c:s)                 //  Loop over the characters of the input as integers
    if((t=c&95)==65)           //   If the current character is an 'A' or 'a':
      f^=1;                    //    Toggle the flag (0→1 or 1→0)
    else                       //   Else:
      System.out.printf("%c",  //    Print integer as character
        f<1|                   //     If the flag-integer is 0,
        t<66|t>90?             //     or the current character isn't a letter:
         c                     //      Simply output the character as is
        :                      //     Else (the flag it 1 and it's a letter)
         c^32);}               //      Print it with its case reversed

Kevin Cruijssen

Posted 2018-03-14T18:15:11.627

Reputation: 67 575

1Damn imperatives... they forbade me to post my answer before yours... Anyways, here's my answer, 11 bytes shorter: s->{int z=0,d;for(int c:s)if((d=c&95)==65)z^=1;else System.out.printf("%c",z<1|d<66|d>90?c:c<91?c|32:c&95);} – Olivier Grégoire – 2018-03-15T10:51:37.570

@OlivierGrégoire Nice answer! And what do you mean by forbade me to post? Is your work-network that strict? – Kevin Cruijssen – 2018-03-15T11:16:26.653

My answer was ready for a while: I was just polishing the test cases before posting but then suddenly endless meetings happened. – Olivier Grégoire – 2018-03-15T11:19:15.073

@OlivierGrégoire Ah of course.. The meetings that always come at the wrong time.. I know the feeling sometimes. ;) – Kevin Cruijssen – 2018-03-15T11:21:05.540

No, it's just that your answer was so similar to mine that it wouldn't have been correct to post mine ;) – Olivier Grégoire – 2018-03-15T11:22:55.190

@OlivierGrégoire If you want I can delete mine and you can post it as yours. You've got my upvote if you do. :) – Kevin Cruijssen – 2018-03-15T12:59:04.873

1No, it's ok, I only have to blame myself for not being quick enough before the meetings ;-) But thank you for suggesting this! – Olivier Grégoire – 2018-03-15T13:23:39.207

298 bytes: s->{int f=0,t;for(int c:s)if((t=c&95)==65)f^=1;else System.out.printf("%c",f<1|t<66|t>90?c:c^32);} – Nevay – 2018-03-17T17:01:00.903


Python, 63 bytes

f=lambda s:s and[s[0]+f(s[1:]),f(s[1:]).swapcase()][s[0]in"aA"]

Another Python solution, works in Python 2 and 3. Takes a very long time for all but small inputs.


Posted 2018-03-14T18:15:11.627

Reputation: 481


C, 167 168 158 131 bytes

Thanks for @Martin Ender for the code review: I've switched the stream processing for string processing to help with reusability. Also many thanks to @RiaD and @ceilingcat for their suggestions.


Try it online!

How does it work?

/* int c is the input character,
   int d is the Caps Lock flag (1=off, 0=on)  starting as "Off". */
int c, d;
/* array of comparison functions and transformation functions for each state */
(*t[][2])() = {{isupper, tolower}, {islower, toupper}};

f(char *s) {
  /* Loop if we haven't hit the terminator */
  for(d = 1; c = *s++;)
    t[0][1](c) == 97 ?
      /* If tolower(c)=='a' then flip the Caps Lock state */
      /* Otherwise, convert character according to the following table:

                       Character case
         Caps Lock  UPPER       LOWER
                ON  tolower()   toupper()
               OFF  toupper()   tolower()


  • s[][] is where the magic happens: [][0] is the comparison function and [][1] is the related transformation function for each state.
  • ! is applied to the comparison function to force it into the range [0,1].


Posted 2018-03-14T18:15:11.627

Reputation: 2 149

Welcome to PPCG! Unfortunately, you can't rely on the initialisation of d like this because it means that your function is not reusable. A simple d=0; should fix it.

– Martin Ender – 2018-03-16T10:01:12.883

I wasn't sure if reusability or maintaining state was more important in this case. If reusability is more important, I would move the variable declarations inside the function so the beginning would read void f(){int c,d=0;[...]. In any case, the stream dies, so an edit is in order! – ErikF – 2018-03-16T11:05:06.710

do you need s in your while loop? It can't became NULL unless you called with f(NULL) – RiaD – 2018-03-16T14:42:41.660

d=!d for flipping – RiaD – 2018-03-16T14:43:30.913

!! will be ! if you flip order of t, and start d with 1 – RiaD – 2018-03-16T14:54:20.293

you may also just remove first 2 – RiaD – 2018-03-16T17:21:20.857

Suggest 1[*t] instead of t[0][1] – ceilingcat – 2018-12-30T17:58:55.003


6502 machine code routine (C64), 51 bytes

A0 00 84 FE B1 FC F0 2A C9 41 F0 06 90 1A C9 C1 D0 08 A9 80 45 FE 85 FE B0 11
B0 06 C9 5B B0 08 90 04 C9 DB B0 02 45 FE 20 16 E7 C8 D0 D6 E6 FD D0 D2 60

Expects a pointer to a 0-terminated input string in $fc/$fd, outputs to the screen.

