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.
• This is code-golf, so the shortest code in bytes wins

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

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 7

Python 2, 32 bytes

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

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Answer 8

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 9

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 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

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 16

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 17

SWI PROLOG 22 bytes

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

Answer 18

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 19

K (ngn/k), 8 7 bytes

1_',\0,

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Answer 20

Brachylog, 9 8 bytes

{∧Ė|a₀}ᶠ

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Explanation

{     }ᶠ    Find all:
 ∧Ė          The empty list
   |         Or
    a₀       A prefix of the input

Answer 21

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 22

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 23

05AB1E, 3 bytes

η¯š

Explanation:

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

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Answer 24

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 25

Groovy, 37 bytes

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

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Answer 26

Japt, 5 bytes

Êò@¯Y

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Answer 27

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 28

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 29

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

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

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Answer 30

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.