RogerBW's Blog

Perl Weekly Challenge 107: Self-Descriptive Methods 07 April 2021

I’ve been doing the Perl Weekly Challenges. The latest involved several sorts of introspection. (Note that this is open until 11 April 2021.)

TASK #1 › Self-descriptive Numbers

Write a script to display the first three self-descriptive numbers.

This is of course similar to task 2 of challenge 43, but not identical, as we need multiple answers; I rewrote the code from scratch. I eschewed most of the optimisations and just took the one that the SDN must end in zero. The example makes it clear that what's wanted is the first three SDNs in any base, and it's easiest just to assume base 10 and ignore the "number must have length equal to the base" constraint.

The algorithm is basically the same in each language: I have an array ns which is each digit in order, and an array d which is the number of times each digit occurs (so d[0] is the number of non-leading zeroes, etc.). If the number is self-describing these arrays will match (with any d in excess of the length of ns being zero). This code won't find SDNs above base 10, and it'll take a long time there. (But I was polite and didn't just pull results from OEIS A138480 or OEIS A108551 which would get the answer with much less work.)

In Raku:

sub sdn($count) {
  my @r;
  my $n=10;
  while (@r.elems < $count) {
    my @ns=map {$_+0}, sprintf('%d',$n).comb;
    my @d=(0) xx 10;
    map {@d[$_]++},@ns;
    my $sd=1;
    for 0..@ns.elems-1 -> $i {
      if (@d[$i] != @ns[$i]) {
    if ($sd && @ns.elems <= 9) {
      for @ns.elems..9 -> $i {
        if (@d[$i] != 0) {
    if ($sd) {
      push @r,$n;
  return @r;

The others are basically the same, except that in Rust I built the two arrays in parallel

        let mut ns: Vec<usize>=vec![];
        let mut d: Vec<usize>=vec![0;10];
        for i in n.to_string().chars() {
            let k=i.to_digit(10).unwrap() as usize;
            d[k] += 1;

because it felt clearer.

TASK #2 › List Methods

Write a script to list methods of a package/class.

Eugh. If I need to do this it is generally a sign that something has gone Badly Wrong and I need to rethink my approach; it's a definite bad code smell if I see it in someone else's program. (I did once write a thing that used it, and it worked, but the code never got to the point of usability for people other than me.) So while I know that this is possible in Python and Ruby, though I believe not in Rust, I only actually did it in Perl and Raku.

Perl (making sure that each thing in the module's namespace is a method):

use List::Util;

no strict 'refs';
foreach my $cm (sort keys %List::Util::) {
  if (exists &{"List::Util::$cm"}) {
    print "$cm\n";

Probably there's some way of getting at %modulename:: where "modulename" is a string variable rather than a literal piece of text, but I wasn't able to find it on a quick search and frankly wasn't enthusiastic enough to look very hard. (Also one would ideally want the contents of @modulename::ISA in order to get all the methods of the things one's inheriting from.)


#! /usr/bin/perl6

use Pod::To::Text;

say Pod::To::Text.^methods.perl;

Full code on github.

See also:
Perl Weekly Challenge 43: rings and self-descriptive numbers

  1. Posted by RogerBW at 10:01am on 12 April 2021

    Part 1: mostly brute-forcing, some use of the table in Wikipedia but this isn't necessarily exhaustive. If I had a use for these I'd probably write a parser for the OEIS internal format. (Which I could use to solve a lot of these problems, really, but it wouldn't be much fun.)

    Part 2: many people feel less revulsion than I do. Though in any case


    will apparently get the job done

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