Commented disassembly

A0 00       LDY #$00
84 FE       STY $FE             ; init capslock state
B1 FC       LDA ($FC),Y         ; next char from string
F0 2A       BEQ .done           ; NUL -> we're done
C9 41       CMP #$41            ; compare to 'a'
F0 06       BEQ .isa            ; if equal, toggle capslock
90 1A       BCC .out            ; if smaller, direct output
C9 C1       CMP #$C1            ; compare to 'A'
D0 08       BNE .ctog           ; if not equal, check for letter
A9 80       LDA #$80            ; toggle bit 7 in caps lock state
45 FE       EOR $FE
85 FE       STA $FE
B0 11       BCS .next           ; and go on
B0 06       BCS .cZ             ; if char larger 'A', check for 'Z'
C9 5B       CMP #$5B            ; compare with 'z'+1
B0 08       BCS .out            ; larger or equal -> direct output
90 04       BCC .tog            ; smaller -> apply capslock
C9 DB       CMP #$DB            ; compare with 'Z'+1
B0 02       BCS .out            ; larger or equal -> direct output
45 FE       EOR $FE             ; toggle bit from capslock state
20 16 E7    JSR $E716           ; output char
C8          INY                 ; and loop to next char
D0 D6       BNE .loop
E6 FD       INC $FD
D0 D2       BNE .loop
60          RTS

Example assembler program using the routine:

Online demo


Code in ca65 syntax:

.import caps ; link with routine above

.segment "BHDR" ; BASIC header
                .word   $0801           ; load address
                .word   $080b           ; pointer next BASIC line
                .word   2018            ; line number
                .byte   $9e             ; BASIC token "SYS"
                .byte   "2061",$0,$0,$0 ; 2061 ($080d) and terminating 0 bytes

string:         .res    $800

prompt:         .byte   $d, "input> ", $0

                lda     #$17            ; set upper/lower mode
                sta     $d018

                lda     #<prompt        ; display prompt
                ldy     #>prompt
                jsr     $ab1e

                lda     #<string        ; read string into buffer
                sta     $fc
                lda     #>string
                sta     $fd
                jsr     readline

                lda     #>string        ; call our caps routine on buffer
                sta     $fd
                jmp     caps

; read a line of input from keyboard, terminate it with 0
; expects pointer to input buffer in $fc/$fd
; NO protection agains buffer overflows !!!
.proc readline
                ldy     #$0
                sty     $cc             ; enable cursor blinking
                sty     $fe             ; temporary for loop variable
                lda     $fd
                sta     $2              ; initial page of string buffer
getkey:         jsr     $f142           ; get character from keyboard
                beq     getkey
                sta     $fb             ; save to temporary
                and     #$7f
                cmp     #$20            ; check for control character
                bcs     prepout         ; no -> to normal flow
                cmp     #$d             ; was it enter/return?
                beq     prepout         ; -> normal flow
                cmp     #$14            ; was it backspace/delete?
                bne     getkey          ; if not, get next char
                lda     $fe             ; check current index
                bne     prepout         ; not zero -> ok
                lda     $2              ; otherwise check if we're in the
                cmp     $fd             ;    first page of the buffer
                beq     getkey          ; if yes, can't use backspace
prepout:        ldx     $cf             ; check cursor phase
                beq     output          ; invisible -> to output
                sei                     ; no interrupts
                ldy     $d3             ; get current screen column
                lda     ($d1),y         ; and clear 
                and     #$7f            ;   cursor in
                sta     ($d1),y         ;   current row
                cli                     ; enable interrupts
output:         lda     $fb             ; load character
                jsr     $e716           ;   and output
                ldx     $cf             ; check cursor phase
                beq     store           ; invisible -> to store
                sei                     ; no interrupts
                ldy     $d3             ; get current screen column
                lda     ($d1),y         ; and show
                ora     #$80            ;   cursor in
                sta     ($d1),y         ;   current row
                cli                     ; enable interrupts
                lda     $fb             ; load character
store:          cmp     #$14            ; was it backspace/delete?
                beq     backspace       ; to backspace handling code
                ldy     $fe             ; load buffer index
                sta     ($fc),y         ; store character in buffer
                cmp     #$d             ; was it enter/return?
                beq     done            ; then we're done.
                iny                     ; advance buffer index
                sty     $fe
                bne     getkey          ; not zero -> ok
                inc     $fd             ; otherwise advance buffer page
                bne     getkey
done:           lda     #$0             ; terminate string in buffer with zero
                ldy     $fe             ; get buffer index
                bne     termidxok       ; and advance ...
                inc     $fd
termidxok:      sta     ($fc),y         ; store terminator in buffer
                inc     $cc             ; disable cursor blinking
                rts                     ; return
backspace:      ldy     $fe             ; load buffer index
                bne     bsidxok         ; if zero
                dec     $fd             ;   decrement current page
bsidxok:        dey                     ; decrement buffer index
                sty     $fe
                bcs     getkey          ; and get next key

Felix Palmen

Posted 2018-03-14T18:15:11.627

Reputation: 3 866

I just have to say I admire you went to the effort to write in assembly. I don’t think this has much to do with the fact I used to really enjoy asm but maybe having the experience makes me more aware of what it entails. Experience or ease is besides the point to me. It brightens my day just a bit to see such enthusiasm too. – Pryftan – 2018-03-18T21:23:58.290

@Pryftan thanks :) It's just a nice way to keep in practice, I'm working on some game and recently also demo code for this nice old machine :) – Felix Palmen – 2018-03-19T08:20:10.763

Well it's great to see! Keep it up; I remember enjoying asm but I don't think I'd enjoy it so much nowadays (unless maybe I had an old machine like you do, that is but possibly not even then) - C is my favourite all time and it's what I primarily use. Anyway, won't let this evolve into chat - just wanted to say I appreciated the answer! – Pryftan – 2018-03-19T20:56:35.967


Jelly, 14 bytes


Try it online!

Full program.


Œu=”Aœp⁸ŒsJḤ$¦ Arguments: x
Œu             Uppercase x
  =”A          ^ Equals 'A' (vectorizes)
     œp⁸       ^ Partition ⁸ [⁸=x]
             ¦ Apply link A, keep results at specific indices B
        Œs     A: Swap case
            $  B: Form a >=2-link monadic chain
          JḤ      Arguments: y
          J       Get list indices ([1, length(list)]) of y
           Ḥ      Double (vectorizes) ^
                  This way, we only "apply" link A to even indices, so every second
                  element, starting from the secondd one.

Erik the Outgolfer

Posted 2018-03-14T18:15:11.627

Reputation: 38 134

Explanation of the code? – SK19 – 2018-03-16T22:00:40.027

1@SK19 Added an explanation. – Erik the Outgolfer – 2018-03-19T10:07:51.550


Haskell, 92 bytes

import Data.Char
g x|x<'['=toLower x|1>0=toUpper x
f(a:b)|elem a"aA"=f$g<$>b|1>0=a:f b
f x=x

Try it online!


