Flipping easy and

distributing fairly

Weekly challenge 192 — 21 November 2022

Week 192 - 21 Nov 2022

Task 1

You are given a positive integer, `$n`

.
Write a script to find the binary flip.

Example 1Input: $n = 5 Output: 2 First find the binary equivalent of the given integer, 101. Then flip the binary digits 0 -> 1 and 1 -> 0 and we get 010. So Binary 010 => Decimal 2.Example 2Input: $n = 4 Output: 3 Decimal 4 = Binary 100 Flip 0 -> 1 and 1 -> 0, we get 011. Binary 011 = Decimal 3Example 3Input: $n = 6 Output: 1 Decimal 6 = Binary 110 Flip 0 -> 1 and 1 -> 0, we get 001. Binary 001 = Decimal 1

Our first task this week is to take a positive integer, render it in binary, swap the zeroes for ones and vice versa, and output the result in decimal.

It's tempting to use the Perl bitwise not operator (~) to flip the bits, but inspection of the examples shows that's not quite what's wanted. The reason is that while we normally express 4, for example, as 100 in binary, the computer considers it to be 0000000000000100. Applying the ~ operator to that gives us 1111111111111011, which isn't the desired answer.

To get around that, note that the supplied number will always start with a 1 in its simple binary representation, so its flipped value will start with 0. And in its 16 (or whatever) bit representation, that will be preceded by a lot of 1s. So as it's just a string, we can safely just strip off all the leading 1s and the algorithm boils down to just two lines:

$flip = sprintf('%b', ~ $test); $flip =~ s|^1+||;

#!/usr/bin/perl # Peter Campbell Smith - 2022-11-21 # PWC 192 task 1 use v5.28; use utf8; use warnings; my (@tests, $test, $flip); @tests = (5, 4, 6, 127, 128, 1234, 999999, 7294019678); # loop over tests while ($test = shift @tests) { $flip = sprintf('%b', $test); say qq[\nInput: $test = 0b$flip]; # 'not' the input and remove leading ones $flip = sprintf('%b', ~ $test); $flip =~ s|^1+||; say qq[Output: ] . oct('0b' . $flip) . qq[ = 0b$flip]; }

Input: 5 = 0b101 Output: 2 = 0b010 Input: 4 = 0b100 Output: 3 = 0b011 Input: 6 = 0b110 Output: 1 = 0b001 Input: 127 = 0b1111111 Output: 0 = 0b0000000 Input: 128 = 0b10000000 Output: 127 = 0b01111111 Input: 1234 = 0b10011010010 Output: 813 = 0b01100101101 Input: 999999 = 0b11110100001000111111 Output: 48576 = 0b00001011110111000000 Input: 7294019678 = 0b110110010110000011110100001011110 Output: 1295914913 = 0b001001101001111100001011110100001

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Peter Campbell Smith is hereby placed in the public domain