P Pr Pre Pref Prefi Prefix Prefixe Prefixes

Question

Given some finite list, return a list of all its prefixes, including an empty list, in ascending order of their length.

(Basically implementing the Haskell function inits.)

Details

• The input list contains numbers (or another type if more convenient).
• The output must be a list of lists.
• The submission can, but does not have to be a function, any default I/O can be used.
• There is a CW answer for all trivial solutions.

Example

[] -> [[]]
[42] -> [[],[42]]
[1,2,3,4] -> [[], [1], [1,2], [1,2,3], [1,2,3,4]]
[4,3,2,1] -> [[], [4], [4,3], [4,3,2], [4,3,2,1]]

Answer 1

Haskell, 20 bytes

Edit: Yet a byte shorter with a completely different scan.

An anonymous function slightly beating the trivial import.

scanr(\_->init)=<<id

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• Uses =<< for the abbreviation (scanr(\_->init)=<<id) l = scanr(\_->init) l l.
• Scans a list l from right to left, collecting intermediate results with the function \_->init.
• That function ignores the elements scanned through (they're only used to get the right total length for the collected results), so really iterates applying init to the initial value of the scan, which is also l.

Answer 2

brainfuck, 21 12 bytes

-9 bytes thanks to Arnauld suggesting the separator ÿ instead of newlines

-[[<]>[.>],]

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Takes bytes through STDIN with no null bytes and prints a series of prefixes separated by the ÿ character with a leading ÿ character. For example, for the input Prefixes, the output is ÿÿPÿPrÿPreÿPrefÿPrefiÿPrefixÿPrefixeÿPrefixes.

For readability, here's a version with newlines instead.

Explanation:

-              Create a ÿ character in cell 0
 [        ,]   While input, starting with the ÿ
  [<]>           Go to the start of the string
      [.>]       Print the string
          ,      Append the input to the end of the string

Answer 3

JavaScript (ES6), 33 bytes

a=>[b=[],...a.map(n=>b=[...b,n])]

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How?

+--- a = input array
|
|       +--- initialize b to an empty array and include it as the first entry
|       |    of the output (whatever the input is)
|       |
|       |          +--- for each value n in a[]:
|       |          |
|       |          |        +--- append n to b[] and include this new array in
|       |          |        |    the final output
|       |          |        |
a => [b = [], ...a.map(n => b = [...b, n])]
               |                  |
               +---------+--------+
                         |
      spread syntax: expands all elements of
      the child array within the parent array

Answer 4

CW for all trivial entries

Clean, 19 bytes

Haskell version works in Clean too.

import StdLib
inits

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Haskell, 22 bytes

import Data.List
inits

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Prolog (SWI), 6 bytes

prefix

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Answer 5

Jelly, 3 bytes

ṭṖƤ

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How it works

ṭṖƤ  Main link. Argument: A

  Ƥ  Map the link to the left over all non-empty(!) prefixes of A.
 Ṗ       Pop; remove the last element.
ṭ    Tack; append A to the resulting list.

Answer 6

Japt, 4 bytes

²£¯Y

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Explanation:

²       :Add an arbitrary extra item to the end of the array
 £      :For each item in the new array:
  ¯Y    : Get an array of the items that are before it

Answer 7

Python 2, 32 bytes

f=lambda l:(l and f(l[:-1]))+[l]

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Answer 8

Perl 6, 13 bytes

{(),|[\,] @_}

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To explain:

In Perl 6 you can wrap an operator in square brackets as an alternate way to write a list reduction. [+] @array returns the sum of the elements in @array, [*] @array returns the product, etc. You can also precede the operator with a backslash to make a "triangular" reduction, which some languages call "scan." So [\+] @array returns a list consisting of the first element of @array, then the sum of the first two elements, then the sum of the first three elements, etc.

Here [\,] @_ is a triangular reduction over the input array @_ using the list construction operator ,. So it evaluates to a lists of lists: the first element of @_, the first two elements of @_, etc. That's almost what's needed, but the problem calls for a single empty list first. So the first element of the return list is a literal empty list (),, then the reduction over the input list is flattened into the rest of the return list with |.