First we declare g to be the function that maps lowercase to upper case and uppercase to lowercase. This is actually the majority of our bytecount. Then we define the function f. If the input to f is of the form a:b we do

 |elem a"aA"=f$g<$>b
 |1>0=a:f b

a and A match the first pattern and thus we apply f to the input with it's case inverted. Otherwise we move a out front and apply f to b.

Post Rock Garf Hunter

Posted 2018-03-14T18:15:11.627

Reputation: 55 382


PHP 101 99 bytes


Run like this:

echo '[the input]' | php -nR '[the code]'


for ($s = $argn; $i < strlen($s); $i++) {
    if (lcfirst($s[$i]) == 'a') {
        $s = strtolower($s) ^ strtoupper($s) ^ $s; // Flip the whole string's case.
    } else {
        print $s[$i]; // Print the current letter.

This just loops through the string with a for loop, and on each iteration it checks if the current letter is a, if so, then flip the case of the whole string (method from here), and if not, then print the current letter.


Posted 2018-03-14T18:15:11.627

Reputation: 435

1The convention for code golf is that all code must be included. That means you have to take input as function parameter and actually declare a function (via the function keyword in php) or have a complete script (e.g. using $argn, $argv, $_GET). So at the moment this is not a correct submission. Return must be echoed or returned (only allowed for functions ofc). – Christoph – 2018-03-15T12:10:32.873

1Thanks for that @Christoph, I'm kind of new to golfing :). I've updated my answer now, just let me know if there's anything else that's wrong. – Ethan – 2018-03-15T20:14:33.897

@Christoph Wow! 75! Very nice! You have my +1 :) – Ethan – 2018-03-16T16:07:07.897


Ruby, 42 41 bytes


Try it online!

A lambda accepting a string, mutating the string in place, and returning it. The trick here is that sub returns the string (a truthy value) if a substitution was made, and returns nil otherwise. The existence of swapcase is pretty handy, too.

-1 byte: Replace boolean logic with ternary operator, thanks to Asone Tuhid

  s.sub!(/a(.*)/i){     # Replace "a" followed by anything with
    $1.swapcase         #   the case-swapped capture group
  } ? redo              # If a match was found, restart the block
    : s                 # Otherwise, return the modified string


Posted 2018-03-14T18:15:11.627

Reputation: 801

save 1 byte. The link was too long if I included all the test cases. – Asone Tuhid – 2018-03-16T06:17:40.600

@AsoneTuhid Thanks... One of these days I'll remember to use the ternary operator straight away, so you won't have to remind me. – benj2240 – 2018-03-16T15:01:22.933


Wolfram Language (Mathematica), 70 bytes


Try it online!

Takes input and output as a list of characters. For convenience I've added code in the footer to convert this from and back to a string.

How it works

The #//.{x___,"a"|"A",y___}:>Join[{x},...{y}]& part is standard: we find the first A (uppercase or lowercase), reverse case of that comes after the A, and repeat until there are no more A's to be found.

The interesting part is how we reverse case: the function ToUpperCase@# + ToLowerCase@# - #&. We add together the upper-cased version of the input and the lower-cased version of the input, then subtract the actual input. For example, given the list {"I","n","P","u","T"} this computes


which threads over lists as


and although Mathematica doesn't have any particular way of adding two strings, it's smart enough to simplify a+b-a to b for any values of a and b, including string values, so this simplifies to {"i","N","p","U","t"}.

Misha Lavrov

Posted 2018-03-14T18:15:11.627

Reputation: 4 846


MATL, 23 20 bytes

'a A'Yb&Ybt2L)Yo2L(g

Try it online!


'a A'Yb   % form a cell array containing {'a', 'A'}
&Yb       % split input into substrings, with either of those ('a' or 'A') as delimiters
t2L)      % extract out the even positioned cells from that result
Yo        % switch the case of those substrings
2L(       % place the result back in even positioned cells of the original cell array
g         % convert cell array to matrix, concatenating all substrings in the process
          % implicit output

Older answer (23 bytes):


Other methods I tried:


sundar - Reinstate Monica

Posted 2018-03-14T18:15:11.627

Reputation: 5 296


SNOBOL4 (CSNOBOL4), 141 92 bytes


Try it online!

Assumes a single line of input.

A whopping 49 bytes saved by @ninjalj!

Line S does all the work, explained below:

I                    # in the subject string I match the following PATTERN:
 ANY("Aa")           # match A or a and
 REM . R             # match the remainder of I, assigning this to R
 =REPLACE(           # replace the PATTERN above with
          R, ...)    # R with swapped cases.
   :S(S)             # and if there was a match, goto S, else goto next line 


Posted 2018-03-14T18:15:11.627

Reputation: 21 077

This gives the wrong answer (as you stated in your comment, case is swapped when CapsLock is on) – mbomb007 – 2018-03-14T18:34:36.687

I've edited the post to require case-swapping (rather than simply uppercase) when CapsLock is enabled because I never realized that my machine does this. – Broadwell – 2018-03-14T18:35:52.927

@mbomb007 ah, I hadn't realized OP had changed it; I'm editing for an explanation now so I'll include that in the explanation. – Giuseppe – 2018-03-14T18:35:57.230

I =INPUT;S I ANY("Aa") REM . R =REPLACE(R,&LCASE &UCASE,&UCASE &LCASE) :S(S); OUTPUT =I;END – ninjalj – 2018-03-16T01:02:02.957

@ninjalj are you also a SNOBOL golfer or am I just terrible at golfing it?? – Giuseppe – 2018-03-16T13:35:25.717

@Giuseppe: I used to golf occasionally in SNOBOL at – ninjalj – 2018-03-16T19:27:23.190


Husk, 15 bytes


Try it online!


