by Arne Sommer

# Attractive Leonardo Numbers with Raku

[50] Published 31. December 2019. Updated 3. January 2020

 Welcome to this 50th article in my Perl6/Raku blog! This is my response to the Perl Weekly Challenge #41.

## Challenge #41.1: Attractive Numbers

 Write a script to display attractive number between 1 and 50. A number is an attractive number if the number of its prime factors is also prime number. The number 20 is an attractive number, whose prime factors are 2, 2 and 5. The total prime factors is 3 which is also a prime number.

Several of the weekly challenges has touched prime numbers, and we sort of computed the divisors in challenge #8.1. This version does things in a slightly different way, just for fun.

File: divisors ```unit sub MAIN (Int \$limit = 50); # [1] for 1 .. \$limit -> \$current # [2] { say "\$current: with divisors: { divisors(\$current) }"; # [2] } sub divisors (Int \$number is copy) # [3] { return (1) if \$number == 1; # [4] return (\$number) if \$number.is-prime; # [5] my @divisors; # [6] for 2 .. (\$number -1) -> \$candidate # [7] { next unless \$candidate.is-prime; # [8] while (\$number %% \$candidate) # [9] { @divisors.push: \$candidate; # [9a] \$number div= \$candidate; # [9b] } if \$number.is-prime # [10] { @divisors.push: \$number; # [10a] last; # [10b] } } return @divisors; # [11] } ```

[1] We can override the default limit (0f 50) if we want to.

[2] For each integer from 1 to the upper limit, print the number and its divisors.

[3] This one returns a list of the divisors for the given number. Note «is copy», as we change the value (in [9b]).

[4] Return «(1)» if the number is 1.

[5] Return the number, if it is a prime number. (Note that «1» is not a prime number, so the previous line is required.)

Note that `(1)` is not a list with one element, but a single value. The parens are the grouping operator, nothing more. The comma is the list (creating) operator (so `(1,)` is a list with one element). The non-list return value doesn't matter here, as the receiving code stringifies the result. (Note that assigning a scalar to an array also works, e.g. `my @array = 1` gives an array with one element.) [Added 3. Jan 2020]

[6] We collect the divisors here.

[7] We loop through the integers from 2 up to the number itself (not including it).

[8] Skip non-prime numbers.

[9] As long as the number is divisible by the candidate (i.e. that it is a divisor of the number), add the candidate to the list of divisors [9a] and remove the divisor from the number. Note the loop as the divisors can come several times, e.g. «27» has three divisors: «3», «3» and «3».

[10] If the number (after we have removed some divisors) is a prime number, add it ot the list of divisors [10a] and exit the loop [10b].

[11] Return the divisors.

Note that the upper limit in [7] could just as well have been written as «Inf», as «last» (in [10b]) does the actual loop termination before we come to the number itself. We can actually replace lines [7] and [8] with «`for (^Inf).grep(*.is-prime) -> \$candidate`».

Running it:

```\$ raku divisors 1: with divisors: 1 2: with divisors: 2 3: with divisors: 3 4: with divisors: 2 2 5: with divisors: 5 6: with divisors: 2 3 7: with divisors: 7 8: with divisors: 2 2 2 9: with divisors: 3 3 10: with divisors: 2 5 11: with divisors: 11 12: with divisors: 2 2 3 13: with divisors: 13 14: with divisors: 2 7 15: with divisors: 3 5 16: with divisors: 2 2 2 2 17: with divisors: 17 18: with divisors: 2 3 3 19: with divisors: 19 20: with divisors: 2 2 5 21: with divisors: 3 7 22: with divisors: 2 11 23: with divisors: 23 24: with divisors: 2 2 2 3 25: with divisors: 5 5 26: with divisors: 2 13 27: with divisors: 3 3 3 28: with divisors: 2 2 7 29: with divisors: 29 30: with divisors: 2 3 5 31: with divisors: 31 32: with divisors: 2 2 2 2 2 33: with divisors: 3 11 34: with divisors: 2 17 35: with divisors: 5 7 36: with divisors: 2 2 3 3 37: with divisors: 37 38: with divisors: 2 19 39: with divisors: 3 13 40: with divisors: 2 2 2 5 41: with divisors: 41 42: with divisors: 2 3 7 43: with divisors: 43 44: with divisors: 2 2 11 45: with divisors: 3 3 5 46: with divisors: 2 23 47: with divisors: 47 48: with divisors: 2 2 2 2 3 49: with divisors: 7 7 50: with divisors: 2 5 5 ```

That looks correct.

On to the challenge, then:

File: attractive-numbers ```unit sub MAIN (Int \$limit = 50, :\$verbose); # [1] for 1 .. \$limit -> \$current # [2] { my @divisors = divisors(\$current); # [2] my \$elems = @divisors.elems; # [2] say ": \$current: Divisors: { divisors(\$current) } Elements: \$elems" if \$verbose; # [3] say "\$current is an attractive number (with divisors: @divisors[])" if \$elems.is-prime; # [4] } sub divisors (Int \$number is copy) # [5] { ... } ```

[1] I have added verbose mode, to make it easier to see what is going on (and debug).

[2] The same as in the «divisors» program.

[3] Verbose output everything; the number, the divisors, and the divisor count.

[4] We have an attractive number if the divisor count is a prime number.

[5] The same as in the «divisors» program.

Running it:

