Cheating a multiple choice test, part 2

Question

This is sequel to this challenge by Adnan. If you like this challenge, chances are you'll like the other one too. Check it out!

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A multiple choice test with 8 questions each with 4 choices might have the answers: BCADBADA. Converted to four different arrays, with true and false if the current letter is the answer, it will look like this

Q#: 1  2  3  4  5  6  7  8
    B  C  A  D  B  A  D  A
A: [0, 0, 1, 0, 0, 1, 0, 1]
B: [1, 0, 0, 0, 1, 0, 0, 0]
C: [0, 1, 0, 0, 0, 0, 0, 0]
D: [0, 0, 0, 1, 0, 0, 1, 0]

This can be compressed using a bit of logic. Each of the choices A, B, C and D can be represented by two true/false values shown below:

A: 1 0
B: 0 1
C: 0 0
D: 1 1

Using this logic, we can compress the four vectors above to just two:

 1  2  3  4  5  6  7  8
 B  C  A  D  B  A  D  A
[0, 0, 1, 1, 0, 1, 1, 1]
[1, 0, 0, 1, 1, 0, 1, 0]

That is, the solution to your test is simply: 00110111, 10011010. By concatenating these, we get the binary number 0011011110011010, or 14234 in decimal. Use this decimal value to cheat on your test!

Challenge

Take a number N in the (inclusive) range [0, 65535], and output a string with the answer to the multiple choice test.

Test cases:

14234
BCADBADA

38513
ABBDCAAB    

0
CCCCCCCC

120
CBBBBCCC

65535
DDDDDDDD

39253
ABCDABCD

The output may be in upper or lower case letters, but you can not use other symbols.

Answer 1

Jelly, 14 bytes

d⁹+⁹BZḄḊị“BADC

Try it online! or verify all test cases.

How it works

d⁹+⁹BZḄḊị“BADC  Main link. Argument: n

d⁹              Divmod 256; yield [n : 256, n % 256].
  +⁹            Add 256; yield [n : 256 + 256, n % 256 + 256].
    B           Binary; convert both integers to base 2.
     Z          Zip; group the quotient bits with corresponding remainder bits.
      Ḅ         Unbinary; convert from base 2 to integer.
       Ḋ        Dequeue; discard the first integer, which corresponds to the
                dummy value introduced by adding 256 to quotient and remainder.
        ị“BADC  Index into that string, mapping [1, 2, 3, 0] to "BADC".

Answer 2

05AB1E, 19 18 16 bytes

Code:

žH+b¦2äøC’c‰±’sè

Uses the CP-1252 encoding. Try it online!

Explanation:

First, we add 65536 to the number (žH is a constant defined to 65536), which is also 10000000000000000 in binary. This is to pad the number with zeroes. Let's take the number 14234 as an example. 14234 + 65536 is equal to 79770. Which in binary is:

10011011110011010

We remove the first character, resulting in:

0011011110011010

We split the string into two pieces using 2ä:

00110111, 10011010

After that, we zip the array with ø:

01, 00, 10, 11, 01, 10, 11, 10

Converting them back into decimal (using C) results in:

1, 0, 2, 3, 1, 2, 3, 2

Now, we only need to index it with the string cbad. The compressed version for this string is ’c‰±’, which can also be tested here. Finally, we get the characters at the index of the above array. For the above example, this results in:

1, 0, 2, 3, 1, 2, 3, 2
b  c  a  d  b  a  d  a

Answer 3

JavaScript (ES6), 55 48 bytes

f=(n,i=8)=>i--?"CBAD"[n>>i&1|n>>i+7&2]+f(n,i):''

console.log(f(14234)); // BCADBADA
console.log(f(38513)); // ABBDCAAB
console.log(f(0));     // CCCCCCCC
console.log(f(120));   // CBBBBCCC
console.log(f(65535)); // DDDDDDDD
console.log(f(39253)); // ABCDABCD

Non-recursive version (55 bytes)

Using a regular expression, we can do:

n=>"76543210".replace(/./g,i=>"CBAD"[n>>i&1|n>>+i+7&2])

Answer 4

Python 2, 53 bytes

f=lambda n,k=8:k*'_'and f(n/2,k-1)+'CBAD'[n>>7&2|n&1]

Test it on Ideone.