ω(F·+(tm\)↕·≠_'a) -- example input: "Bar, baz and Foo."
ω(              ) -- apply the following, until fixpoint is reached:
          ↕       -- | split string with predicate
           · _    -- | | the lower-cased character
            ≠ 'a  -- | | is not 'a'
                  -- | : ("B","ar, baz and Foo.")
  F               -- | apply the following to the tuple
    +             -- | | join the elements with..
   · (   )        -- | | ..the second element: "ar, baz and Foo."
       m\         -- | | | swap case: "AR, BAZ AND fOO."
      t           -- | | | tail: "R, BAZ AND fOO."
                  -- | : "BR, BAZ AND fOO."
                  -- : "BR, Bz ND fOO."


Posted 2018-03-14T18:15:11.627

Reputation: 15 345


Stax, 12 bytes


Run and debug it online

It splits on a regex, and then alternately toggles case. Here's the same program, unpacked, ungolfed, and commented.

"a|A"|s split on regex /a|A/
rE  reverse and explode array to stack
W   repeat forever...
p   print top of stack with no newline
:~p print top of stack, case inverted, with no newline

Run this one


Posted 2018-03-14T18:15:11.627

Reputation: 8 616

I somehow cannot relate your explanation to your code. – SK19 – 2018-03-16T21:59:14.943

Try stepping through the commented one, and watching the internal state of the interpreter. Does that help? – recursive – 2018-03-16T22:29:54.607

1@SK19: Oh, I think I see the problem. I didn't mention that stax programs have two representations. Ascii and packed. There's a lossless conversion between the two. Ascii is easy to type, but wasteful for golf, since there are only 95 symbols. The golfed program is packed, so it looks different, but it's the same program. – recursive – 2018-03-16T23:45:17.110


05AB1E, 12 bytes


Try it online!


õ?             # print an empty string (to account for the special case of only A's)
  „AaS¡        # split on occurrences of "A" or "a"
       D       # duplicate
        v      # for each element in the top copy
         ć?    # extract and print the head of the other copy
           š   # switch the case of the rest of the other copy


Posted 2018-03-14T18:15:11.627

Reputation: 50 798


Japt v2.0a0, 16 bytes


Try it


e                   :Recursively replace
 /a.*/i             :RegEx /a.*/gi
       _            :Pass each match through a function
        Å           :  Slice off the first character
         r          :  Replace
          \l        :  RegEx /[A-Za-z]/g
            _       :  Pass each match though a function
             c^     :    Bitwise XOR the character code
               H    :    With 32


Posted 2018-03-14T18:15:11.627

Reputation: 24 623


Fortran (GFortran), 307 bytes

G=' '
DO I=1,999

Try it online!

Since Fortran has not "advanced" tools for dealing with strings, I came up with this little monster.

Indented and commented:

CHARACTER(999)F,G	!Define input and output strings (up to 999 characters)
G=' '			!Fill output with spaces
READ(*,'(A)')F		!Take input
N=1			!Represent the position to be written in output string
M=1			!M=-1: Change case; M=1: Do not change case
DO I=1,999
	IF(F(I:I)=='a'.OR.F(I:I)=='A')THEN	!If the character is A...
		M=-M				!Ah-ha - you pressed cPS-LOCK!
	ELSEIF(M==1)THEN			!Case the character is not A, and do not need to change case...
		G(N:N)=F(I:I)			!...only copy the character
	ELSE !Otherwise...
		J=IACHAR(F(I:I))			!...get ascii of current character
			CASE(65:90)			!If is upper case,
				G(N:N)=ACHAR(J+32)	!now is lower case
			CASE(97:122)			!If is lower case,
				G(N:N)=ACHAR(J-32)	!now is upper case
			CASE DEFAULT			!If do not belong to alphabet,
				G(N:N)=F(I:I)		!simply copy the character
PRINT*,TRIM(G) !Trim out trailing spaces
END !That's all folks!


Posted 2018-03-14T18:15:11.627

Reputation: 310


Javascript (ES6), 80 79 bytes

(Partly based off of this answer by Rick Hitchcock. Posting as a separate answer because I don't have sufficient reputation to comment.)

(Saved 1 byte thanks to @l4m2's post here.)


Yair Rand

Posted 2018-03-14T18:15:11.627

Reputation: 381

Welcome to PPCG! – Laikoni – 2018-03-19T09:16:45.290


Dirty, 55 bytes

    \   ␛◌Ẃ!w1/      \1wX/

Try it online!

Can probably be about a third shorter.
I'll write up an explanation and golf some more when I'm on a desktop.


Posted 2018-03-14T18:15:11.627

Reputation: 7 916


Python 3, 78 72 bytes

import re
lambda x:re.sub("[Aa](.*?)(a|A|$)",lambda m:m[1].swapcase(),x)


Posted 2018-03-14T18:15:11.627

Reputation: 3 987

You can use m[1] in place of on Python 3.6+. – Bubbler – 2018-03-15T00:04:11.573

Why was this flagged as low quality…? – Nissa – 2018-03-15T00:21:33.527

I have no idea ... – pppery – 2018-03-15T00:22:21.723

1New posts are flagged automatically if they are short and contain no text. Adding a description usually prevents that. – mbomb007 – 2018-03-15T00:35:48.463

And now I wonder how many "A"s there are supposed to be in "ppperry". – Mr Lister – 2018-03-15T11:03:51.517

Um, why should there be any? – pppery – 2018-03-17T17:38:24.583


PHP 4, 77 76 75


Split into substrings by A (case insensitive) then toogle every second case.

Try it out here.

Old version


walks over the string and toogles a flag if the current char is a or A else the char gets toogled depending on the flag and echoed.


