Duck, Duck, Josephus

Question

Given an Integer array:

Start from the first number
Jump forward n positions where n is the value of the current position
Delete the current position, making what was the next position the current position.
Goto step 2 until there is one number remaining
Print that number

Rules

The array wraps-around (the next number after the last number in the array is the first number).

A zero removes itself (Obviously).

Negative numbers are not allowed as input.

Test Cases

[1] => 1
[1,2] => 1
[1,2,3] => 3
[1,2,2] => 1
[1,2,3,4] => 1
[6,2,3,4] => 4
[1,2,3,4,5] => 5
[0,1] => 1
[0,0,2,0,0] => 0

Step-by-step example

[1,4,2,3,5]
 ^          start from the first position
   ^        jump 1 position (value of the position)
[1,  2,3,5] remove number in that position
     ^      take next position of the removed number (the 'new' 'current' position)
         ^  jump 2 positions
[1,  2,3  ] remove number in that position
 ^          take next position (looping on the end of the array)
     ^      jump 1 position
[1,    3  ] remove number in that position
       ^    take next position (looping)
 ^          jump 3 positions (looping on the end of the array)
[      3  ] remove number in that position
print 3

Example #2

[4,3,2,1,6,3]
 ^            start from the first position
         ^    jump 4 positions
[4,3,2,1,  3] remove number in that position    
           ^  take next position
     ^        jump 3 positions
[4,3,  1,  3] remove number in that position    
       ^      take next position
           ^  jump 1 positions
[4,3,  1    ] remove number in that position    
 ^            take next position
   ^          jump 4 positions
[4,    1    ] remove number in that position    
       ^      take next position
 ^            jump 1 position
[      1    ] remove number in that position
print 1

This is code-golf, the shortest answer in bytes wins!

@Jenny_mathy I didn't think there would be a similar one, but as Luis said, the wraparound array makes an interesting challenge for golfing. I think :/ — workoverflow, Commented Dec 12, 2017 at 14:00
@EriktheOutgolfer Not really a dupe. The elements there are indistinguishable and the step size is fixed. Luis's is much closer, but still sufficiently different I think. — Martin Ender, Commented Dec 12, 2017 at 14:12
Does it need to actually print the final number, or can it just return it? Does it need to actually return the number, or can it just operate on the array in-place so the after the function is run, the array contains just the number? — Reinstate Monica -- notmaynard, Commented Dec 12, 2017 at 16:48

Community · Accepted Answer · 2020-06-17 09:04:33Z

11

Husk, 7 bytes

This returns the result as a singleton list

ΩεSotṙ←

Try it online!

Explanation

Ω               Until
 ε              the result is a singleton list
     ṙ          Rotate left by
  S   ←         the first element
   ot           Then remove the first element

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 12, 2017 at 17:30

H.PWiz

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Laikoni · Accepted Answer · 2017-12-12 16:50:00Z

7

Haskell, 54 50 48 bytes

f[x]=x
f(x:r)=f$snd<$>zip r(drop(x+1)$cycle$x:r)

Try it online!

Explanation:

f[x]=x: If the given list is a singleton list, return its element.
f(x:r)=f$ ...: Otherwise recursively apply f to the following list:
- The elements of the current list cycled infinitely (cycle$x:r),
- with the first x+1 elements removed (drop(x+1)$),
- and truncated to length of r. (snd<$>zip r is a shorter alternative to take(length r)).

Previous 54 byte version:

f=(%)=<<head
_%[x]=x
n%(x:r)|n<1=f r|s<-r++[x]=(n-1)%s

Try it online!

edited Dec 12, 2017 at 16:50

answered Dec 12, 2017 at 15:57

Laikoni

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Reinstate Monica -- notmaynard · Accepted Answer · 2017-12-12 19:43:50Z

7

Ruby, 37 bytes

->r{r.rotate!(r[0]).shift while r[1]}

Modifies the array in-place, which appears to be acceptable as output. Try it online!

answered Dec 12, 2017 at 19:43

Reinstate Monica -- notmaynard

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Dennis · Accepted Answer · 2017-12-13 01:01:16Z

6

Python 3, 54 51 bytes

f=lambda x:x and f((x+x*x[0])[x[0]:][1:len(x)])or x

Output is a singleton list.

