PWC193 - Binary String


Here we are with TASK #1 from The Weekly Challenge #193. Enjoy!

The challenge

You are given an integer, $n > 0.

Write a script to find all possible binary numbers of size $n.

Example 1

Input: $n = 2
Output: 00, 11, 01, 10

Example 2

Input: $n = 3
Output: 000, 001, 010, 100, 111, 110, 101, 011

The questions

I think that by “all possible binary numbers” we mean “all possible strings of length $n that can be build over an alphabet comprising 0 and 1 only”. This is because I’m not sure I would consider 0123 a decimal number of size 4, to be honest.

Also… I’m not sure how we’re supposed to produce the strings. I guess any order will do.

The solution

Each challenge lives on its own little monad, and has its own rules. But (you knew there was a but) I can’t help observing that the binary strings here can happily start with a string of 0 characters, while no later than the last week they had to definitely start with a 1, or bad things would have happened.

So shouldn’t this go in the questions sections somehow? Well, no. The challenge is subtly worded as to find all possible binary numbers of size $n, which means that we don’t get to do any conversion if we don’t want to.

OK, as usual let’s start with Raku:

#!/usr/bin/env raku
use v6;
sub MAIN (Int:D $n where * > 0 = 2) { .put for binary-strings($n) }

sub binary-strings (Int:D $n where * > 0) {
   my $prefix = '0' x ($n - 1);
   my $i = 0;
   return gather loop {
      my $raw = ($i++).base(2).Str;
      my $len = $raw.chars;
      last if $len > $n;
      take ('0' x ($n - $len)) ~ $raw;

We’re just counting from 0 upwards, stopping when the binary representation gets too long. To cope with the length requirement, we just pre-pend each produced string with a suitable number of 0 characters.

I know, this solution is neither efficient nor scalable. Raku has built-in big integers, but we’re computing/accumulating all strings up-front and we’re also using gather/take, which is not the best efficiency-wise. I like it too much though.

For the Perl alternative we move on to good old iterators - they play nicer for bigger values of the input $n, as we start seeing stuff immediately and we keep memory consumption to a minimum.

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';
use Math::BigInt;

my $it = binary_strings_iterator(shift // 2);
while (defined(my $binary_string = $it->())) {
   say $binary_string;

sub binary_strings_iterator ($n) {
   my $i = Math::BigInt->bzero;
   return sub {
      return unless defined $i;
      my $raw = ($i++)->to_bin;
      my $len = length $raw;
      return $i = undef if $len > $n;
      return ('0' x ($n - $len)) . $raw;

So go ahead, feed it with 1000 as input and stop it when you’ve got enough!

Stay safe!

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