Answer 9

R, 40 39 bytes

function(L)lapply(0:length(L),head,x=L)

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-1 byte thanks to digEmAll

The output of R's list type is a bit weird; it uses sequential indexing, so for instance, the output for

list(1,2) is

[[1]]                     # first list element
list()

[[2]]                     # second list element
[[2]][[1]]                # first element of second list element
[1] 1


[[3]]                     # third list element
[[3]][[1]]                # first element of third list element
[1] 1

[[3]][[2]]                # etc.
[1] 2

Taking input as a vector instead gives a neater output format, although then the inputs are not technically lists.

Answer 10

JavaScript, 36 bytes

a=>[...a,0].map((x,y)=>a.slice(0,y))

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Answer 11

Mathematica, 22 21 bytes

^{-1 byte thanks to Misha Lavrov!}

{}~FoldList@Append~#&

Pure function. Takes a list as input and returns a list of lists as output. I believe this is the shortest possible solution.

Answer 12

Husk, 2 bytes

Θḣ

Gets all the ḣeads and then prepends Θ (in this case []):

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^{(needs type annotation for empty list: Try it online!)}

Answer 13

J, 5 bytes

a:,<\

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Answer 14

PowerShell, 65 bytes

param($a)'';$x=,0*($y=$a.count);0..--$y|%{$x[$_]=@($a[0..$_])};$x

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PowerShell helpfully unrolls lists-of-lists when the default Write-Output happens at program completion, so you get one item per line. Tack on a -join',' to see the list-of-lists better, by converting the inner lists into strings.

(Ab)uses the fact that attempting to output an empty array (e.g., @()) results in no output, so an empty array input just has '' as the output, since the $a[0..$_] will result in nothing. It will also throw out some spectacular error messages.

Answer 15

K (ngn/k), 8 7 bytes

1_',\0,

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Answer 16

Common Lisp, 39 bytes

(defun f(l)`(,@(if l(f(butlast l))),l))

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Explanation

(defun f(l)                           )  ; Define a function f
           `(                        )   ; With the list (essentially capable of interpolation), containing:
             ,@                          ;     The value of, flattened to one level
               (if l              )      ;         If l is not the empty list (which is the representation of nil, i.e. the only falsy value)
                    (f(butlast l))       ;         Recurse with all of l but the tail
                                   ,l    ;     The value of l

Answer 17

F#, 53 bytes

I've actually got two pretty similar answers for this, both the same length. They both take a generic sequence s as a parameter.

First solution:

let i s=Seq.init(Seq.length s+1)(fun n->Seq.take n s)

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Seq.take takes the first n elements of the sequence. Seq.init creates a new sequence with a count (in this case) of the length of sequence s plus 1, and for each element in the sequence takes the first n elements in s.

Second solution:

let i s=Seq.map(fun n->Seq.take n s){0..Seq.length s}

Similar to before, except it creates a sequence from 0 to the length of s. Then takes that number of elements from s.

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Answer 18

MATL, 15 12 bytes

3 bytes saved thanks to @Giuseppe

vin:"G@:)]Xh

Try it at MATL Online.

Due to the way that MATL displays the output, you can't explicitly see the empty array in the cell array. Here is a version that shows the output a little more explicitly.

Explanation

v       # Vertically concatenate the (empty) stack to create the array []
i       # Explicitly grab the input
n       # Compute the number of elements in the input (N)
:       # Create an array from [1, ..., N]
"       # Loop through this array
  G     # For each of these numbers, M
  @:    # Create an array from [1, ..., M]
  )     # Use this to index into the initial array
]       # End of the for loop
Xh      # Concatenate the entire stack into a cell array

Answer 19

SWI PROLOG 22 bytes

i(X,Y):-append(X,_,Y).

Answer 20

Charcoal, 6 bytes

Ｅθ…θκθ

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

 θ      Input array
Ｅ       Map over elements
   θ    Input array
  …     Moulded to length
    κ   Current loop index
        Implicitly print each array double-spaced
     θ  Input array
        Implicitly print

It's possible at a cost of 1 byte to ask Charcoal to print an n+1-element array which includes the input as its last element, but the output is the same, although the cursor position would be different if you then went on to print something else.

Answer 21

05AB1E, 3 bytes

η¯š

Explanation:

η    Prefixes
  š  Prepend
 ¯   Global array (empty by default)

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Answer 22

RAD, 7 bytes

(⊂⍬),,\

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This also works in Dyalog APL as a function.