Try it online!

edited Dec 13, 2017 at 1:01

answered Dec 13, 2017 at 0:44

Dennis

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\$\begingroup\$ Completely unrelated, but I like your unicorn hat, Dennis. xD (And nice answer of course, as always!) \$\endgroup\$
– Kevin Cruijssen
Commented Dec 18, 2017 at 10:02

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Martin Ender · Accepted Answer · 2017-12-12 14:15:30Z

CJam, 15 bytes

l~_,({_0=m<1>}*

Try it online!

Explanation

Instead of keeping track of a pointer, I just shift the array cyclically so that the current element is always at the front.

l~     e# Read and evaluate input.
_,(    e# Get its length L and decrement to L-1.
{      e# Run this block L-1 times...
  _0=  e#   Get the first element X.
  m<   e#   Rotate the array left by X positions.
  1>   e#   Discard the first element.
}*
       e# The final element remains on the stack and gets printed implicitly.

A fun alternative which unfortunately doesn't save any bytes:

l~_{;m<1>_0=}*;

Community · Accepted Answer · 2020-06-17 09:04:33Z

MATL, 21 bytes

1`yy)+ynX\[]w(5Mynq]x

Try it online! Or verify all test cases.

Explanation

1        % Push 1: current position in the array
`        % Do...while
  yy     %   Duplicate top two elements in the stack. Takes input implicitly
         %   in the first iteration.
         %   STACK: array, position, array, position
  )      %   Get specified entry in the array
         %   STACK: array, position, selected entry
  +      %   Add
         %   STACK: array, position (updated)
  y      %   Duplicate from below
         %   STACK: array, position, array
  n      %   Number of elements of array
         %   STACK: array, position, number of elements or array
  X\     %   1-based modulus
         %   STACK: array, position (wrapped around)
  []     %   Push empty array
         %   STACK: array, position, []
  w      %   Swap
         %   STACK: array, [], position
  (      %   Write value into specified entry in array. Writing [] removes
         %   the entry
         %   STACK: array (with one entry removed)
  5M     %   Push latest used position. Because of the removal, this now
         %   points to the entry that was after the removed one
         %   STACK: array, position
  y      %   Duplicate from below
         %   STACK: array, position, array
  n      %   Number of elements of array
         %   STACK: array, position, number of elements of array
  q      %   Subtract 1
         %   STACK: array, position, number of elements of array minus 1
]        % End. If top of the stack is nonzero, proceed with next iteration
         % STACK: array (containing 1 entry), position
x        % Delete. Implicitly display
         % STACK: array (containing 1 entry)

Note: using list rotations instead of keeping a pointer will probably make this much shorter. — Erik the Outgolfer, Commented Dec 12, 2017 at 18:02
@Erik Thanks. But now that I added the explanation I think I’ll leave it like this — Luis Mendo, Commented Dec 12, 2017 at 19:18
Well, you can always remove the explanation, it will be kept in the history :) — Erik the Outgolfer, Commented Dec 12, 2017 at 19:22

Erik the Outgolfer · Accepted Answer · 2017-12-12 14:40:52Z

5

Jelly, 7 bytes

ṙḷ/ḊµḊ¿

Try it online!

Full program.

answered Dec 12, 2017 at 14:40

Erik the Outgolfer

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3

\$\begingroup\$ That ḷ/ is damn clever. \$\endgroup\$
– Mr. Xcoder
Commented Dec 12, 2017 at 15:20
\$\begingroup\$ Could you add an explanation, please? I've now looked at the Quicks and Atoms on the linked GIT-pages, and +1-ed based on that, but I can imagine not everyone having the patients to do the same. ;) \$\endgroup\$
– Kevin Cruijssen
Commented Dec 18, 2017 at 10:10

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hyperneutrino · Accepted Answer · 2017-12-12 15:06:32Z

5

Jelly, 9 bytes

ṙḷ/ḊµL’$¡

Try it online!

-2 bytes thanks to user202729

Explanation

ṙḷ/ḊµL’$¡  Main Link
     L’$¡  Repeat <length - 1> times
ṙ          Rotate left by
 ḷ/        The first element (from JHT; thanks to user202729)
   Ḋ       Take all but the first element

edited Dec 12, 2017 at 15:06

answered Dec 12, 2017 at 14:14

hyperneutrino♦

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Community · Accepted Answer · 2020-06-17 09:04:33Z

5

Brain-Flak, 88 bytes

([[]]()){({}<(({})){({}<({}<([]){({}{}<>)<>([])}{}>)<>{({}[<>[]])<>}<>>[()])}{}{}>())}{}

Try it online!

