PWC133 - Integer Square Root


Here we are with TASK #1 from The Weekly Challenge #133. Enjoy!

The challenge

You are given a positive integer $N.

Write a script to calculate the integer square root of the given number.

Please avoid using built-in function. Find out more about it here.


Input: $N = 10
Output: 3

Input: $N = 27
Output: 5

Input: $N = 85
Output: 9

Input: $N = 101
Output: 10

The questions

Not much to ask for this challenge, as it’s actually the gamification of some study. And I like both studying and games, so I can only thank our host here.

Were it an interview, though, I’d probably ask this:

  • how hard should we validate our inputs? Input validation is a good thing, but it might also imply a performance cost which might be avoided by doing proper validation only at the beginning of some data munging. Hence, it might be avoided if the function we’re after will only be used on valid data.

  • what is the maximum value for the input $N? Very big numbers might imply the adoption of specific techniques; moreover it’s good to test with a few values in that ballpark.

The solution

Speaking of validation, Raku basically gives it for free programmer time, so why not?

#!/usr/bin/env raku
use v6;
sub integer-square-root (Int:D $n where * >= 0) {
   return $n if $n < 2;
   my $x = $n +> 1;  # first estimate
   my $y = $x + 1;   # just to get started with $x < $y
   ($x, $y) = (($x + ($n / $x).Int) +> 1, $x) while $x < $y;
   return $y;
sub MAIN (Int:D $n where * >= 0) { put integer-square-root($n) }

I don’t know about the performance hit of using incremental typing here, though.

Looking at the algorithm in C proposed here, it’s easy to spot that x0 is just a way to memorize the previous value that was calculated. For this reason, I changed the variables’ names to reflect it: $x evolves only in terms of itself, and $y gets the previous value.

To get the ball rolling, $y is arbitrarily initialized to be greater than $x. This ensures that the while is triggered at least once, giving $y a proper value.

The same algorithm is also implemented in Perl:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';
sub integer_square_root ($n) {
   return $n if $n < 2;
   my $x = $n >> 1;  # first estimate
   my $y = $x + 1;   # just to get started with $x < $y
   ($x, $y) = (($x + int($n / $x)) >> 1, $x) while $x < $y;
   return $y;
say integer_square_root(shift // die "$0 <n>\n");

Here I’m giving up on input validation and only accounting for the user forgetting to pass a value on the command line. For the rest, it’s the same as the Raku version, with due changes in the right places (e.g. >> instead of +>, as well as int instead of .Int).

I considered putting a use integer inside the function, but eventually decided to avoid it for the explicit int(...) - at the end of the day, it’s just one single place where I have to put it, so there’s little space for getting this wrong.

I hope it’s everything for this post because I’m closing it and… wishing you to have a nice and safe day!

Comments? Octodon, , GitHub, Reddit, or drop me a line!