Posted 2018-03-14T18:15:11.627

Reputation: 1 489


><>, 139 129 bytes

1vo   <
^o+< >::"aA"@=?^=?^$v
^o-^?  ("^"$*48:v?@:<
^ vv? )"Z":v?(< >>o
  >  ^v  < >:0 )?^;
  :< ^v?( "a"

A language with no concept of "characters" is surely the right tool for the job :)

SE - stop firing the good guys

Posted 2018-03-14T18:15:11.627

Reputation: 529


Julia 1.0, 82 bytes

s->foldl((b,c)->c∈"aA" ? !b : (print(c+32isletter(c)sign('_'-c)b);b),s,init=1<0)



function f(s::String)::Bool
    foldl(s, init=false) do b, c
        if c ∈ "aA"
            return !b
            print(c + 32 * (isletter(c) & b) * sign('_'-c))
            return b

foldl(f, s, init=false): takes a function f that maps a state and a Char c to a new state. Applys f repeatedly over each Char of the string s, always passing the state previously returned by f back to f. init is the initial state. Here the state represents whether caps-lock is on.

if c in "aA": If c is an upper- or lowercase 'a', just return the opposite state.

isletter(c) & b: Bool, returns true iff c is a letter and b indicates, that caps-lock is on.

sign('_'-c): -1 if c is lowercase, 1 if c is uppercase.

print(c + 32 * (isletter(c) & b) * sign('_'-c)): Bools act like 0/1 under simple arithmetic operations, so if caps-lock should have an effect, this either adds or substracts 32 from c, returning a Char with opposite case. Then just print that.

Try it online!


Posted 2018-03-14T18:15:11.627

Reputation: 381


JavaScript (ES6), 62 60 bytes




s.split(/a|A/) split string into array removing a's at the same time

.map((x,i)=>(i%2?x.toUpperCase():x)) make every other string in the array upper case

.join`` convert the array to a string. without any joining character



console.log(f("The quick brown fox jumps over the lazy dog."));
console.log(f("Compilation finished successfully."));
console.log(f("What happens when the CapsLock key on your keyboard doesn't have a notch in it?"));
console.log(f("The end of the institution, maintenance, and administration of government, is to secure the existence of the body politic, to protect it, and to furnish the individuals who compose it with the power of enjoying in safety and tranquillity their natural rights, and the blessings of life: and whenever these great objects are not obtained, the people have a right to alter the government, and to take measures necessary for their safety, prosperity and happiness."));
console.log(f("CapsLock locks cAPSlOCK"));
console.log(f("wHAT IF cAPSlOCK IS ALREADY ON?"));


Posted 2018-03-14T18:15:11.627

Reputation: 21

1Welcome to Code Golf! You can save a couple bytes by replacing .join('') with .join\`` – Oliver – 2019-08-08T19:50:37.580

Thanks! I will make the edit. – Joel – 2019-08-08T20:26:07.063

1Not correct (although pretty nicely golfed), when you enter Capslock mode you need to flip the capitalisation, not just make it uppercase. – IQuick 143 – 2019-08-09T12:32:56.353


Python 3, 114, 101, 97 bytes

import re
print(''.join(i.swapcase()if j%2else i for j,i in enumerate(re.split('a|A',input()))))

Split on all a's and toggle the case on the odd parts

Thanks @ElPedro !

Rick Rongen

Posted 2018-03-14T18:15:11.627

Reputation: 223

Can reduce to 96 by not assigning input() to the variable s. Try it online!

– ElPedro – 2018-03-15T13:11:30.400

Good one @ElPedro I will edit! – Rick Rongen – 2018-03-15T13:13:12.247


CJam, 31 bytes



Loops through searching for a and non-a

Try it online!

CJam, 47 bytes

Slightly more fun version


Try it online!


This code has two parts.

               Part 1
 q                          -> Read all input as a single string
 'a'Aer                     -> Replace 'a' in string with 'A'
 'A/                        -> Split by 'A' leaving empty sets
 _                          -> Duplicate
 2%                         -> Get all rows where i%2 is 0
 \                          -> Swap top two elements of stack
 2/                         -> Split into array with groups of length 2
 z                          -> Zip/Transpose
 )\;                        -> Right uncons, swap and pop.

               Part 2
  _[el_eu]                  -> Create an array containing the lower case and upper case version
  z                         -> Zip/Transpose
   \                        -> Swap top two
   (                        -> Uncons left
   @                        -> Rotate top three elements of stack
   |                        -> Set union
   1=                       -> Get element at array indice 1 (Wraps)
   \                        -> Swap top two
  }%                        -> Map onto every element of string
  e_                        -> Flatten
 }%                         -> Map onto every element of array
]z                          -> Zip/Transpose

The first part splits the string by 'A' leaving any empty sets behind. For the string "baacadE" that will give the following array.

["b", "", "c", "dE"]

That way, all normal case elements are at even indices and reverse case are odd. Which After executing the rest of the first part gives the following.

[["b", "c"], ["", "dE"]]

The second part will take the odd half and reverse the case of every string. This is done with the set union operator |. So for the element "dE" It will do the following. Since the set union operator preserves the order of the elements found in the first string/array we can always assume the reverse case element will be the second one in the string/array.

["dE", "dD"]
["dD", "dE"]
["dD", "E", "d"]
["E", "d", "dD"]
["E", "dD"]
["E", "D"]
["D", "E", "eE"]
["D", "Ee"]
["D", "e"]

All That is left to do is zip up the two halves.


Posted 2018-03-14T18:15:11.627

Reputation: 171


Rust, 330 bytes

fn main(){let mut i=String::new();std::io::stdin().read_line(&mut i);let mut o=vec![];let mut c=false;for l in i.trim().as_bytes(){if*l==65||*l==97{c=!c;}else if c{if l.is_ascii_uppercase(){o.push((*l).to_ascii_lowercase());}else{o.push((*l).to_ascii_uppercase());}}else{o.push(*l);}}println!("{}",String::from_utf8(o).unwrap());}


fn main() {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input);
    let mut output_chars = vec![];
    let mut capslock = false;
    for letter in input.trim().as_bytes() {
        if *letter == 65 || *letter == 97 {
            capslock = !capslock;
        } else if capslock {
            if letter.is_ascii_uppercase() {
            } else {
        } else {
    println!("{}", String::from_utf8(output_chars).unwrap());

Since this uses bytes instead of chars in the loop, 65 and 97 are the byte values for 'A' and 'a'.