Explanation

([[]]())                      Push negative N: the stack height - 1
{({}< … >())}{}               Do N times
     (({}))                     Duplicate M: the top of the stack
     {({}< … >[()])}{}          Do M times 
                                  Rotate the stack by 1:
          ({}< … >)               Pop the top of the stack and put it back down after
          ([]){({}{}<>)<>([])}{}  Pushing the rest of the stack on to the other one, in reverse, with the stack height added to each element (to ensure that all are positive)
          <>{({}[<>[]])<>}<>      Push the rest of the stack back, unreversing, and subtracting the stack height from each element
                      {}        Pop the top of stack

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 12, 2017 at 18:17

H.PWiz

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1

\$\begingroup\$ A very strange golf but here it is in 88 bytes. \$\endgroup\$
– Wheat Wizard ♦
Commented Dec 12, 2017 at 18:35
1

\$\begingroup\$ @WheatWizard Nice, surprisingly I tried something like that earlier on. \$\endgroup\$
– H.PWiz
Commented Dec 12, 2017 at 18:37
\$\begingroup\$ I can never know how people can code like that! is there a pseudo-code translator or something? \$\endgroup\$
– workoverflow
Commented Dec 18, 2017 at 13:16
1

\$\begingroup\$ @workoverflow no, it's honestly easier than it looks. It was very daunting before I actually started , but the when the commands are this simple, it's easy to learn. \$\endgroup\$
– H.PWiz
Commented Dec 18, 2017 at 13:22

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Mr. Xcoder · Accepted Answer · 2017-12-13 05:20:07Z

5

Python 2, 55 bytes

def f(a):
 while a[1:]:l=a[0]%len(a);a[:]=a[-~l:]+a[:l]

Try it online!

Outputs as a singleton list, as allowed by default. Saved a few bytes thanks to Dennis, by reminding me that modifying the function argument is allowed.

How it works

def f(a) - Defines a function with a parameter a.
while a[1:]: - While a with the first element removed is truthy, run the block of code to follow. A list with one element or more is truthy, and empty lists are falsy in Python, hence this will stop once a reaches a length of 1.
l=a[0]%len(a) - Take the first element, and get the remainder of its division by the length of a. Assign the result to l.
a[:]=a[-~l:]+a[:l] - Rotate a to the left by l elements, and remove the first one, while assigning this to a in place.

Python 2, 63 bytes

f=lambda a,i=0:a[1:]and f(a,a.pop(((a*-~i)[i]+i)%len(a))+1)or a

Try it online!

Although longer, this seems much more elegant. Also thanks to ovs for helping in chat.

edited Dec 13, 2017 at 5:20

answered Dec 12, 2017 at 14:14

Mr. Xcoder

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1

\$\begingroup\$ Couldn't you do something like a,*b=input() (python3) and save few bytes? However I'm not sure how that would affect l and the slice \$\endgroup\$
– Rod
Commented Dec 12, 2017 at 14:44
1

\$\begingroup\$ @Rod I don't think so, I would need to evaluate the input too in Python 3 \$\endgroup\$
– Mr. Xcoder
Commented Dec 12, 2017 at 14:46

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Erik the Outgolfer · Accepted Answer · 2017-12-12 20:16:13Z

3

APL (Dyalog), 20 18 bytes

{1<≢⍵:∇1↓⊖∘⍵⊃⍵⋄⊃⍵}

Try it online!

edited Dec 12, 2017 at 20:16

answered Dec 12, 2017 at 19:26

Erik the Outgolfer

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hyperneutrino · Accepted Answer · 2017-12-13 02:08:49Z

3

Python 3, 60 bytes

def f(a):b=a[0]%len(a);return f(a[-~b:]+a[:b])if a[1:]else a

Try it online!

-3 bytes thanks to ovs

edited Dec 13, 2017 at 2:08

answered Dec 12, 2017 at 14:23

hyperneutrino♦

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Galen Ivanov · Accepted Answer · 2017-12-13 11:53:22Z

3

J, 21 17 bytes

-4 bytes thanks to FrownyFrog

((1<#)}.{.|.])^:_

Try it online!

