### Octal Representation

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.)

Challenge 1 this week is an easy one:

Write a script to print decimal number 0 to 50 in [the] Octal Number System.

## Perl and Raku

We can solve this with the following polyglot (runs in both languages at once):

printf "Decimal %2d = Octal %2o\n", $_,$_ for 0..50;


Still, in Raku, we can do the following:

### Leonardo 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.)

Happy new year! We are on Week 41, and this is Challenge #2.

The Leonardo Numbers (A001595) are a simple recursively defined sequence:

$$L(n) = \begin{cases} 1 & \text{if } n \lt 2 \\ 1 + L(n – 1) + L(n – 2) & \text{if } n \geq 2 \end{cases}$$

The sequence starts: 1, 1, 3, 5, 9, 15, 25, 41, 67, 109, 177, 287, 465, 753, 1219, 1973, 3193, 5167, 8361, 13529, 21891, …

You’ll note this sequence is very similar to the well-known Fibonacci sequence, which differs only in that the Fibonacci sequence does not have the + 1 term, and starts at F(0) = 0.

### Attractive 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.)

This week, Mohammad asks us to output a list of all Attractive Numbers between 1 and 50. Attractive numbers, as described by the Online Encyclopedia of Integer Sequences (OEIS) are:

Numbers with a prime number of prime divisors (counted with multiplicity)

OEIS Sequence A063989

The first numbers are 4, 6, 8, 9, 10, 12, 14, 15, 18, 20, 21, 22, 25, 26, 27, 28, 30, 32, 33, 34, 35, 38, 39, 42, 44, 45, 46, 48, 49, 50, …

This is a straightforward problem, but I’m not going to let that stop me from finding an interesting way to solve it. If I were looking for a sensible way to solve it, I’d do something like this:

use Math::Prime::Util ':all';
say for grep { is_prime( factor(\$_) ) } 1..50;


### Zip6

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.)

The zip6 function has long been a staple of the List::MoreUtils CPAN module. The Week 40 challenge #1 describes it very well. But even more succinctly: zip6 takes an array of arrays (AoA) and returns another AoA of all the 1st elements, then the 2nd, and so on.

It’s not a difficult algorithm by any stretch. In fact, the description more or less tells you how to do it. Simply iterate through all of the arrays in parallel, building up the resulting array as you go.

### Reverse Polish Notation

This is my first blog post regarding the Perl Weekly Challenge tirelessly maintained by Mohammad Anwar. Although I’ve been doing the challenge for a while now, I haven’t maintained my blog for years. Let’s change that!

This week’s challenge #2 from the Perl Weekly Challenge is a computer science classic: create a Reverse Polish Notation (RPN) calculator. This brings back fond memories.

## Basic Algorithm

My Perl 5 and Raku solutions will both use the same basic algorithm. First, we tokenize the input into numbers and operators. Then, we loop through every token. For each token, we either:

1. If the token is a number, we add it to the stack
2. Otherwise, it’s an operator:
1. Pull zero or more items off the stack depending on the operator
2. Perform the required operation (e.g., addition, subtraction, etc.)
3. Push the result onto the stack