I'm new to Rust, so this might be golfable further.


Posted 2018-03-14T18:15:11.627

Reputation: 91

2Welcome to PPCG! – Giuseppe – 2018-03-15T12:09:49.960


Chip, 64 bytes


Try it online!

How it works

This is where I wish I could color-code the source. However, all of the below are full programs, and may be run on their own.

First things first, write a (slightly mangled) cat program.



Now invert capitalization of all characters A-Za-z.

This is determined by:

  1. c & 0x1F != 0
    • Filter out anything whose low 5 bits are zero
    • This is done by the left most column and upper ~
  2. ((c & 0x1F) + 0x5) & ~0x1F == 0
    • Filter out anything, that when added to 5, carries into or beyond the 6th bit
    • This is done by the two # columns
  3. c & 0x40 != 0
    • Filter out anything that is less than 64
    • This is done by the lower ~

The program assumes it will receive no characters above 127.

The results are aggregated and, the inversion is done by the { at the bottom.

>C#. Bb
>D## Cc
>E## Dd
`~+' Ee

But attach that to a toggle (T flip-flop) that is initially off. Now, the toggle decides whether to invert the capitalization, and the filter from above decides which characters to apply it to. Put a * above the m to toggle on every cycle.

>C#. Bb
>D## Cc
>E## Dd
`~+' Ee

Toggle the toggle when we see an Aa, and suppress that character too. We have an Aa if the filter above says we have A-Za-z, and the low five bits equal one (c & 0x1F == 1).



Posted 2018-03-14T18:15:11.627

Reputation: 1 366


Javascript (ES6), 64 bytes


Let me know if you think this is broken :)

(edit: simplified ternary)

Nick Ellsworth

Posted 2018-03-14T18:15:11.627

Reputation: 111

1This doesn't seem to work with letters that are already uppercase. – Yair Rand – 2018-03-25T19:59:43.710


x86-64, 31 bytes

Conforms to the System-V calling convention, tested in Ubuntu 16.04. Takes input from a pointer to a null-terminated in rsi, and outputs the result as a null-terminated string in rdi. rdi is expected to pointer to a buffer that's already been allocated with sufficient size.

Disassembly for byte count:

   0:       31 d2                   xor    edx,edx
   2:       ac                      lods   al,BYTE PTR ds:[rsi]
   3:       88 c1                   mov    cl,al
   5:       24 df                   and    al,0xdf
   7:       3c 41                   cmp    al,0x41
   9:       75 05                   jne    10 <not_A>
   b:       80 f2 20                xor    dl,0x20
   e:       eb f2                   jmp    2 <loop>
  10:       2c 41                   sub    al,0x41
  12:       3c 19                   cmp    al,0x19
  14:       77 02                   ja     18 <not_ascii>
  16:       30 d1                   xor    cl,dl
  18:       91                      xchg   ecx,eax
  19:       aa                      stos   BYTE PTR es:[rdi],al
  1a:       84 c0                   test   al,al
  1c:       75 e4                   jne    2 <loop>
  1e:       c3                      ret

Commented Assembly (GAS):

.intel_syntax noprefix
// rsi has the input char*, rdi has the output char*.
// Input char* is \0-terminated.
.global caps
    // Use this for xor mask.
    xor     edx, edx
// Loop is set up as a do-while loop.
    // al = *rdi++

    // Save the character we read into cl.
    // We use al because the instructions are shorter.
    mov     cl, al

    // Make upper case
    and     al, 0b11011111

    // 65 = 'A'
    cmp     al, 65
    jne     not_A
    // Invert capitialization mask
    xor     dl,0x20
    // Continue
    jmp     loop

    // Now we check if input character is ascii.
    // Basically, if (al-'A' > 'Z'-'A'), it's not ascii
    sub     al, 65
    cmp     al, 25

    ja      not_ascii
    // Flip capitilization if necessary.
    xor     cl, dl

    // Restore saved character that we read.
    // xchg is 1 byte, as opposed to mov, which is 2.
    xchg    ecx, eax

    // Write out the character to the output buffer.

    // If al == 0, break
    test    al, al
    jnz     loop

    // Just return, we've already written out the null
    // character to the output string.

Testing code in C, takes the input string via the first command line argument, prints out the result string:

#include <stdio.h> //puts
#include <stdlib.h> //malloc, free
#include <string.h> //strlen

void caps(char* output, char* input);

int main(int argc, char** argv) {
    char* instr = argv[1];
    char* buf = malloc(strlen(instr) + 1);
    caps(buf, instr);

    // Print converted string.


Ian Chew

Posted 2018-03-14T18:15:11.627

Reputation: 171


Python 3, 110 bytes

code_golf=lambda s:"".join([j.swapcase()if i&1else j for i,j in  enumerate(__import__('re').split(r"a|A",s))])

Kuldeep Gajera

Posted 2018-03-14T18:15:11.627

Reputation: 11


You should use codeblock formatting so that the underscores in __import__ don't get interpreted as bolding. As for actual golfing, you can remove the code_golf= part, as anonymous functions are acceptable. You can also use an online interpreter like TIO so that people can test your code and it can auto-format your CGCC submission. Try It Online

– Jo King – 2019-08-08T04:20:06.603

Some other golfing tips, the [] in the join is not needed and it is shorter to import the module outside of the lambda. Additionally, could you please add the score of your submission to the header? – Jo King – 2019-08-08T04:56:26.630

Welcome! It is recommended to add an explanation for your code, because code-only answers are usually automatically flagged as low-quality. – mbomb007 – 2019-08-08T14:41:02.617

... and there are two spaces between for i,j in and enumerate( – pppery – 2019-08-08T14:42:35.177


