Advent Calendar - December 24, 2019

Tuesday, Dec 24, 2019| Tags: Raku

Advent Calendar 2019

| Day 23 | Day 24 |

The gift is presented by Jo Christian Oterhals. Today he is talking about his solutions to Task #1: Niven Numbers of “The Weekly Challenge - 007”.

After a week’s hiatus I’ve returned to the Perl Weekly Challenge. This is the seventh challenge so far. As before there are two excercises. The first one is to calculate all Niven Numbers between 0 and 50. Niven numbers are integers that are divisible by the sum of its digits. I.e. 47 is a niven number if 47 / (4 + 7) is an integer without remainder (it’s not, as the result is 4.2727; 48 is, as 48/(4+8) is 4).

These kinds of operations are called modulo operations or mods. Most programming languages use the operator % for this, so that you can check for this by testing whether 47 % (4 + 7) == 0. But Raku has an additional operator, a shorthand for the former, and is written as 47 %% (4 + 7). %% returns True or False and lets you ignore the == 0 part.

To find the individual digits of a number we use the .comb method. That returns a list with the individual digits (I’ve used .comb on earlier challenges as well, but then to divide strings into their individual characters). On the combed list we use something called a reduction operator, specifically [+]. What this does is that it loops through the list and adds all of the numbers within. Where you once would have written something like

my $result = 0;
$result += $_ for 1, 2, 3, 4, 5;
say $result;

you can now do the same with a simple

say [+] 1, 2, 3, 4, 5 .

There are lots of these reduction operators. They save time, so look into them.

Knowing these two things it becomes apparaent that calculating Niven numbers with Raku can be done with a simple and reasonably readable one-liner.

.say if $_ %% [+] .comb for 0..50;

That’s it.

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Advent Calendar 2019


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