# ETOOBUSY ðŸš€ minimal blogging for the impatient

# PWC204 - Monotonic Array

**TL;DR**

Here we are with TASK #1 from The Weekly Challenge #204. Enjoy!

# The challenge

You are given an array of integers.

Write a script to find out if the given array is Monotonic. Print 1 if it is otherwise 0.

An array is Monotonic if it is either monotone increasing or decreasing.

`Monotone increasing: for i <= j , nums[i] <= nums[j] Monotone decreasing: for i <= j , nums[i] >= nums[j]`

Example 1`Input: @nums = (1,2,2,3) Output: 1`

Example 2`Input: @nums (1,3,2) Output: 0`

Example 3`Input: @nums = (6,5,5,4) Output: 1`

# The questions

No questions asked! (Maybe we could know if thereâ€™s any limit to the
*integers*, so that we can understand if big integer support is needed
in Perl. But this is looking for trouble).

# The solution

Iâ€™m *almost* happy with the following solution, I only suspect that the
`map`

might be expressed differently and more idiomatically. Overall,
anyway, I think itâ€™s good:

```
#!/usr/bin/env raku
use v6;
sub MAIN (*@args) { put monotonic-array(@args) ?? 1 !! 0 }
sub monotonic-array (@array) {
([*] @array.rotor(2 => -1).map({[-] $_}).minmax[0, *-1]) >= 0
}
```

In a nutshell:

- the array elements are taken two by two, with overalaps. This means taking each possible pair of consecutive elements;
- the difference of each pair is calculated
- the minimum and maximum differences are extracted (
`minmax`

returns a range, so we take the two extremes)

If these two values have different signsâ€¦ itâ€™s not monotonic. Otherwise it is!

More or less the same goes in Perl, except that to do the check element by element - arguably this is more efficient, because it does not require calculating all the differences if the first elements already allow taking a decision.

```
#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';
say monotonic_array(@ARGV) ? 1 : 0;
sub monotonic_array (@array) {
my $direction = 0;
for my $i (1 .. $#array) {
my $delta = $array[$i] - $array[$i - 1];
return 0 if $direction * $delta < 0;
$direction ||= $delta;
}
return 1;
}
```

I like this short-circuiting too, so overall Iâ€™m happy with the more low-level solution.

Stay safe!

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