Answer 5

CP-1610 assembly, 24 DECLEs (30 bytes)

This code is intended to be run on an Intellivision. ⁽¹⁾

A CP-1610 opcode is encoded with a 10-bit value, known as a 'DECLE'. The actual function is 24 DECLEs long, starting at $4809 and ending at $4820.

The CPU registers are however 16-bit wide, so it will support any input value in 0x0000 .. 0xFFFF.

                            ROMW  10            ; use 10-bit ROM
                            ORG   $4800         ; start program at address $4800
4800 0002                   EIS                 ; enable interrupts (to enable display)

                    ;; ---- usage example
4801 0001                   SDBD                ; load parameter in R0
4802 02B8 009A 0037         MVII  #14234, R0    ;
4805 0004 0148 0009         CALL  cheat         ; call function
4808 0017                   DECR  PC            ; infinite loop

                    ;; ---- 'Cheat Your Test' function
                    cheat   PROC  

4809 0082                   MOVR  R0,     R2    ; copy R0 to R2
480A 0040                   SWAP  R0            ; swap LSB/MSB in R0
480B 02BC 0214              MVII  #$214,  R4    ; R4 = pointer to 2nd row of screen memory

480D 01DB           @@loop  CLRR  R3            ; clear R3
480E 0052                   RLC   R2            ; extract highest bit of R2 to carry
480F 0053                   RLC   R3            ; inject carry into R3
4810 0050                   RLC   R0            ; extract highest bit of R0 to carry
4811 0053                   RLC   R3            ; inject carry into R3
4812 0001                   SDBD                ; add pointer to lookup table to R3
4813 02FB 001D 0048         ADDI  #@@tbl, R3    ;
4816 029B                   MVI@  R3,     R3    ; read character value
4817 0263                   MVO@  R3,     R4    ; write it to screen memory (also does R4++)
4818 037C 021C              CMPI  #$21C,  R4    ; 8 characters written? ...
481A 0225 000E              BLT   @@loop        ; ... if not, jump to @@loop

481C 00AF                   JR    R5            ; return

481D 011F 0117      @@tbl   DECLE $11F, $117    ; characters 'B', 'C', 'A' and 'D'
481F 010F 0127              DECLE $10F, $127    ; in white, using the built-in font

                            ENDP

Output

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_{(1) Granted that at least one compiler, several emulators and copyright-free replacement ROM files are freely available, I think that it doesn't infringe any PPCG submission rule. But please let me know if I'm wrong.}

Answer 6

PHP, 57 Bytes

for($i=8;$i--;)echo CBAD[($n=$argv[1])>>$i+7&2|$n>>$i&1];

Version without Bitwise operators 70 Bytes

for(;$i<8;)echo CABD[($s=sprintf("%016b",$argv[1]))[$i]+$s[8+$i++]*2];

Answer 7

Mathematica, 75 73 68 66 bytes

StringPart["CBAD",#+##+1]&@@IntegerDigits[#,2,16]~Partition~8<>""&

Thanks to @MartinEnder for saving 2 bytes.

Answer 8

JavaScript, 113 93 90 88 bytes

A big thanks to @Neil for helping me save 20 bytes!
-3 bytes thanks to @Cyoce

n=>{r="";b=("0".repeat(15)+n.toString(2)).slice(-16);for(i=0;i<8;i++)r+="CBAD"[parseInt(b[i]+b[i+8],2)];return r}

n=>{r="";b=(65536+n).toString(2).slice(1);for(i=0;i<8;i++)r+="CBAD"[+b[i+8]+2*b[i]];return r}

n=>eval('r="";b=(65536+n).toString(2).slice(1);for(i=0;i<8;i++)r+="CBAD"[+b[i+8]+2*b[i]]')

n=>eval('r="";b=n.toString(2).padStart(16,0);for(i=0;i<8;i++)r+="CBAD"[+b[i+8]+2*b[i]]')

Sadly, JavaScript lacks functions like decbin, bindec, and str_pad that PHP has.