```\$ raku attractive-numbers 4 is an attractive number (with divisors: 2 2) 6 is an attractive number (with divisors: 2 3) 8 is an attractive number (with divisors: 2 2 2) 9 is an attractive number (with divisors: 3 3) 10 is an attractive number (with divisors: 2 5) 12 is an attractive number (with divisors: 2 2 3) 14 is an attractive number (with divisors: 2 7) 15 is an attractive number (with divisors: 3 5) 18 is an attractive number (with divisors: 2 3 3) 20 is an attractive number (with divisors: 2 2 5) 21 is an attractive number (with divisors: 3 7) 22 is an attractive number (with divisors: 2 11) 25 is an attractive number (with divisors: 5 5) 26 is an attractive number (with divisors: 2 13) 27 is an attractive number (with divisors: 3 3 3) 28 is an attractive number (with divisors: 2 2 7) 30 is an attractive number (with divisors: 2 3 5) 32 is an attractive number (with divisors: 2 2 2 2 2) 33 is an attractive number (with divisors: 3 11) 34 is an attractive number (with divisors: 2 17) 35 is an attractive number (with divisors: 5 7) 38 is an attractive number (with divisors: 2 19) 39 is an attractive number (with divisors: 3 13) 42 is an attractive number (with divisors: 2 3 7) 44 is an attractive number (with divisors: 2 2 11) 45 is an attractive number (with divisors: 3 3 5) 46 is an attractive number (with divisors: 2 23) 48 is an attractive number (with divisors: 2 2 2 2 3) 49 is an attractive number (with divisors: 7 7) 50 is an attractive number (with divisors: 2 5 5) ```

With verbose mode:

```\$ raku attractive-numbers --verbose : 1: Divisors: 1 Elements: 1 : 2: Divisors: 2 Elements: 1 : 3: Divisors: 3 Elements: 1 : 4: Divisors: 2 2 Elements: 2 4 is an attractive number (with divisors: 2 2) : 5: Divisors: 5 Elements: 1 : 6: Divisors: 2 3 Elements: 2 6 is an attractive number (with divisors: 2 3) : 7: Divisors: 7 Elements: 1 : 8: Divisors: 2 2 2 Elements: 3 8 is an attractive number (with divisors: 2 2 2) : 9: Divisors: 3 3 Elements: 2 9 is an attractive number (with divisors: 3 3) : 10: Divisors: 2 5 Elements: 2 10 is an attractive number (with divisors: 2 5) : 11: Divisors: 11 Elements: 1 : 12: Divisors: 2 2 3 Elements: 3 12 is an attractive number (with divisors: 2 2 3) : 13: Divisors: 13 Elements: 1 : 14: Divisors: 2 7 Elements: 2 14 is an attractive number (with divisors: 2 7) : 15: Divisors: 3 5 Elements: 2 15 is an attractive number (with divisors: 3 5) : 16: Divisors: 2 2 2 2 Elements: 4 : 17: Divisors: 17 Elements: 1 : 18: Divisors: 2 3 3 Elements: 3 18 is an attractive number (with divisors: 2 3 3) : 19: Divisors: 19 Elements: 1 : 20: Divisors: 2 2 5 Elements: 3 20 is an attractive number (with divisors: 2 2 5) : 21: Divisors: 3 7 Elements: 2 21 is an attractive number (with divisors: 3 7) : 22: Divisors: 2 11 Elements: 2 22 is an attractive number (with divisors: 2 11) : 23: Divisors: 23 Elements: 1 : 24: Divisors: 2 2 2 3 Elements: 4 : 25: Divisors: 5 5 Elements: 2 25 is an attractive number (with divisors: 5 5) : 26: Divisors: 2 13 Elements: 2 26 is an attractive number (with divisors: 2 13) : 27: Divisors: 3 3 3 Elements: 3 27 is an attractive number (with divisors: 3 3 3) : 28: Divisors: 2 2 7 Elements: 3 28 is an attractive number (with divisors: 2 2 7) : 29: Divisors: 29 Elements: 1 : 30: Divisors: 2 3 5 Elements: 3 30 is an attractive number (with divisors: 2 3 5) : 31: Divisors: 31 Elements: 1 : 32: Divisors: 2 2 2 2 2 Elements: 5 32 is an attractive number (with divisors: 2 2 2 2 2) : 33: Divisors: 3 11 Elements: 2 33 is an attractive number (with divisors: 3 11) : 34: Divisors: 2 17 Elements: 2 34 is an attractive number (with divisors: 2 17) : 35: Divisors: 5 7 Elements: 2 35 is an attractive number (with divisors: 5 7) : 36: Divisors: 2 2 3 3 Elements: 4 : 37: Divisors: 37 Elements: 1 : 38: Divisors: 2 19 Elements: 2 38 is an attractive number (with divisors: 2 19) : 39: Divisors: 3 13 Elements: 2 39 is an attractive number (with divisors: 3 13) : 40: Divisors: 2 2 2 5 Elements: 4 : 41: Divisors: 41 Elements: 1 : 42: Divisors: 2 3 7 Elements: 3 42 is an attractive number (with divisors: 2 3 7) : 43: Divisors: 43 Elements: 1 : 44: Divisors: 2 2 11 Elements: 3 44 is an attractive number (with divisors: 2 2 11) : 45: Divisors: 3 3 5 Elements: 3 45 is an attractive number (with divisors: 3 3 5) : 46: Divisors: 2 23 Elements: 2 46 is an attractive number (with divisors: 2 23) : 47: Divisors: 47 Elements: 1 : 48: Divisors: 2 2 2 2 3 Elements: 5 48 is an attractive number (with divisors: 2 2 2 2 3) : 49: Divisors: 7 7 Elements: 2 49 is an attractive number (with divisors: 7 7) : 50: Divisors: 2 5 5 Elements: 3 50 is an attractive number (with divisors: 2 5 5) ```

That looks correct as well.

A fun fact about attractive numbers: whilst prime factors are a major (or «prime») ingredient, the numbers themselves are not prime.

## Challenge #41.2: Leonardo Numbers

 Write a script to display first 20 Leonardo Numbers. Please checkout wiki page for more information. For example: ``````L(0) = 1 L(1) = 1 L(2) = L(0) + L(1) + 1 = 3 L(3) = L(1) + L(2) + 1 = 5 `````` and so on.

This looks familiar to the Fibonacci Sequence (which hasn't been used in any of the challenges so far), and we can use a Sequence here as well:

