RogerBW's Blog

Perl Weekly Challenge 30 15 October 2019

I've been doing the Perl Weekly Challenges. This one dealt with date calculations and exhaustive number searches.

Write a script to list dates for Sunday Christmas between 2019 and 2100. For example, 25 Dec 2022 is Sunday.

I was going to do this straight, but then it seemed like fun to make it a one-liner. Well, almost.

use Time::Local;

print map {"$_\n"} grep {(gmtime(timegm(0,0,12,25,11,$_)))[6]==0} (2019..2100);

Note that timegm matches gmtime's 0-based month numbering. (And it's a habit of mine to use timegm, and noon, to minimise the possibility of time-zone-related confusion, though in this case it shouldn't make any difference.)

In Perl6 this is just as straightforward:

map {say "$_"}, grep {Date.new($_,12,25).day-of-week==7}, (2019..2100);

I do like Perl6's built-in Date type, but I'm sure that having to load it (and all the rest) for every program is the reason startup is so much slower.

Write a script to print all possible series of 3 numbers, where in each series at least one of the number is even and sum of the three numbers is always 12. For example, 3,4,5.

Since this is clearly intended to be a finite list I choose not to consider negative numbers and non-integers. An optimisation: an even sum of three integers must by definition contain at least one even integer, so there's no need to check for it. I optimise further by generating the numbers in ascending order, so that each combination will only be displayed once.

my $target=12;
foreach my $a (0..$target) {
  foreach my $b ($a..$target) {
    my $c=$target-$a-$b;
    if ($c>=$b) {
      print "$a $b $c\n";
    }
  }
}

and converting this to Perl6 is just a matter of changing the loop syntax.

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