Original:

([:}.{.|.])^:(1<#)^:_

How it works:

^:_ repeat until the result stops changing

^:(1<#) if the length of the list is greater than 1

{.|.] rotate the list to the left its first item times

[:}. drop the first element and cap the fork

Try it online!

edited Dec 13, 2017 at 11:53

answered Dec 13, 2017 at 8:18

Galen Ivanov

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Arnauld · Accepted Answer · 2017-12-13 13:35:15Z

3

JavaScript (ES6), 54 60 bytes

Saved 1 byte thanks to @Shaggy
Fixed version (+6 bytes)

Modifies the input array, which is reduced to a singleton.

f=(a,p=0)=>1/a||f(a,p=(p+a[p%(l=a.length)])%l,a.splice(p,1))

Test cases

f=(a,p=0)=>1/a||f(a,p=(p+a[p%(l=a.length)])%l,a.splice(p,1))

;[
  [1],         // [1]
  [1,2],       // [1]
  [1,2,3],     // [3]
  [1,2,2],     // [1]
  [1,2,3,4],   // [1]
  [6,2,3,4],   // [4]
  [1,2,3,4,5], // [5]
  [0,1],       // [1]
  [0,0,2,0,0], // [0]
  [3,5,7,9]    // [5]
].forEach(a => f(a) && console.log(JSON.stringify(a)))

How?

We recursively apply the algorithm described in the challenge. Only the stop condition 1/a may seem a bit weird. When applying an arithmetic operator:

Arrays of more than one element are coerced to NaN and 1/NaN is also NaN (falsy).
Arrays of exactly one integer are coerced to that integer, leading to either 1/0 = +Infinity or 1/N = positive float for N > 0 (both truthy).

f = (a, p = 0) =>                 // a = input array, p = pointer into this array
  1 / a ||                        // if a is not yet a singleton:
    f(                            //   do a recursive call with:
      a,                          //     a
      p = (                       //     the updated pointer
        p + a[p % (l = a.length)] //
      ) % l,                      //
      a.splice(p, 1)              //     the element at the new position removed
    )                             //   end of recursive call

edited Dec 13, 2017 at 13:35

answered Dec 12, 2017 at 14:29

Arnauld

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\$\begingroup\$ Seeing as splice modifies the original array, you could do f=(a,p=0)=>1/a||f(a,p=p+a[p]%a.length,a.splice(p,1)) for 52 bytes \$\endgroup\$
– Shaggy
Commented Dec 12, 2017 at 17:06
\$\begingroup\$ it seems it doesn't give the right result for the second step by step example, f=(a,p=0)=>1/a?a:f(a,p=(p%a.length+a[p%a.length])%a.length,a.splice(p,1)) is ok but may be optimized \$\endgroup\$
– Nahuel Fouilleul
Commented Dec 12, 2017 at 17:13
\$\begingroup\$ @NahuelFouilleul Oops. I thought at some point that the parentheses around p+a[p] could be removed. Which -- of course -- is not the case. Thanks for reporting this! \$\endgroup\$
– Arnauld
Commented Dec 12, 2017 at 17:19
\$\begingroup\$ See this consensus, which @Neil brought to my attention here. \$\endgroup\$
– Shaggy
Commented Dec 12, 2017 at 17:20
\$\begingroup\$ @Shaggy Oh, I see. Thank you! (I missed your TIO link the 1st time...) \$\endgroup\$
– Arnauld
Commented Dec 12, 2017 at 17:26

| Show 2 more comments

LukeS · Accepted Answer · 2017-12-20 20:19:54Z

3

Julia 0.6, 46 42 bytes

!x=length(x)>1?!circshift(x,-x[])[2:end]:x

Try it online!

