Prime step

You are given a string with English alphabetic characters only. What is the absolute difference of the sum of the ASCII values of the characters in the string to the nearest prime number?

Examples

Example 1
Input: $str = 'hello'
Output: 9
The ordinal values of 'hello' are [104,101,108,108,111], summing up to
   532.
The nearest prime number to 532 is 523, resulting in an absolute
   difference of 9.

Example 2
Input: $str = 'football'
Output: 2
Starting with the values [102,111,111,116,98,97,108,108] and the sum
   841.
We find 839 as the nearest prime number, so the difference is 2.

Example 3
Input: $str = 'a'
Output: 0

Example 4
Input: $str = 'challenge'
Output: 2
The ordinal values of 'challenge' are [99, 104, 97, 108, 108, 101,
   110, 103, 101], which sum up to 931.
The nearest prime number to 931 is 929, so the difference is 2.

Example 5
Input: $str = 'perl'
Output: 2
The ordinal values of 'perl' are [112, 101, 114, 108], summing up to
   435.
Nearest prime is 433, so the difference is 2.

Analysis

I've used Math::Prime::Util for this as I've submitted Sieve of Eratosthenes solutions to at least 11 earlier challanges.

The summation of the Unicode code points - ie what ord($a) returns - is an easy single line.

It's then just a case of stepping up and down from that sum to find the nearest prime in both directions, and choosing the closer.

I could of course have stopped as soon as I found a prime in either direction, but it seems more satisfying to check both.

Peter has come up with an efficient and performance-oriented solution for Task 1 of Perl Weekly Challenge 393. He precomputes a square lookup table and symmetry bounds (a < b) that makes it possible for him to get rid of costly square root and integer operations with the help of precomputation.

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

Try it 

Your input:

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2026-09-28
use utf8;     # Week 393 - task 2 - Prime step
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Math::Prime::Util 'is_prime';
use Encode;

prime_step('football');
prime_step('hello');
prime_step('challenge');
prime_step('supercalifragilisticexpialidocious');
prime_step('ⱠⱡⱢⱣⱤⱥⱦ');

sub prime_step {
    
    my ($string, $answer, $j, $sum, $explain, $above, $below);
    
    $string = $_[0];
    $answer = -1;
    $below = $above = 0;
    
    # sum the ordinal values of the string
    $sum += ord($_) for split(//, $string);
    
    # work up and down until we find a prime
    for $j (0 .. 1000) {
        $below = $j if is_prime($sum - $j);
        $above = $j if is_prime($sum + $j);
        
        # found surrounding primes
        if ($below and $above) {
            $answer = ($below < $above) ? $below : $above;
            $explain = qq[sum = $sum: primes below and above = ] . 
                ($sum - $below) . ', ' . ($sum + $above);
            last;
        }
    }
    
    say qq[\nInput:  '$string'];
    say qq[Output: $answer\n     $explain]; 
}

16 lines of code

Output from script

Input:  'football'
Output: 12
     sum = 851: primes below and above = 839, 863

Input:  'hello'
Output: 9
     sum = 532: primes below and above = 523, 541

Input:  'challenge'
Output: 2
     sum = 931: primes below and above = 929, 937

Input:  'supercalifragilisticexpialidocious'
Output: 12
     sum = 3643: primes below and above = 3631, 3659

Input:  'ⱠⱡⱢⱣⱤⱥⱦ'
Output: 4
     sum = 79541: primes below and above = 79537, 79549