Task — Peak positions

You are given an array of integers, @ints. Find all the peaks in the array, a peak is an element that is strictly greater than its left and right neighbours. Return the indices of all such peak positions.

Examples

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

Example 2
Input: @ints = (2, 4, 6, 5, 3)
Output: (2)

Example 3
Input: @ints = (1, 2, 3, 2, 4, 1)
Output: (2, 4)

Example 4
Input: @ints = (5, 3, 1)
Output: (0)

Example 5
Input: @ints = (1, 5, 1, 5, 1, 5, 1)
Output: (1, 3, 5)

Analysis

I can't think of a better way than simply looping over the elements of @ints from the second to the second last one, pushing the index onto @peaks if the current element exceeds both neighbours.

Clearly there need to be at least 3 elements in @ints, so I check for that and return an empty @peaks if there aren't enough.

The algorithm works for integers less, greater or equal to zero, and - although not required - for non-integers too.

Try it 

#!/usr/bin/perl

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

use v5.26;    # The Weekly Challenge - 2025-10-27
use utf8;     # Week 345 - task 1 - Peak positions
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

peak_positions(1, 2, 3, 2, 1);
peak_positions(1, 2, 3, 4, 5);
peak_positions(3, 3, 3, 3, 3);
peak_positions(3, 2, 1, 2, 3);
peak_positions(1, 2, 1, 3, 1, 4, 1, 6, 1);
peak_positions(1, 2);
peak_positions(-3, -2, -4, 0, 9, -100);

sub peak_positions {
    
    my (@ints, @peaks, $j);
    
    # initialise
    @ints = @_;
    
    # do as instructed
    if ($#ints > 2) {
        for $j (1 .. $#ints - 1) {
            push @peaks, $j if ($ints[$j] > $ints[$j - 1]
                and $ints[$j] > $ints[$j + 1]);
        }
    }
        
    say qq[\nInput:  (] . join(', ', @ints) . ')';
    say qq[Output: (] . join(', ', @peaks) . ')';
}

Output

Input:  (1, 2, 3, 2, 1)
Output: (2)

Input:  (1, 2, 3, 4, 5)
Output: ()

Input:  (3, 3, 3, 3, 3)
Output: ()

Input:  (3, 2, 1, 2, 3)
Output: ()

Input:  (1, 2, 1, 3, 1, 4, 1, 6, 1)
Output: (1, 3, 5, 7)

Input:  (1, 2)
Output: ()

Input:  (-3, -2, -4, 0, 9, -100)
Output: (1, 4)