Count prefixes

You are given an array of words and a string which contains only lowercase English letters.

Write a script to return the number of words in the given array that are a prefix of the given string.

Examples

Example 1
Input: @array = ('a', 'ap', 'app', 'apple', 'banana'),
   $str = 'apple'
Output: 4

Example 2
Input: @array = ('cat', 'dog', 'fish'), $str = 'bird'
Output: 0

Example 3
Input: @array = ('hello', 'he', 'hell', 'heaven', 'he'),
   $str = 'hello'
Output: 4

Example 4
Input: @array = ('', 'code', 'coding', 'cod'),
   $str = 'coding'
Output: 3

Example 5
Input: @array = ('p', 'pr', 'pro', 'prog', 'progr',
   'progra', 'program'), $str = 'program'
Output: 7

Analysis

This is an easy one for Perl, the answer being:

grep { $string =~ m|^$_| } @words

The expression in curly brackets tests whether each word (mapped to $_) is or isn't a prefix of $string. It returns a list of those that are, or, as it's assigned to a scalar, a count of such words.

And that's the answer we want.

Peter Campbell Smith's Week 366 Write-up provides an unambiguous, pragmatic solution style representing a strong real-world Perl mindset. The emphasis is placed on solving the problem in an accurate and efficient manner through simple implementation methods. The provided solution is straightforward and effective; he understands the relevant tasks thoroughly and prefers to solve issues clearly and without complexity (typical of all Weekly Challenges).

This review may cover either or both challenges for this week.

Try it 

Try running the script with any input:

Your @words =

example: ca can dog cant cantelo

Your $string =

example: canteloupe

⤡

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2026-03-23
use utf8;     # Week 366 - task 1 - Count prefixes
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

count_prefixes(['a', 'ap', 'app', 'apple', 'banana'], 'apple');
count_prefixes(['cat', 'dog', 'fish'], 'bird');
count_prefixes(['hello', 'he', 'hell', 'heaven', 'he'], 'hello');
count_prefixes(['', 'code', 'coding', 'cod'], 'coding');
count_prefixes(['p', 'pr', 'pro', 'prog', 'progr', 'progra', 'program'], 'program');

sub count_prefixes {
    
    my (@words, $string);
    
    # initialise
    @words = @{$_[0]};
    $string = $_[1];
    
    # report
    say qq[\nInput:  \$words = ('] . join(q[', '], @words) . 
        qq['), \$string = '$string'];
    say qq[Output: ] . grep { $string =~ m|^$_| } @words;
}

7 lines of code

Output from script

Input:  $words = ('a', 'ap', 'app', 'apple', 'banana'),
   $string = 'apple'
Output: 4

Input:  $words = ('cat', 'dog', 'fish'), $string = 'bird'
Output: 0

Input:  $words = ('hello', 'he', 'hell', 'heaven', 'he'),
   $string = 'hello'
Output: 4

Input:  $words = ('', 'code', 'coding', 'cod'),
   $string = 'coding'
Output: 3

Input:  $words = ('p', 'pr', 'pro', 'prog', 'progr',
   'progra', 'program'), $string = 'program'
Output: 7