```> my \$leonardo := (1, 1, { \$^a + \$^b +1 } ... Inf); ################## A B # C ######### # D ## ```

The first number is 1 (#A), the second is (also) 1 (#B). Then the third, fourth and so on up to infinity (#D) number is the sum of the two previous numbers + 1 (#C).

Obtaining the first 20 numbers is as easy as:

```> say \$leonardo[^20]; (1 1 3 5 9 15 25 41 67 109 177 287 465 753 1219 1973 3193 5167 8361 13529) ```

As a one-liner (REPL, and on the command line):

```> say (1, 1, { \$^a + \$^b +1 } ... Inf)[^20] (1 1 3 5 9 15 25 41 67 109 177 287 465 753 1219 1973 3193 5167 8361 13529) ``` ```\$ raku -e "say (1, 1, { \$^a + \$^b +1 } ... Inf)[^20]" (1 1 3 5 9 15 25 41 67 109 177 287 465 753 1219 1973 3193 5167 8361 13529) ```

If you want a program, here it is:

File: leonardo ```my \$leonardo := (1, 1, { \$^a + \$^b +1 } ... Inf); unit sub MAIN (\$limit = 20); say "\$_: \$leonardo[\$_]" for ^\$limit; ```

Running it:

```\$ raku leonardo 0: 1 1: 1 2: 3 3: 5 4: 9 5: 15 6: 25 7: 41 8: 67 9: 109 10: 177 11: 287 12: 465 13: 753 14: 1219 15: 1973 16: 3193 17: 5167 18: 8361 19: 13529 ```

And with a user specified upper limit:

```\$ raku leonardo 50 0: 1 1: 1 2: 3 3: 5 4: 9 5: 15 6: 25 7: 41 8: 67 9: 109 10: 177 11: 287 12: 465 13: 753 14: 1219 15: 1973 16: 3193 17: 5167 18: 8361 19: 13529 20: 21891 21: 35421 22: 57313 23: 92735 24: 150049 25: 242785 26: 392835 27: 635621 28: 1028457 29: 1664079 30: 2692537 31: 4356617 32: 7049155 33: 11405773 34: 18454929 35: 29860703 36: 48315633 37: 78176337 38: 126491971 39: 204668309 40: 331160281 41: 535828591 42: 866988873 43: 1402817465 44: 2269806339 45: 3672623805 46: 5942430145 47: 9615053951 48: 15557484097 49: 25172538049 ```

You can read more about the Fibonacci Numbers in my Beginning Raku book. (Look it up in the Index.) As procedures as well as sequences.

And that's it.