Straightforward recursive Julia version. x[] accesses the first element of x.

edited Dec 20, 2017 at 20:19

answered Dec 17, 2017 at 19:22

LukeS

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Jakob · Accepted Answer · 2018-05-08 03:35:39Z

3

Java 8, 79 bytes

This lambda accepts a Stack<Integer> and returns an int or Integer.

l->{for(int i=0,s=l.size();s>1;)l.remove(i=(i+l.get(i%s))%s--);return l.pop();}

Try It Online

Ungolfed

l -> {
    for (
        int i = 0, s = l.size()
        ; s > 1
        ;
    )
        l.remove(
            i = (i + l.get(i % s)) % s--
        );
    return l.pop();
}

Acknowledgments

-2 bytes thanks to Nahuel Fouilleul

edited May 8, 2018 at 3:35

answered Dec 12, 2017 at 19:44

Jakob

2,7691 gold badge13 silver badges24 bronze badges

1

\$\begingroup\$ i%=s may be removed if l.get(i) changed by l.get(i%s) \$\endgroup\$
– Nahuel Fouilleul
Commented Dec 13, 2017 at 8:54

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Community · Accepted Answer · 2020-06-17 09:04:33Z

Pyth, 9 bytes

.WtHt.<Zh

Try it here!

This outputs the result as a singleton list, as allowed by default.

How it works

.WtHt.<Zh ~ Full program.

.W        ~ Functional while. It takes three arguments, two functions: A and B
            and a starting value, which in this case is automatically assigned
            to the input. While A(value) is truthy, value is set to B(value).
            Returns the ending value. A's argument is H and B's is Z.
  tH      ~ A (argument H): Remove the first element of H. A singleton list
            turns into [], which is falsy and thus breaks the loop. Otherwise,
            it is truthy and the loops goes on until the list reaches length 1.
     .<Zh ~ B (argument Z): Cyclically rotate Z by Z[0] places, whereas Z[0]
            represents the first element of Z.
    t     ~ And remove the first element.

Note: If you don't want to see those brackets, just add h or e in front of the whole code.

Endenite · Accepted Answer · 2017-12-12 18:17:38Z

2

Swift, 87 bytes

func f(a:inout[Int]){var i=0,c=0;while(c=a.count,c>1).1{i=(i+a[i%c])%c;a.remove(at:i)}}

Returns as a singleton list by modifying the input. Try it online!

Explanation

func f(a:inout[Int]){
  var i=0,c=0;            // Set the index i to 0
  while(c=a.count,c>1).1{ // While the length of the list > 0:
    i=(i+a[i%c])%c;       //  Add a[i] to i and loop back using modulo
    a.remove(at:i)        //  Remove a[i]
  }
}

edited Dec 12, 2017 at 18:17

answered Dec 12, 2017 at 17:54

Endenite

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Community · Accepted Answer · 2020-06-17 09:04:33Z

2

Mathematica, 36 bytes

uses Martin's algorithm

#//.l:{x_,__}:>Rest@RotateLeft[l,x]&

-5 bytes from Misha Lavrov && Martin Ender

Try it online!

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 12, 2017 at 15:30

ZaMoC

24.9k2 gold badges30 silver badges81 bronze badges

1

\$\begingroup\$ You can save two bytes by using the pattern to pick out the first element #//.{x_,y__}:>Rest@RotateLeft[{x,y},x]&. (This stops when there is only one element because {a} no longer matches the pattern {x_,y__}.) \$\endgroup\$
– Misha Lavrov
Commented Dec 12, 2017 at 22:12
1

\$\begingroup\$ @MishaLavrov can't test right now, but you can probably shorten that further by dropping the y, calling the entire list l and then using l instead of {x,y}. \$\endgroup\$
– Martin Ender
Commented Dec 12, 2017 at 23:08
1

\$\begingroup\$ @MartinEnder You mean like this - #//.l:{x_,__}:>Rest@RotateLeft[l,x]&? \$\endgroup\$
– Misha Lavrov
Commented Dec 12, 2017 at 23:14
1

\$\begingroup\$ @MishaLavrov yep. \$\endgroup\$
– Martin Ender
Commented Dec 12, 2017 at 23:27

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Wheat Wizard · Accepted Answer · 2017-12-14 08:57:42Z

2

Brain-Flak, 104 bytes

H.PWiz has a shorter answer here that I helped to make, you should check it out.

([[]]()){({}()<(({})){({}[()]<({}<(([])<{{}({}<>)<>([])}{}<>>)<>>)<>{({}[()]<({}<>)<>>)}{}<>>)}{}{}>)}{}

Try it online!

