Task — 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 Prel, 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.

Try it 

#!/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;
}

last updated 2026-03-23 — 7 lines of code

Output

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