Forth, 225 bytes

: d DUP ; : x SWAP ; : w WITHIN ; : a d d 65 = x 97 = OR ; : y d d 65 91 w x 97 123 w OR ; : ~ INVERT ; x VALUE s 0 VALUE f : l 0 DO I s + C@ a IF f ~ TO f THEN y IF f IF 32 XOR d THEN a ~ IF EMIT THEN ELSE EMIT THEN LOOP ; l

What happening:

: d DUP ;                     \ shortcut for duplicating value on stack
: a d d 'A' = SWAP 'a' = OR ;             \ testing, if char is 'a' or 'A'
: y d d 'A' '[' WITHIN SWAP 'a' '{' WITHIN OR ;   \ testing, if char is within letters
: ~ INVERT ;                      \ shortcut for negation
SWAP                          \ swap addr and len of the string after s" command
VALUE s                       \ pop addr to s
0 VALUE f                     \ write 0 to f(lag)
: l 0 DO                      \ declaration of loop named 'l' from 0 to len (len is laying on top of the stack, remember)
I s + C@                      \ push iteration number on stack and load char from s + iteration

        \ this IF is searching for 'a' or 'A'
a IF
    f ~ TO f              \ if we found 'a' or 'A', then flip the f(lag)
y IF                      \ if char is letter
    f IF                  \ and f is set
    32 XOR d THEN             \ change case of symbol

    a ~ IF                \ if it's not 'a' or 'A' then print char
    EMIT                  \ print char


Try it online!

Actually code in tio is 285 bytes, but this is because Footer didn't work for defining string, so i had to include string definition in actual code.


Posted 2018-03-14T18:15:11.627

Reputation: 129


Pyth, 21 bytes


Try it here


           scR\AcQ\a   Split the input on 'a' and 'A'
          c         2  Split the blocks into pairs.
  m  d   d             For each pair...
   +P r2e              ... cAPSlOCK the second block.
ss                     Join the blocks together.


Posted 2018-03-14T18:15:11.627



Pyth, 19 bytes


Try it here.

Saved one byte thanks to mnemonic.

Mr. Xcoder

Posted 2018-03-14T18:15:11.627

Reputation: 39 774


Python 3, 100 bytes

edit: save 3 bytes thanks to sonrad10! Thanks!

import re
print(''.join(map(lambda s:x.index(s)%2and s.swapcase()or s,x)))

Try it online!

To be further golfed (probably).


Posted 2018-03-14T18:15:11.627

Reputation: 391

1You could change from from re import * to import re and x=re.split(...) to save 3 bytes. – sonrad10 – 2018-03-15T11:25:24.593


Charcoal, 21 bytes


Try it online! Link is to verbose version of code. Explanation:


Loop through each character in the line of input.


Test whether it equals a in lower case.


If so then logically negate k. (Note that for some reason Charcoal considers k's default value None to be truthy.)


Otherwise check whether k is currently truthy.


If so then just print the character.


Otherwise check whether the current character is an upper case letter.


If it is then print it in lower case.


Otherwise print it in upper case.


Posted 2018-03-14T18:15:11.627

Reputation: 95 035


Python 2, 103, 97 bytes

import re
def f(s):print"".join(j.swapcase()if i%2else j for i,j in enumerate(re.split('a|A',s)))


Posted 2018-03-14T18:15:11.627

Reputation: 535


APL (Dyalog Classic), 46 41 40 bytes


Try it online!


  • 'aA'~⍨ - Remove all a's from ...
  • ⊢819⌶⍨¨2|(⊢≠819⌶¨) - ... the alternating upper-and-lower-case of the argument ...
  • +≠\∘ ... based upon ...
  • (∨⌿'aA'∘.=⊢) ... the position of a and/or A.


Posted 2018-03-14T18:15:11.627

Reputation: 5 710


Yabasic, 121 bytes

An anonymous function that takes input as a line of text from STDIN and outputs to STDOUT.

Line Input""s$
For i=1To Len(s$)
If u$="A"Then
ElsIf d Then?u$;Else?Lower$(c$);Fi

Try it online!

Taylor Scott

Posted 2018-03-14T18:15:11.627

Reputation: 6 709


Excel VBA, 115 bytes

A declared subroutine that takes input, s of expected type variant/string and outputs to the range [A1].

Sub f(s)
For i=1To Len(s)
If u="A"Then d=Not d Else[A1]=[A1]+IIf(d,u,LCase(c))
End Sub

Taylor Scott

Posted 2018-03-14T18:15:11.627

Reputation: 6 709




Test it here

Basically it decides to write in uppercase depending on a flag everytime it encounters an 'a'

Francisco Hahn

Posted 2018-03-14T18:15:11.627

Reputation: 591


Welcome to PPCG! It seems that your code assumes that the input is given in a hardcoded variable. However, all answers need to be either full programs or callable functions. Wrapping your code in a function which takes $str as a parameter would be the simplest fix, but I don't know if PHP has shorter ways to read the input from STDIN or a command-line argument instead.

– Martin Ender – 2018-03-16T17:11:18.027


pwsh, 109 bytes

$Input|%{$i=$_[0]-ceq'a';$r='';foreach($s in $_.split('a')){$r+=($s.ToUpper(),$s.ToLower())[$i];$i=!$i};$r}


Posted 2018-03-14T18:15:11.627

Reputation: 111


Python 3, 100 bytes

def f(x):x=x.replace('A','a').split('a');return''.join(i.swapcase()if i in x[1::2]else i for i in x)

Try it online!


Posted 2018-03-14T18:15:11.627

Reputation: 879


Haskell, ~~199~~ 133 bytes

import Data.Char
l c|isUpper c=toLower c|1<2=toUpper c
f _""=""
f b(c:cs)|toLower c=='a'=f(not b)cs|1<2=(if b then l else id)c:f b cs

Run using f False "<string>"

Ungolfed version:

import Data.Char

flipCase c
    | isUpper c = toLower c
    | otherwise = toUpper c

f "" _ = ""
f b (c:cs)
    | toLower c == 'a' = f cs $ not b
    | True = (if b then flipCase else id) c:f b cs


Posted 2018-03-14T18:15:11.627

Reputation: 251

You can replace cs by a single letter variable name and define f as an infix function.