How?

This works the same for both APL and RAD, given their close relation.

• (⊂⍬) the empty array
• , prepended to
• ,\ the prefixes (which exclude the empty array.)

Answer 23

Groovy, 37 bytes

{x->(0..x.size()).collect{x[0..<it]}}

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Answer 24

Java 8+, 86 77 bytes

^{-9 bytes thanks to Kevin Cruijssen (getting rid of the import)!}

x->java.util.stream.IntStream.range(0,x.size()+1).mapToObj(t->x.subList(0,t))

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Alternative, 65 bytes

The following will print the results to stdout (due to Olivier Grégoire):

x->{for(int i=0;i<=x.size();)System.out.print(x.subList(0,i++));}

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Answer 25

Japt, 5 bytes

Êò@¯Y

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Answer 26

brainfuck, 43 bytes

Take a list of non-null characters as input and returns all prefixes separated by newline. Requires double-infinite or wrapping tape.

,>-[+>,]<[-<]<<++++++++++[[<]>[.>]>[-<+>]<]

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Answer 27

C# (Visual C# Interactive Compiler), 39 bytes

x=>x.Select((_,i)=>x.Take(i)).Append(x)

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Answer 28

F# (Mono), 45 bytes

fun x->List.mapi(fun i y->List.take i x)x@[x]

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I am not totally sure if this is valid, but it seems like it follows the same "anonymous lambda" syntax that I've seem used in several other languages.

Answer 29

Ruby, 31 29 bytes

->a{[a*i=0]+a.map{a[0,i+=1]}}

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Explanation:

->a{             # take array input a
  [a*i=0]+       # set i to 0 and add whatever comes next to [[]] (a*0 == [])
  a.map{         # for every element in a (basically do a.length times)
    a[0,i+=1]  # increment i and return the first i-1 elements of a to map
  }
}

Answer 30

C (gcc), 102 97 bytes

Pretty long, as C quite obviously isn't well-suited to manipulating lists of lists. Dependent on certain behavior; running on some platforms requires changing int to short (for +2 bytes), due to wchar_t being UTF-16 rather than UTF-32. Should work on Linux as-is.

Takes in a pointer to the first element of the input array, the length of the array, and a pointer to the variable in which the pointer to output will be stored. Produces a list of lists such that the first integer in the linear array stores the number of sub-lists. Sub-lists start with the number of elements, and are stored in memory in a contiguous manner.

p(r,e,f,i,x)int*e,**r,*x;{x=*r=malloc(4-++f*~f*2);*x++=f;for(i=0;i<f;x+=++i)wmemcpy(x+1,e,*x=i);}

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Degolf

p(r,e,f,i,x) int**r,*e,*x; // Function p(), where r is a pointer to the output variable,
                           // e is a pointer to the input location and f is the size of
                           // the input.
{ 
  x=*r=malloc(4-++f*~f*2); // Allocate 4*(x+1)*(x+2)/2+4 bytes of memory. 
                           // This is the exact amount of memory needed.
  *x++=f;                  // Store n+1 (number of sub-lists) in the first four bytes. 
  for(i=0;i<f;x+=++i)      // Iterate over [0, n]->i and 
                           // increment the pointer to x by i+1 every iteration.
    wmemcpy(x+1,e,*x=i);   // Set first element at location pointed to by x to i,
                           // then copy i of either 2 or 4 byte elements, depending on
                           // the system wchar implementation, to location pointed to
                           // by x+1.
}

Answer 31

Brachylog, 9 bytes

a₀ᶠ~b.hĖ∧

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Explanation

a₀ᶠ           Find all prefixes of the input
   ~b         Add an element at the beginning of that list of prefixes
      hĖ      This element is the empty list
     .  ∧     (the list with the additional empty list is the output)

Answer 32

05AB1E, 8 bytes

That was my first succesfull golf! Thanks for being such a nice community!

