PWC 047 › Gapful Numbers

This post is part of a series on Mohammad Anwar’s excellent Weekly Challenge, where hackers submit solutions in Perl, Raku, or any other language, to two different challenges every week. (It’s a lot of fun, if you’re into that sort of thing.)

Task 2 this week has us print the first 20 “gapful numbers,” as described by OEIS sequence A108343. Gapful numbers are numbers greater than 99 that are evenly divisible by their first and last digits combined. For example, 132 is a gapful number because 132 ÷ 12 = 11.

This is certainly the easier of the two tasks, as both Perl and Raku have convenient ways to index and concatenate string fragments.


In Perl, we can check if a number is gapful by splitting the number and joining the first and last characters:

sub is_gapful(_) { $_ = pop; not $_ % join '', (split '')[0,-1] }

The modulus operation (%) will return 0 if the number is gapful, so we invert the result. The is_gapful(_) prototype will allow us to use is_gapful without arguments if we want to operate on $_:

sub first_n_gapful {
    my $N = shift;
    my @r;
    for ($_ = 100; @r < $N; $_++) {
        push @r, $_ if is_gapful;

I of course could have done without the prototype and used is_gapful($_), but this reads a little bit better.


Raku makes this ridiculously easy. The is-gapful sub is similar:

sub is-gapful( Int \n ) { n ≥ 100 and n %% n.comb[0,*-1].join }

Note the %% divisibility operator makes the logic a little bit cleaner. We can make a lazy list of every gapful number:

my @gapful = (100..∞).grep: &is-gapful; [1]

Then, printing the first 20 looks like this:

say @gapful[^20];

[1] The is meant to be an ampersand (&), but my blogging software and syntax highlighting plugin have their relationship status listed as “it’s complicated”, so I had to use the “fullwidth ampersand” Unicode character here instead.

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