PWC196 - Range List


On with TASK #2 from The Weekly Challenge #196. Enjoy!

The challenge

You are given a sorted unique integer array, @array.

Write a script to find all possible [Number Range i.e [x, y] represent range all integers from x and y` (both inclusive).

Each subsequence of two or more contiguous integers

Example 1

Input: @array = (1,3,4,5,7)
Output: [3,5]

Example 2

Input: @array = (1,2,3,6,7,9)
Output: [1,3], [6,7]

Example 3

Input: @array = (0,1,2,4,5,6,8,9)
Output: [0,2], [4,6], [8,9]

The questions

I guess either everything is clear, or I’m softening in my nitpicking. I’d only ask if it’s necessary to print out exactly as in the example… I’ll assume not.

The solution

For me, the tricky part was filtering out ranges where the begin and the end are equal. This makes my code… inelegant.


The approach is this: iterate through the list, keeping track of the latest range. If a value is out of the previous range (or there’s no previous range), we add another one.

Here’s Perl, which I hope makes the algorithm above more readable:

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

say join ', ',
  map { '[' . join(',', $_->@*) . ']'}
  range_list(@ARGV ? @ARGV : (1, 3, 4, 5, 7));

sub range_list (@array) {
   my @retval;
   for my $v (@array) {
      if (@retval && $retval[-1][1] == $v - 1) {
         $retval[-1][1] = $v;
      else {
         pop @retval if @retval && $retval[-1][0] == $retval[-1][1];
         push @retval, [$v, $v];
   pop @retval if @retval && $retval[-1][0] == $retval[-1][1];
   return @retval;

The Raku rendition is almost the same, only with slightly different machinery. I liked the idea to use gather/take. Still, there’s the need to take the last range into account, which bothers me a bit because of the code repetition.

#!/usr/bin/env raku
use v6;
sub MAIN (*@args) { say range-list(@args) }

sub range-list (@array) {
   gather {
      my ($begin, $end);
      for @array -> $v {
         if defined($end) && $end == $v - 1 {
            $end = $v;
         else {
            take [$begin, $end] if defined($begin) && $begin < $end;
            $begin = $end = $v;
      take [$begin, $end] if defined($begin) && $begin < $end;

So well, this makes it for this week’s challenges… Stay safe!

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