Explanation

([[]]())   #Push 1 minus stackheight
{({}()<    #N times
 (({}))    #Get a copy of the top
 {({}[()]< #N times
  ({}<(([])<{{}({}<>)<>([])}{}<>>)<>>)<>{({}[()]<({}<>)<>>)}{}<>
           #Roll the top to the bottom (From the wiki)
 >)}{}     #End loop
 {}        #Remove one value
>)}{}      #End loop

edited Dec 14, 2017 at 8:57

answered Dec 12, 2017 at 17:33

Wheat Wizard♦

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\$\begingroup\$ I thought I would compete. Then I realised that mine was almost exactly the same as yours, apart from a different "top roll" \$\endgroup\$
– H.PWiz
Commented Dec 12, 2017 at 18:24
\$\begingroup\$ I saw that ;). Using the fact that everything is non-negative is rather clever. \$\endgroup\$
– Wheat Wizard ♦
Commented Dec 12, 2017 at 18:25

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Sean · Accepted Answer · 2017-12-24 16:48:29Z

2

Perl 6, 46 45 bytes

(-1 byte thanks to Brad Gilbert)

{($_,{(|$_ xx*)[.[0]+(1..^$_)]}...1)[*-1][0]}

Try it online!

($_, { ... } ... 1) generates a sequence of lists, starting with the input list $_, each successive element being generated by the brace expression, and terminating when the list smart-matches 1--ie, has a length of 1. The trailing [* - 1] obtains the final element, and the final [0] takes the sole element out of that singleton list.

(|$_ xx *) generates a flat, infinitely replicated copy of the current element. This list is indexed with the range .[0] + (1 ..^ $_) to extract the next finite list in the series.

edited Dec 24, 2017 at 16:48

answered Dec 12, 2017 at 18:37

Sean

8,03611 silver badges15 bronze badges

1

\$\begingroup\$ mind blown o.O \$\endgroup\$
– Adrian
Commented Dec 12, 2017 at 20:01
\$\begingroup\$ [*-1][0] can be combined into [*-1;0] saving a byte. Also 1..$_-1 is better written as 1..^$_ again saving a byte. \$\endgroup\$
– Brad Gilbert b2gills
Commented Dec 17, 2017 at 22:47
\$\begingroup\$ @BradGilbertb2gills I tried [*-1;0], but it seems not to be equivalent somehow. The function then returns a list rather than a number. \$\endgroup\$
– Sean
Commented Dec 18, 2017 at 0:02
\$\begingroup\$ That doesn't stop the 1..^$_ optimization \$\endgroup\$
– Brad Gilbert b2gills
Commented Dec 21, 2017 at 22:53

Add a comment |

Xcali · Accepted Answer · 2017-12-12 15:31:12Z

1

Perl 5, 47 43 41 + 2 (`-ap`) = 43 bytes

$\=splice@F,($_+=$F[$_%@F])%@F,1while@F}{

Try it online!

Takes input as space separated numbers.

edited Dec 12, 2017 at 15:31

answered Dec 12, 2017 at 14:21

Xcali

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\$\begingroup\$ it seems it does not exactly the same as the step by step example following does but is longer $x%=@F,splice@F,$x=($x+$F[$x])%@F,1while$#F;$_="@F" \$\endgroup\$
– Nahuel Fouilleul
Commented Dec 12, 2017 at 16:55
1

\$\begingroup\$ wow o.O Need to get my game up. \$\endgroup\$
– Adrian
Commented Dec 12, 2017 at 20:00

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totallyhuman · Accepted Answer · 2017-12-12 16:03:43Z

1

Haskell, 56 bytes

f[x]=x
f l|m<-head l`mod`length l=f$drop(m+1)l++take m l

Try it online!

answered Dec 12, 2017 at 16:03

totallyhuman

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\$\begingroup\$ 55 bytes \$\endgroup\$
– H.PWiz
Commented Dec 13, 2017 at 1:01

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Rod · Accepted Answer · 2017-12-12 16:44:11Z

1

Python 3, 57 56 bytes

f=lambda a,*b:f(*([a,*b]*-~a)[a+1:a-~len(b)])if b else a

Try it online!

edited Dec 12, 2017 at 16:44

answered Dec 12, 2017 at 16:09

Rod

18.4k4 gold badges31 silver badges89 bronze badges

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Mr. Xcoder · Accepted Answer · 2017-12-12 17:26:13Z

Java 8, 325 Bytes

Golfed:

static void n(Integer[]j){Integer[]h;int a=0;h=j;for(int i=0;i<j.length-1;i++){if(h.length==a){a=0;}a=(a+h[a])%h.length;h[a]=null;h=m(h);}System.out.print(h[0]);}static Integer[] m(Integer[]array){Integer[]x=new Integer[array.length-1];int z=0;for(int i=0;i<array.length;i++){if(array[i]!=null){x[z]=array[i];z++;}}return x;}

