PWC106 - Maximum Gap


Here we are with TASK #1 from the Perl Weekly Challenge #106. Enjoy!

The challenge

You are given an array of integers @N. Write a script to display the maximum difference between two successive elements once the array is sorted. If the array contains only 1 element then display 0.

The questions

As a paranoid of corner cases, my first question would be… what if the array is empty?. I’ll take that 0 is a valid answer here too.

The other question might also be… is there an efficient way to do this in better than $O(N log(N))$ ?!? 😅

The solution

I waited. A long time. And no answer came to my last question.

Well, at least if you consider 5 seconds a long time to say… to hell with it, let’s just use the brute force and spare precious developer time.

At this point, the path is clear. Though no eyes can see. Now I’m really digressing.

So, boring solution here:

  • first, let’s sort the input data;
  • then, scan every consecutive pair to look for the maximum gap.

All of this… with a readability twist!

sub maximum_gap (@N) {
   return 0 if @N <= 1;
   (my $p, @N) = sort { $a <=> $b } @N;
   return max(map { (my $d, $p) = ($_ - $p, $_); $d } @N);

When sorting the inputs, we take care to remove the first item and put it in variable $p. This is our previous element in each pair.

At this point, we can transform the sorted input data in a sequence of gaps:

map { (my $d, $p) = ($_ - $p, $_); $d } @N

The assignment here is between two lists; this allows us to both calculate the difference (in $d) and update the value of $p for the next iteration. As we want to generate the list of gaps, the last expression in map’s body is the difference we calculated in $d.

At this point we just have to get the max out of it!

The whole script, should you be curious:

#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;

sub maximum_gap (@N) {
   return 0 if @N <= 1;
   (my $p, @N) = sort { $a <=> $b } @N;
   return max(map { (my $d, $p) = ($_ - $p, $_); $d } @N);

my @input = @ARGV ? @ARGV : (1, 3, 8, 2, 0);
say maximum_gap(@input);

Stay safe and have fun, hopefully!

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