PWC147 - Truncatable Prime


Here we are with TASK #1 from The Weekly Challenge #147. Enjoy!

The challenge

Write a script to generate first 20 left-truncatable prime numbers in base 10.

In number theory, a left-truncatable prime is a prime number which, in a given base, contains no 0, and if the leading left digit is successively removed, then all resulting numbers are primes.


9137 is one such left-truncatable prime since 9137, 137, 37 and 7
are all prime numbers.

The questions

I have two questions:

  1. are single-digit prime numbers considered left-truncatable? I would say no, because there is no “resulting” number after chopping off the first digit. Anyway, my first intuition about what $0!$ should be failed me, so who knows? This will be my working assumption, though.

  2. why stop so low at 20? We’re not even reaching the example!

While we’re at it, a small personal reflection too.

I enjoy these challenges, in their unique shape of being sometimes quite vague and relying heavily on examples. I think our fine host is a fox disguised as a lamb, throwing us puzzles that sometimes give us that distinct wtf?!? moment that often arises when we’re dealing with customers. It’s a stimulus to go beyond, ask questions, gather more requirements, try to see things from different perspectives, make assumptions… in summary, architecting a solution.

Thanks manwar.

The solution

OK, let’s get this started. I initially thought about a constructive approach, which ended up with a good amount of code and being sufficiently complicated to leave me with a question… is it even correct?

I mean, I know that the constructive approach (more on this in a minute, promised!), but is my implementation right? No clue.

So I also implemented a dumber, brute-force exclusive approach based on iterating over all odd values and filtering stuff out if the required condition does not apply. As a comparison. As a test. As a reference.

You know what? In the end, the original constructive approach was correct since the beginning, while the “simpler” exclusive approach had a bug!

Anyway, I thought of baking the choice between them in the code:

#!/usr/bin/env raku
use v6;

sub MAIN (Int:D $n = 20, :$exclusive = False) {
   $exclusive ?? exclusive($n) !! constructive($n);

Let’s start with the constructive approach. We’re aiming for a nth-left-truncatable($n) function that will give us the n-th number that applies, so we just have to print all values from the first to the twentieth:

sub constructive ($n) { put nth-left-truncatable($_) for 1 .. $n }

As a matter of fact, it’s the underlying function that is constructive. The main insight here is that, for a number to be left-truncatable, the left truncation must either be a single-digit prime number, or it must be left-truncatable itself.

Hence, it’s easy to find new candidates for left-truncatable numbers: just start from the ones you have and try adding all possible digits from 1 to 9 in front of them. If we end up with a prime… it’s also left-truncatable.

sub nth-left-truncatable ($nth) {
   state @cache = (10..99).grep({ .is-prime && .substr(1, 1).is-prime });
   state $prefix = 1;
   state $first-id = 0;
   state $next-first-id = @cache.elems;
   state $id = $first-id;
   while @cache < $nth { # find moar!
      my $candidate = ($prefix ~ @cache[$id++]).Int;
      @cache.push($candidate) if $;
      if $candidate.chars == @cache[$id].chars { # toppled over!
         if $prefix < 9 {
         else {
            $prefix = 1;
            ($first-id, $next-first-id) = ($next-first-id, $id);
         $id = $first-id; # just reset the cursor
   return @cache[$nth - 1];

The implementation aims at giving something similar to a lazy thing. If we already have what we need in the @cache… well, it’s a good day! Let’s return it and go take an ice cream. Well, if you’re in Australia, anyway, otherwise a hot chocolate will do for me, thanks.

Otherwise, we go looking for more lef-truncatable numbers until we have enough to cover the request. To do so, then, we keep some state variables so that we remember where we stopped in our previous search and we can restart from there.

The @cache is pre-warmed with all the two-digits left-truncatable numbers. This is necessary because we’re excluding single-digit primes from the lot, so we use the two-digits ones as a starting point.

OK, time for the exclusive approach. Here we have a is-left-truncatable($n) test function, which tells us whether a given input $n is indeed left-truncatable or just something else. This is used to print values as we find them, until we hit the limit:

sub exclusive (Int:D $n is copy = 20) {
   my $i = 9;
   while $n > 0 {
      next unless is-left-truncatable($i = $i + 2);

The implementation actually uses the same insight as before: the characteristic of being left-truncatable is inherently recursive, so why not?

sub is-left-truncatable ($n) {
   return False if $n < 10 || $n ~~ /0/;
   return False unless $;
   state %cache;
   if %cache{$n}:!exists {
      my $truncated = $n.substr(1);
      return $ if $truncated < 10;
      %cache{$n} = is-left-truncatable($truncated);
   return %cache{$n};

Just as we’re at it, I decided to throw some %cache just because memoization is so cool. Totally overkill and unneeded in this case, isn’t overengineering so funny?!?

For the Perl counterpart I decided to go only with the constructive alternative. At the end of the day, it’s the one I personally like more, and I already had plenty of references to compare to.

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

say nth_left_truncatable($_) for 1 .. shift // 20;

sub nth_left_truncatable ($nth) {
   state $cache = [ grep { is_prime($_) && is_prime(substr $_, 1) } 10 .. 99 ];
   state $prefix = 1;
   state $first_id = 0;
   state $next_first_id = $cache->@*;
   state $id = $first_id;
   while ($cache->@* < $nth) {
      my $candidate = $prefix . $cache->[$id++];
      push $cache->@*, $candidate if is_prime($candidate);
      if (length($candidate) == length($cache->[$id])) { # toppled over!
         if ($prefix < 9) {
         else {
            $prefix = 1;
            ($first_id, $next_first_id) = ($next_first_id, $id);
         $id = $first_id; # just reset the cursor
   return $cache->[$nth - 1];

sub is_prime { #
   return if $_[0] < 2;
   return 1 if $_[0] <= 3;
   return unless ($_[0] % 2) && ($_[0] % 3);
   for (my $i = 6 - 1; $i * $i <= $_[0]; $i += 6) {
      return unless ($_[0] % $i) && ($_[0] % ($i + 2));
   return 1;

Alas, we lack a primality test in the language here, so we have to code one. This is taken from Wikipedia, there’s probably better stuff around but this does the job in decent time for our purposes.

OK, enough for this post… stay safe everybody!

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