)UŒ¹g£Xš

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Explanation:

)UŒ¹g£Xš
)        : Create an empty list
 U       : Save it to variable ˙X˙
  Π     : Make the "substrings" of the implicit input
   ¹g£   : Take the first n elements of the result where n is the lenght of input
      Xš : Add the empty list to the beginning       

Answer 33

C# (Visual C# Interactive Compiler), 48 bytes

l=>new int[l.Count()+1].Select((_,n)=>l.Take(n))

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Explanation

l=>                                                // Function with list l as argument
   new int[l.Count()+1]                            // A new array (int is shortest type available) of length l + 1
                       .Select((_,n)=>l.Take(n))   // Where every element is mapped to the first n elements of the input list, where n is the index

Answer 34

Lua 5.3, 92 bytes

function f(...)t=...if#t>0then return f({table.unpack(t,1,#t-1)},...)else return{...}end end

Keeps creating a copy of the array-like table with the element at the end removed and prepending it to the varargs until the length of the table is 0, then puts the varargs in a table. Relies on defining the variable f to allow recursion.

Lua 5.1, 87 bytes

function f(...)t=...if#t>0 then return f({unpack(t,1,#t-1)},...)else return{...}end end

unpack is a global variable rather than a member of the table library. 0then is syntactically invalid in Lua 5.1, but strangely not in Lua 5.3.

Answer 35

Pari/GP, 23 bytes

a->[a[1..n]|n<-[0..#a]]

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Answer 36

Pip, 16 bytes

{h@<a}M,1+#(h:a)

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Explanation

      M            Map
{h@<a}                 Function returning global variable h, cut at index a, and left part taken (0-indexed)
       ,               Range from 1 to
        1+#(   )           1 + the length of
            h:a                Assign global variable h to a, evaulating to the value of h (which is also the value of a - the first parameter to the function)

Answer 37

Powershell, 24 bytes

,@()
$args|%{,($x+=,$_)}

Explanation:

1. The trivial accumulator wrapped to array by unary ,.
2. The script uses splatting. See parameter @a.

Test script:

$f = {

,@()
$args|%{,($x+=,$_)}

}

@(
    ,( @()        , @() )
    ,( @(42)      , @(@(),@(42)) )
    ,( @(1,2,3,4) , @(@(), @(1), @(1,2), @(1,2,3), @(1,2,3,4)) )
    ,( @(4,3,2,1) , @(@(), @(4), @(4,3), @(4,3,2), @(4,3,2,1)) )
) | % {
    $a,$expected = $_
    $result = &$f @a

    $false -notin $(
        $result -is [array]
        $result.Count -eq $expected.Count
        for($j=0; $j-lt$result.Count; $j++){
            "$($result[$j])" -eq "$($expected[$j])"
        }
    )

    # $result   # uncomment this line to display a result

}

Output:

True
True
True
True

Answer 38

Ruby, 34 31 bytes

->l{(0..l.size).map{|i|l[0,i]}}

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Thanks @ConorO'Brien!

Answer 39

Factor, 63 bytes

USE: fry [ [ dup '[ nip _ swap head ] map-index ] keep suffix ]

Keeps a copy of the input to tack on at the end, like C#.

Answer 40

FALSE, 57 bytes

1^[$~][\1+^]#%a:0[$a;1->~][0[$@$@>][\$a;\-ø,1+]#\%10,1+]#

Try it online! (you'll have to copy paste the code. Click "Show" and then run)

Like brainf*ck, FALSE only takes one char at a time as input, so this program splits a string into prefixes, separating them with a newline.

Explanation:

1^         {push 1 {counter) and first character onto stack}
[$~][      {while a character is input:}
  \1+^     {add 1 to the counter}
]#

%          {drop truth value from stack}
a:         {define the counter as the variable "a'}
0          {push 0 onto stack (loopvar)}
[$a;1->~][ {while loopvar is less than a-1:}

  0        {push 0 (loopvar2)}
  [$@$@>][ {while loopvar is greater than loopvar2}
    \$a;\- {a-loopvar2 (to get index of char to print)}
    ø,     {copy value at index to top of stack and print}
    1+     {increment loopvar2}
  ]#
  \%       {swap'n'drop (get rid of loopvar2)}
  10,      {print newline}
  1+       {increment loopvar}
]#

A FALSE expert could probably do this using no variables (pure stack) but since I only learnt this language yesterday I think this is alright.

Answer 41

Scala, 21 bytes

_.inits.toSeq.reverse

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Not quite a trivial answer, since I had to deal with the fact that the inits method returns an iterator in the reverse of the required order.

