Task — Swap nibbles

You are given a positive integer $N. Write a script to swap the two nibbles of the binary representation of the given number and print the decimal number of the new binary representation. A nibble is a four-bit aggregation, or half an octet. To keep the task simple, we only allow integers less than or equal to 255.

Examples

Input: $N = 101
Output: 86

Binary representation of decimal 101 is 1100101 or as 2 
nibbles (0110)(0101). The swapped nibbles would be 
(0101)(0110) same as decimal 86.

Input: $N = 18
Output: 33

Binary representation of decimal 18 is 10010 or as 2 
nibbles (0001)(0010). The swapped nibbles would be 
(0010)(0001) same as decimal 33.

Analysis

To do this we first convert $N to its binary representation using sprintf('%08b', $N). The nibbles are then easily switched, and the new binary number is converted to decimal by multiplying the 1 bits by the appropriate power of 2 and adding those together.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2021-06-28
use utf8;     # Week 119 - task 1 - Swap nibbles
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

swap_nibbles(101);
swap_nibbles(18);
swap_nibbles(243);

sub swap_nibbles {
    
    my ($input, $text, $flipped, $output, $n, $k);
    
    # initialise
    $input = $_[0];
    
    $text = sprintf('%08b', $input);
    $text =~ m|(..)(..)(..)(..)|;
    $flipped = $3 . $4 . $1 . $2;
    $n = 128;
    for $k (split('', $flipped)) {
        $output += $n * $k;
        $n /= 2;
    }
    
    say qq[\nInput:  $input = $1 $2 $3 $4];
    say qq[Output: $output = $3 $4 $1 $2];
}

last updated 2026-03-14 — 12 lines of code

Output

Input:  101 = 01 10 01 01
Output: 86 = 01 01 01 10

Input:  18 = 00 01 00 10
Output: 33 = 00 10 00 01

Input:  243 = 11 11 00 11
Output: 63 = 00 11 11 11