Task — Swap odd/even bits

You are given a positive integer $N less than or equal to 255. Write a script to swap the odd positioned bit with even positioned bit and print the decimal equivalent of the new binary representation.

Examples

Input: $N = 101
Output: 154

Binary representation of the given number is 01 10 01 01.
The new binary representation after the odd/even swap is 
10 01 10 10. The decimal equivalent of 10011010 is 154.

Input: $N = 18
Output: 33

Binary representation of the given number is 00 01 00 10.
The new binary representation after the odd/even swap is 
00 10 00 01. The decimal equivalent of 100001 is 33.

Analysis

The answer is given either ~ $N or equivalently 255 - $N. Displaying the input and output numbers as paired binary numbers can be done with:

	$text = sprintf('%08b', $N);
	$text =~ s|(\d\d)|$1 |g;

And that's it.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2021-07-05
use utf8;     # Week 120 - task 1 - Swop odd/even bits
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

swap_odd_even_bits (101);
swap_odd_even_bits (255);
swap_odd_even_bits (0);
swap_odd_even_bits (0b10101010);

sub swap_odd_even_bits {
    
    my ($input, $bits, $text, $output);
    
    # initialise
    $input = shift;
    $bits = $input & 255;
    $text = sprintf('%08b', $bits);
    $text =~ s|(\d\d)|$1 |g;
    say qq[\nInput:  $input = $text];
    
    $bits = ~$bits & 255;
    $text = sprintf('%08b', $bits);
    $text =~ s|(\d\d)|$1 |g;
    say qq[Output: $bits = $text];
}

last updated 2026-03-13 — 11 lines of code

Output

Input:  101 = 01 10 01 01 
Output: 154 = 10 01 10 10 

Input:  255 = 11 11 11 11 
Output: 0 = 00 00 00 00 

Input:  0 = 00 00 00 00 
Output: 255 = 11 11 11 11 

Input:  170 = 10 10 10 10 
Output: 85 = 01 01 01 01