Answer 42

MathGolf, 9 7 6 bytes

hÅ_╡]x

-1 byte thanks to @maxb.

Try it online or verify all test cases.

Explanation:

h        # Push the length of the (implicit) input-array, without popping the array
 Å       # Loop this length amount of times,
         # and do the following two commands each iteration:
  _      #  Duplicate the array at the top of the stack
   ╡     #  Remove the right-most item of the array
    ]    # After the loop, wrap everything on the stack in an array
     x   # Reverse it (and output implicitly)

Answer 43

D, 70 bytes (31 imports, 39 expression)

import std.algorithm,std.range;
alias prefixes = t=>(t.length+1).iota.map!(i=>t.take(i));

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edit: Do I count the alias prefixes = part? You can use the lambda without it, but you have to add parens to disambiguate, so should I count it as 42?

Answer 44

MathGolf, 7 bytes

hæ_ï<\]

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Explanation

h         length of array/string without popping
 æ        start block of length 4
  _       duplicate TOS
   ï      index of current loop, or length of last loop
    <     slice list
     \    swap top elements
      ]   end array / wrap stack in array

Answer 45

Clojure, 21 bytes

#(reductions conj[]%)

Answer 46

Gambit Scheme (gsi), 77 72 bytes

(define(p z)(if(pair?(car z))(p(cons(reverse(cdr(reverse(car z))))z))z))

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First time using Scheme. It's a function that takes a wrapped list ((list (list 1 2 3 4))) as an argument.

Answer 47

Haskell, 27 bytes

i[]=[[]]
i l=i(init l)++[l]

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Answer 48

Canvas, 6 5 bytes

）╶［｝）

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Outputs to the stack; Linked with a trailing ｒａｗ so that's visible.

)      wrap the (currently empty) stack in an array
 ╶[}   map nothing over the prefixes of the input
    )  wrap the stack (the empty array and the prefixes) in an array

2 bytes without the starting empty array.

Answer 49

MY, 7 bytes

ωωι0;↑←

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Woah ... never thought MY would actually come in handy.

How?

ωωι0;↑←

• ω = push(arg[0])
• ω = push(arg[0])
• ι = push([1 ... pop()])
• 0 = push(0)
• ; = push(pop() + pop())
• ↑ = does the prefixing work by vecifying in a stupid manner.
• ← output

Answer 50

Tcl, 81 bytes

set i -1
set a {{}}
foreach _ $argv {lappend a [lrange $argv 0 [incr i]]}
puts $a

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Answer 51

Attache, 13 bytes

{_[0...0:#_]}

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Explanation

{_[0...0:#_]}
{           }    block taking `_` as input
 _[        ]     index from the input
       0:#_      range from 0 to the length of `_` inclusive
   0...          range from 0 to each number that range, right exclusive

Answer 52

Red, 76 bytes

func[b][a: copy[[]]c: b until[append/only a copy/part b c: next c tail? c]a]

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Answer 53

Kotlin, 32 bytes

{l->(0..l.size).map{l.take(it)}}

{ list ->
    (0..list.size).map {  // map over range [0, length of list]
        list.take(it)  // first n items of list
    }
}

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Answer 54

Pushy, 7 bytes

1@$O_vI

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Answer 55

Tcl, 65 bytes

proc P {L J\ {{}}} {lmap e $L {lappend J [lappend K $e]}
list $J}

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Answer 56

Z80Golf, 18 bytes

00000000: 2511 00ff 1b1a ffa7 20fa cd03 802b 7730  %....... ....+w0
00000010: f076                                     .v

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Takes bytes through STDIN with no null bytes and prints a series of prefixes, each terminated by a null byte.

Source code:

    dec h
start:
    ld de, $ff00
.printloop:
    dec de
    ld a, (de)
    rst $38
    and a
    jr nz, .printloop
    call $8003
    dec hl
    ld (hl), a
    jr nc, start
    halt

Answer 57

Forth (gforth), 57 bytes

: x here 0 begin 2dup type cr 1+ key dup c, 5 < until ; x

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Takes bytes through STDIN and prints a series of prefixes, each terminated by a newline.

Answer 58

str, 6 bytes

e;:dno

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Each output line corresponds to an entry in the prefix array.