Ungolfed:

 interface ArrayLeapFrog {
static void main(String[] z) throws Exception {
    Integer[] j = {6, 2, 3, 4};
    n(j);
}

static void n(Integer[] j) {
    Integer[] h;
    int a = 0;
    h = j;
    for (int i = 0; i < j.length - 1; i++) {
        if (h.length == a) {
            a = 0;
        }
        a = (a + h[a]) % h.length;
        h[a] = null;
        h = m(h);
    }
    System.out.print(h[0]);
}

static Integer[] m(Integer[] array) {
    Integer[] x = new Integer[array.length - 1];
    int z = 0;
    for (int i = 0; i < array.length; i++) {
        if (array[i] != null) {
            x[z] = array[i];
            z++;
        }
    }
    return x;
  }
}

Welcome! A couple tips: No need to count the static keywords here. Typically multi-method solutions are implemented as non-static members of a class, and main creates an instance for testing. Also, if you do it that way you support Java 7 and can submit as simply a "Java" solution. For future reference, input format tends to be quite flexible here, so for example you could choose to take input as a List (which is quite helpful for this problem). — Jakob, Commented Dec 13, 2017 at 0:43

Community · Accepted Answer · 2020-06-17 09:04:33Z

1

APL+WIN, 36 bytes

¯1↑⍎¨(1⌈¯1+⍴v←,⎕)⍴⊂'v←(1<⍴v)↓v[1]⌽v'

Explanation:

Prompts for screen input.

'v←(1<⍴v)↓v[1]⌽v' Loop logic as a string

 (1<⍴v)↓ only drop the first when number of elements n>1

 (1⌈¯1+⍴v←,⎕)⍴⊂ create a nested vector of logic of length 1 max n-1

 ⍎¨ execute each element of the nested vector in turn

¯1↑ take answer from executing final element

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 12, 2017 at 19:10

Graham

6,8461 gold badge15 silver badges11 bronze badges

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Community · Accepted Answer · 2020-06-17 09:04:33Z

1

Python 2, 61 bytes

def f(x):
 while x[1:]:y=x[0]%len(x);x=x[y+1:]+x[:y]
 print x

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 13, 2017 at 3:10

Riker

7,8504 gold badges39 silver badges73 bronze badges

1

\$\begingroup\$ I know a bunch of python answers exist, but I figured I might as well add my own. \$\endgroup\$
– Riker
Commented Dec 13, 2017 at 3:10

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Community · Accepted Answer · 2020-06-17 09:04:33Z

1

JavaScript, 58 56 59 bytes

let f =

a=>{for(i=0,k=a.length;k>1;)i+=a[i%=k],a.splice(i%=k--,1)}

<h2>Test</h2>
Enter or paste a valid array literal within square brackets and click Run.
<blockquote>
   <input id = "array" type="text" length="20">
   <button type="button" onclick="run()">Run</button>
</blockquote>
Result: <pre id="o"></pre>

<script>
    function run() {
       let a = JSON.parse(array.value);
       f(a);
       o.textContent = a;
    }
</script>

Returns the result as the only element remaining in the input array which is updated in place.

Two bytes saved by using a comma separated statement instead of a block statement in the for loop body! Three bytes lost to skip from an element deleted at end of array (:

Less golfed:

a => {
    for(i=0,k=a.length;k>1;) // once less than array length
        i+=a[i%=k],          // the new index
        a.splice(            // delete an element
           i%=k--,           // ensuring index is within array,
                             // and post decrement loop count
           1
        )
}

edited Jun 17, 2020 at 9:04

CommunityBot

1

answered Dec 13, 2017 at 1:36

traktor

4793 silver badges10 bronze badges

\$\begingroup\$ This seems to give the wrong answer for [3, 5, 7, 9]. \$\endgroup\$
– Neil
Commented Dec 13, 2017 at 9:56
\$\begingroup\$ Wrong for [3,5,7,9]. Expected value 5 \$\endgroup\$
– edc65
Commented Dec 13, 2017 at 10:02
\$\begingroup\$ The function does not return the value, i'm not sure if the byte count is proper keeping that in mind, since it can't work on it's own... \$\endgroup\$
– Brian H.
Commented Dec 13, 2017 at 11:40
\$\begingroup\$ @edc65 and Neil , thanks - the index of an element deleted at end of array was not being adjusted to the start of the shortened array. \$\endgroup\$
– traktor
Commented Dec 13, 2017 at 22:35
\$\begingroup\$ @BrianH. the function modify its parameter, there is consensus about that codegolf.meta.stackexchange.com/a/4942/21348 \$\endgroup\$
– edc65
Commented Dec 13, 2017 at 22:40