Answer 9

CJam, 22 bytes

ri2bG0e[8/:.{1$=)^'A+}

Try it online!

Explanation

Powered by magic...

The mapping of bit pairs to letters in this challenge is a bit arbitrary. If we represent ABCD by 0, 1, 2, 3 (so we can just add them to the character A) then we want the following mapping:

i1   i2   o
0    0    2
0    1    1
1    0    0
1    1    3

This mapping can be computed with a magical little formula: ((i1 == i2) + 1) ^ i1, where the equality check returns 0 or 1. Check out the following table, where each column corresponds to one input, each row corresponds to one operation, and each cell will show the stack at that point:

[i1, i2]:  [0, 0]     [0, 1]     [1, 0]     [1, 1]
copy i1:   [0, 0, 0]  [0, 1, 0]  [1, 0, 1]  [1, 1, 1]
equals:    [0, 1]     [0, 0]     [1, 0]     [1, 1]
inc:       [0, 2]     [0, 1]     [1, 1]     [1, 2]
xor:       [2]        [1]        [0]        [3]

With that in mind here is the full breakdown of the source code:

ri     e# Read input, convert to integer.
2b     e# Get binary representation.
G0e[   e# Pad to 16 bits with zeros.
8/     e# Split into two halves of 8 bits each.
:.{    e# For each pair of bits, i1 and i2...
  1$   e#   Copy i1.
  =    e#   Check equality with i2.
  )    e#   Increment.
  ^    e#   Bitwise XOR.
  'A+  e#   Add to 'A'
}

An alternative solution with the same byte count which is decidedly less magical:

ri2bG0e[8/z2fb"CBAD"f=

And in case it's useful to anyone, if you turn the i1 and i2 bits back into a single number (i.e. when you want the mapping 0 -> 2, 1 -> 1, 2 -> 0, 3 -> 3) this can be computed even more easily as (~n - 1) & 3 or (~n - 1) % 4 if your language gets modulo on negative values right. I think this can be written concisely as 3&~-~n in many languages. In CJam this turns out to be a byte longer, because of the additional conversion back from base 2.

Answer 10

Perl, 42 bytes

Includes +1 for -n

Give input on STDIN:

perl -nE 'say+(A..D)[2-($`>>8-$_&257)%127]for/$/..8' <<< 39253

Just the code:

say+(A..D)[2-($`>>8-$_&257)%127]for/$/..8

Answer 11

MATL, 16 bytes

16&B8eXB'BADC'w)

Try it Online!

or Verify all test cases

Explanation

        % Implicitly grab input
16&B    % Convert to binary string with at least 16 bits
8e      % Reshape the resulting string to have 8 rows and 2 columns
XB      % Convert each row from binary to decimal
'BADC'  % Push this string literal
w)      % Use the decimal numbers to index into this string (modular indexing)
        % Implicitly display the resulting string

Answer 12

Julia, 73 Bytes

Gives a function f taking N as input and returning the answer as string.

f(N)=(b=bin(N,16);join(["CBAD"[parse("0b$(b[i])$(b[i+8])")+1]for i=1:8]))

Try it

Depending if a char array counts as string, one can omit the join (67 Bytes)

f(N)=(b=bin(N,16);["CBAD"[parse("0b$(b[i])$(b[i+8])")+1]for i=1:8])

Try it

Answer 13

R, 110 bytes

Came up with a vectorized solution in R. This should probably by golfable by coming up with a smarter conversion int to binary conversion.

x=as.integer(intToBits(scan()));cat(LETTERS[1:4][match(paste0(x[16:9],x[8:1]),c("10","01","00","11"))],sep="")