Explanation

e;:dno
            -- preamble --
e           push empty string
            -- iteration --
  :         append current char to build string
   d        duplicate it
    n       push "\n"
     o      output it
            (implicitly output duplicated entry)

Answer 59

Perl 5 + -a, 24 bytes

say"@F[0..$_]"for-1..$#F

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Answer 60

Actually, 13 bytes

;╗ru⌠╜H⌡M[]@o

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Explanation:

;╗ru⌠╜H⌡M[]@o
;╗             save a copy of input in register 0
  ru           range(1, len(input)+1)
    ⌠╜H⌡M      for i in range:
     ╜H          first i elements of input
         []@o  prepend an empty list

Answer 61

Elixir, 37 bytes

fn x->Enum.scan [[]|x],&(&2++[&1])end

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Looking at other languages, this looks rather verbose, but I'm definitely not an expert in Elixir, so please let me know if I missed something obvious.

Answer 62

APL (Dyalog Unicode), 12 bytes

(0,∘⍳∘≢⊢)↑¨⊂

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Prefix tacit function. TIO link has boxing enabled to make the output more readable.

How:

(0,∘⍳∘≢⊢)↑¨⊂ ⍝ Tacit fn, taking a vector as argument, which will be called ⍵ for simplicity.
         ↑   ⍝ Take the first ⍺ (left arg) elements of ⍵
          ¨⊂ ⍝ Since the ⍺ for ↑ is a vector, we enclose the ⍵ and apply ↑ to it for each of the elements in ⍺.
(      ⊢)    ⍝ Use ⍵ as argument for
      ≢      ⍝ Tally; yields the number of elements in ⍵
    ⍳∘       ⍝ Then, yield the range [1..≢⍵]
 0,∘         ⍝ and prepend a 0.

The function returns a vector of vectors, each with the first [0..≢⍵] elements of the argument.

Answer 63

Bash, 34 bytes

I output each list element on a new line as someone else did - bash only has 1-D arrays:

for((;c<=$#;c++)){ echo ${*:1:c};}

Could be 1 byte less but this does not work for null case - don't see why... c++ does not get evaluated...

for((;c<=$#;)){ echo ${*:1:c++};}

This can output text as suggested by OP:

C='echo -n ';A=(${1//[],[]/ });$C[[];for((c=0;c<${#A[*]};)){ B=${A[*]::++c};$C,[${B// /,}];};echo ]

No one should have to decode that. Here is annotated version:

C='echo -n '              # basically a macro to improve my golf
A=( ${1//[],[]/ } )       # convert [,] to spaces - like magic this one
$C[[]                     # echo [[] since every list needs this
for((c=0;c<${#A[*]};)){   # for all elements of list
  B=${A[*]::++c}          # make space separated list of first c elements
  $C,[${B// /,}]          # prefix with ,[ and convert space to comma
}
echo ]                    # terminate list with ]

Answer 64

Reticular, 60 bytes

Bql:D=:D`:E=l[:D`@d]~*[:E`b@qM[$]:D`:E`-*:E`1-:E=]:D`1+*lbo;

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Assumes that the input list is at the top of the stack. To test input, run the following code

'1''2''3''4''5'lbBql:D=:D`:E=l[:D`@d]~*[:E`b@qM[$]:D`:E`-*:E`1-:E=]:D`1+*lbo;

Explanation

B                         # Push every element from the input list to the stack.
 q                        # Reverse stack.
  l                       # Push the size of the stack to the stack.
   :D=                    # Save it as the variable D.
      :D`:E=              # Define the variable E = D.
            l             # Push D to the stack.
             [:D`@d]      # Push the function which duplicates the top D items in the stack
                    ~     # Swap the top two items in the stack
                     *    # Call the above function D number of times.



[                          ]            # Push a function that does the following:
 :E`b                                   # Put the top E items in a list.
     @q                                 # Reverse list that is at the top of the stack.
       M                                # Rotate stack so that top of stack -> bottom of stack.
        [$]:D`:E`-*                     # Delete the top D-E items from the stack.
                   :E`1-:E=             # Define E = E - 1.
                            :D`1+*      # Call the above function D+1 number of times.
                                  q     # Reverse stack.
                                   lb   # Put every item in the stack into a list.
                                     o; # Output the resulting list and exit.