Consecutive ones

You are given an array containing only 0s and 1s. Write a script to find out the maximum consecutive 1s in the given array.

Examples

Example 1
Input: @binary = (0, 1, 1, 0, 1, 1, 1)
Output: 3

Example 2
Input: @binary = (0, 0, 0, 0)
Output: 0

Example 3
Input: @binary = (1, 0, 1, 0, 1, 1)
Output: 2

Analysis

The simplest solution to this is to pass once along the array, incrementing $this_run when a 1 is encountered, and when a zero is found setting $max_run to $this_run and $this_run to zero.

This will give the wrong answer if the longest run of 1s finishes with the last element of the array, but that is easily fixed by pushing a 0 onto the end of the array at the start.

I tried that algorithm with a million random 0s and 1s and it produced the result (20) in under 2 seconds so I think that's good enough.

There is one possible optimisation, which is that if the current element is zero and $max_run equals or exceeds the number of elements of @array still to be inspected, you can drop out of the loop because there cannot be a yet-undiscovered longer run. However, in most cases the cost of an extra if for such cases will negate the peformance gain.

Algorithmic purity with mathematical precision. Code clarity is exceptional with pedagogical value.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-06-09
use utf8;     # Week 325 - task 1 - Consecutive ones
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

consecutive_ones(0, 1, 1, 0, 1, 1, 1);
consecutive_ones(0, 0, 0, 0);
consecutive_ones(1, 0, 1, 0, 1, 1);

# bigger example
my @array;
push @array, int(rand($_)) & 1 for 0 .. 100;
consecutive_ones(@array);

sub consecutive_ones {
    
    my(@array, $this_run, $max_run, $j);
    
    # initialise
    @array = @_;
    push @array, 0;
    $max_run = $this_run = 0;
    
    # explore array
    for $j (@array) {
        
        # it's a 1
        if ($j) {
            $this_run ++;
            $max_run = $this_run if $this_run > $max_run;
            
        # it's a 0
        } else {
            $this_run = 0;
        }
    }

    say qq[\nInput:  (] . join(', ', @array[0 .. $#array - 1]) . q[)];
    say qq[Output: $max_run];
}

13 lines of code

Output from script

Input:  (0, 1, 1, 0, 1, 1, 1)
Output: 3

Input:  (0, 0, 0, 0)
Output: 0

Input:  (1, 0, 1, 0, 1, 1)
Output: 2

Input:  (0, 0, 0, 1, 1, 0, 1, 1, 1, 1, 0, 1, 1, 0, 0, 1,
   1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1,
   0, 0, 1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 0,
   1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 0, 1, 1, 0,
   1, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1,
   1, 1, 0, 0, 0, 0, 1, 0, 1, 1, 1, 1, 0)
Output: 5