PWC197 - Wiggle Sort


On with TASK #2 from The Weekly Challenge #197. Enjoy!

The challenge

You are given a list of integers, @list.

Write a script to perform Wiggle Sort on the given list.

Wiggle sort would be such as list[0] < list[1] > list[2] < list[3]….

Example 1

Input: @list = (1,5,1,1,6,4)
Output: (1,6,1,5,1,4)

Example 2

Input: @list = (1,3,2,2,3,1)
Output: (2,3,1,3,1,2)

The questions

Why… oh why these sort of sorts? Sort of strange…

More seriously, what to do with corner cases where e.g. most numbers are the same, and the requested sorting cannot be obtained?

The solution

I’ll call my solutions sloppy to acknowledge the fact that I’m not putting exactly all myself into this solution. Maybe it’s an interview question geared at testing our willingness to do sillyness. Or lack thereof.


We can sort the list and then cut it in half, then interleave the two parts. If the list has an odd number of elements… no worries, let’s just make the second half shorter and interleave them starting from the first.

Why is this sloppy? There might be corner cases, like most numbers being the same, where the requested sorting is simply not possible. I’ll happily waive my hand and leave this as a simple exercise for the reader.

Which brings us to the Perl solution:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

my @wiggled = wiggle_sort_sloppy(@ARGV ? @ARGV : (1, 5, 1, 1, 6, 4));
{local $" = ','; say "(@wiggled)"}

sub wiggle_sort_sloppy (@list) {
   @list = sort { $a <=> $b } @list;
   my @upper = splice @list, int((@list + 1) / 2);
   map { $_, (@upper ? shift(@upper) : ()) } @list;

The Raku solution is very similar, although I like gather/ŧake and use them when possible:

#!/usr/bin/env raku
use v6;
sub MAIN (*@args) {
   @args = 1, 5, 1, 1, 6, 4 unless @args;
   put '(', wiggle-sort-sloppy(@args).join(','), ')';

sub wiggle-sort-sloppy (@list) {
   my @ordered = @list.sort: { $^a <=> $^b };
   my $halfway = (@ordered + 1) div 2;
   gather for ^$halfway {
      take @ordered[$_];
      take @ordered[$_ + $halfway] if $_ + $halfway < @ordered;

Enough! Stay safe!

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