| Show 1 more comment

Οurous · Accepted Answer · 2017-12-14 22:31:12Z

1

Clean, 78 bytes

Uses the same method as Laikoni's Haskell answer.

import StdEnv
f[e]=e
f[x:r]=f(take(length r)(drop(x+1)(flatten(repeat[x:r]))))

Try it online!

answered Dec 14, 2017 at 22:31

Οurous

8,1271 gold badge16 silver badges38 bronze badges

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Mark · Accepted Answer · 2017-12-15 16:01:53Z

1

R, 111 117 126 bytes

Thanks to @Giuseppe for golfing off 11 bytes by changing to a while loop, got another 4 by removing the function and reading user input directly.

I don't feel great about what it took to get there - I'm sure a more elegant solution exists.

i=scan();m=1;while((l=sum(i|1))-1){j=i[m];p=`if`(j+m>l,j%%l+!m-1,j+m);p=`if`(!p,m,p);i=i[-p];m=`if`(p-l,p,1)};i

Try it online!

Ungolfed code

i=scan()
m=1
while((l=sum(i|1))-1){
  j=i[m]
  p=`if`(j+m>l,j%%l+!m-1,j+m)
  p=`if`(!p,m,p)
  i=i[-p]
  m=`if`(p-l,p,1)
}
i

edited Dec 15, 2017 at 16:01

answered Dec 15, 2017 at 15:37

Mark

4312 silver badges5 bronze badges

\$\begingroup\$ 117 bytes -- note that since this is a recursive function, the name f= needs to be included \$\endgroup\$
– Giuseppe
Commented Dec 15, 2017 at 15:45
1

\$\begingroup\$ I found this quite a difficult challenge with a 1 based-index language without array rotations; this is potentially 1-3 bytes shorter with a while loop, I think. \$\endgroup\$
– Giuseppe
Commented Dec 15, 2017 at 15:46
\$\begingroup\$ 115 bytes with a while loop \$\endgroup\$
– Giuseppe
Commented Dec 15, 2017 at 15:50
\$\begingroup\$ my previous 115 byter was invalid since we both forgot the f= part of the recursive function. :( \$\endgroup\$
– Giuseppe
Commented Dec 15, 2017 at 15:51
\$\begingroup\$ I updated the old score and the new score to reflect the recursiveness :) With the 'while' loop I golfed off another 4 bytes using scan. \$\endgroup\$
– Mark
Commented Dec 15, 2017 at 15:59

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Duck, Duck, Josephus

38 Answers 38

Husk, 7 bytes

Explanation

Haskell, 54 50 48 bytes

Previous 54 byte version:

Ruby, 37 bytes

Python 3, 54 51 bytes

CJam, 15 bytes

Explanation

MATL, 21 bytes

Explanation

Jelly, 7 bytes

Jelly, 9 bytes

Explanation

Brain-Flak, 88 bytes

Explanation

Python 2, 55 bytes

How it works

APL (Dyalog), 20 18 bytes

Python 3, 60 bytes

J, 21 17 bytes

How it works:

JavaScript (ES6), 54 60 bytes

Test cases

How?

Julia 0.6, 46 42 bytes

Java 8, 79 bytes

Ungolfed

Acknowledgments

Pyth, 9 bytes

How it works

Swift, 87 bytes

Explanation

Mathematica, 36 bytes

Brain-Flak, 104 bytes

Explanation

Perl 6, 46 45 bytes

Perl 5, 47 43 41 + 2 (-ap) = 43 bytes

Haskell, 56 bytes

Python 3, 57 56 bytes

Java 8, 325 Bytes

APL+WIN, 36 bytes

Python 2, 61 bytes

JavaScript, 58 56 59 bytes

Clean, 78 bytes

R, 111 117 126 bytes

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