– Laikoni – 2018-03-20T11:06:32.290

Also note that when requiring extra arguments for the function, those need to be counted too. In this case the default would be to add 8 bytes to the byte count for f False and a newline. – Laikoni – 2018-03-20T11:10:14.777

Instead of using a boolean it is shorter to use 0 and 1. not b then becomes 1-b, b in the conditional becomes b>0 and f False just f 0. – Laikoni – 2018-03-20T11:14:46.887


Haskell, 96 94 Bytes

(Unix Line Endings)

import Data.Char
i c|c>'Z'=t c|0<1=toLower c
f(x:s)|t x=='A'=f$i<$>s|0<1=x:f s
f x=x


Posted 2018-03-14T18:15:11.627

Reputation: 195

@Laikoni Unfortunate. I've fixed that now at the cost of 16 more bytes, but realised I was counting Windows line endings as 2 bytes, so I've clawed back 3 bytes by switching to Unix line endings. – Pharap – 2018-03-19T23:05:31.300


Pip, 22 bytes

(The SC operator was added because of this challenge.)


Try it online!


                        i is 0, x is empty string (implicit)
                     q  Read a line of input
{                  }M   Map this function to its characters:
 LCa                     The character, lowercased
    Q'a                  Is it equal to lowercase a?
       ?                 If so:
          !:i             Logically negate i in place (0 -> 1, 1 -> 0)
        xX                Repeat the empty string that many times (thus, always return
                          empty string when the character is a/A)
             i?           Is i truthy (1)? If so, caps lock is on, so return:
               SCa         The character, swap-cased
                          Else return:
                  a        The character, unchanged


Posted 2018-03-14T18:15:11.627

Reputation: 21 213


Java (OpenJDK 9), 90 bytes

int j=0;for(char t:c){if(t==65||t==97)j^=1;else System.out.printf("%c",j>0&&t>65?t^32:t);}

Try it online!

anubhav deshwal

Posted 2018-03-14T18:15:11.627

Reputation: 1

Welcome to PPCG! Submissions should be a program or function, and yours is currently a snippet. Adding c->{...} around your code mades it a valid lambda function. You can also golf || and && to | and &, and remove the {} around the for-loop. In addition, char can be int in this case. Try it online 90 bytes. Nice first answer, and enjoy your stay! Oh, and you might find Tips for golfing in Java interesting to read through.

– Kevin Cruijssen – 2018-04-09T08:07:41.760


K4, 49 bytes




q)k){@[x;&.q.mod[;2]@&-':w;.q.upper]_/|w:&"a"=_x,"a"}"The quick brown fox jumps over the lazy dog."
"The quick brown fox jumps over the lZY DOG."
q)k){@[x;&.q.mod[;2]@&-':w;.q.upper]_/|w:&"a"=_x,"a"}"Compilation finished successfully."
q)k){@[x;&.q.mod[;2]@&-':w;.q.upper]_/|w:&"a"=_x,"a"}"What happens when the CapsLock key on your keyboard doesn't have a notch in it?"
"WhT Hppens when the CPSLOCK KEY ON YOUR KEYBOrd doesn't hVE  notch in it?"


Fairly heavy, will look at other answers to see if I can golf this down:

{@[x;&.q.mod[;2]@&-':w;.q.upper]_/|w:&"a"=_x,"a"} / the solution
{                                               } / lambda with implicit input x
                                           x,"a"  / append "a" to input
                                          _       / lowercase
                                      "a"=        / boolean list where input is "a"
                                     &            / indices where true
                                   w:             / save as w
                                  |               / reverse
                                _/                / drop over (remove these indices from...)
 @[ ;                 ;        ]                  / apply [var;indices;function]
                       .q.upper                   / uppercase
                  -':w                            / deltas of w
                 &                                / where, builds list of ascending values
                @                                 / apply
      .q.mod[;2]                                  / mod 2 to generate boolean list
   x                                              / apply uppercase to input x at these locations


Think I may have missed the point. I enabled/disable caps when an a is encountered. Passes the first test cases but not the ones where caps is apparently already enabled...


Posted 2018-03-14T18:15:11.627

Reputation: 3 635


Lua, 152 151 bytes""for c in p:gmatch"."do l=c:lower()if c:find"[aA]"then a=(a+1)%2 else s=s..((c:find"%A"or a==0)and c or l==c and c:upper()or l)end end

Try it online!


p = -- reads the input
a = 0
s = ""
for c in p:gmatch"." do -- loops through the string 'p' (the input) calling each character 'c'
  l = c:lower() -- stores the lowercase version of 'c'
  if c:find("[aA]") then -- if 'c' is a lowercase or uppercase 'A'
    a = (a+1)%2 -- flip the value of 'a' between 0 and 1, from what I've seen it uses less space
    s = s..((c:find("%A") or a==0) and c or l==c and c:upper() or l)
    s = s..   -- append to the string 's'
            (c:find("%A") or a==0)   -- if this is not a letter or the "reversion" is off
                                   and c or   -- then append 'c' else
                                            l==c and c:upper() or l   -- if 'c' is lowercase then append it as uppercase else append it as lowercase

If you have any questions, feel free to ask and i'll try to answer.


Posted 2018-03-14T18:15:11.627

Reputation: 151


Lua 5.3, 137 bytes

a=0s=""".",function(c)if c:find"[aA]"then a=not a else l=c:lower()s=s..(a and c or l==c and c:upper()or l)end end)print(s)

Try it online!


My result was very similar to @Visckmart above. But I included the print(s) in my byte count. Also the try it online test uses Lua 5.1, where my final result is in Lua 5.3. To make my final answer run on the website linked above I had to add a single space in between defining a and s: a=0 s="" which make it 138 bytes when using Lua 5.1


Posted 2018-03-14T18:15:11.627

Reputation: 1