This article has been moved from «perl6.eu» and updated to reflect the language rename in 2019.
This is my response to the Perl Weekly Challenge #8.
Write a script that computes the first five perfect numbers. A perfect number is an integer that is the sum of its positive proper divisors (all divisors except itself). Please check Wiki for more information. This challenge was proposed by Laurent Rosenfeld. 
I'll start with a program that prints the divisors (excluding the number itself) for a given number:
File: perfectdivisorssub MAIN ($number)
{
say "Divisors (excluding the number itself): " ~ properdivisors($number);
}
multi properdivisors (2) { return (1); } # [1]
multi properdivisors (Int $number where $number > 2) # [2]
{
return (1) if $number.isprime; # [3]
my @divisors = (1); # [4]
for 2 .. ($number 1) > $candidate # [5]
{
@divisors.push: $candidate if $number %% $candidate; # [5a]
}
return @divisors;
}
[1] This variant is called for the value 2.
[2] This variant is called when the value is 3 or greater. (Note that integers less than 2, as well as nonintegers, give a runtime error.)
[3] Prime numbers don't have divisors, so avoid trying to compute them.
[4] The first divisor is always 1.
[5] Looping through the possible values, we add it to the list if it is a divisor (5a). Note that we could have written this line like this instead:
for 2 ..^ $number > $candidate
Testing it:
$ raku perfectdivisors 2
Divisors (excluding the number itself): 1
$ raku perfectdivisors 3
Divisors (excluding the number itself): 1
$ raku perfectdivisors 4
Divisors (excluding the number itself): 1 2
$ raku perfectdivisors 12
Divisors (excluding the number itself): 1 2 3 4 6
Extending (and renaming) the program with code to check if the number is a perfect number:
File: perfectnumberstestsub MAIN ($number)
{
say "Divisors (excluding the number itself): " ~ properdivisors($number);
say "Is the number perfect: " ~ isperfect($number);
}
multi properdivisors (2) { return (1); }
multi properdivisors (Int $number where $number > 2)
{
return (1) if $number.isprime;
my @divisors = (1);
for 2 .. ($number 1) > $candidate
{
@divisors.push: $candidate if $number %% $candidate;
}
return @divisors;
}
sub isperfect ($number)
{
return $number == properdivisors($number).sum; # [1]
}
[1] Checking if the number is a Perfect Number is easy. Just add the divisors and compare the sum with the number itself.
And finally as a program that prints the first 5 perfect numbers (or whatever other number we specify on the command line):
File: perfectnumberssub MAIN (Int $count where $count > 0 = 5) # [1]
{
my $numbers := gather # [2]
{
for 2..Inf # [3]
{
take $_ if isperfect($_); # [3]
}
}
say "The first $count perfect numbers: "
~ $numbers[0 .. $count 1].join(', ') ~ "."; # [4]
}
multi properdivisors (2) { return (1); }
multi properdivisors (Int $number where $number > 2)
{
return (1) if $number.isprime;
my @divisors = (1);
for 2 .. ($number 1) > $candidate
{
@divisors.push: $candidate if $number %% $candidate;
}
return @divisors;
}
sub isperfect ($number)
{
return $number == properdivisors($number).sum;
}
[1] A positive integer, with 5 as default value.
[2] You may have noticed (from my other articles) that I have a fondness for
«gather
»/«take
».
[3] Setting up the values.
[4] Fetching the first «$count» number of values.
Searching for the five first perfect numbers is extremely slow, as the fifth number is «33550336» (Source: wikipedia).
Count  Time  Values 
1  0,163s  6 
2  0,173s  6,28 
3  0,249  6,28,496 
4  11,919s  6,28,496,8128 
5  ???  6,28,496,8128,33550336 
It is actually so slow that I gave up after the program had been running for about one day. So I haven't actually verified that the program gives the correct answer.
multi properdivisors (Int $number where $number > 2)
{
return (1) if $number.isprime;
my @divisors = (1);
for 2 .. ($number / 2) > $candidate
{
@divisors.push: $candidate if $number %% $candidate;
}
return @divisors.sort;
}
Timing it with the value 4 gives us 17,649s, so this version is actually slower than the original one (adding about 50% to the time used).
It turns out that the problem is the upper limit for the Range in the
«for» loop. If we ensure that the upper limit is an integer, e.g. by using integer
division with div
(instead of /
), the time is as low as
5,723s:
for 2 .. ($number div 2) > $candidate
See docs.raku.org/routine/div for more information about «div».
Write a function, ‘center’, whose argument is a list of strings, which will be
lines of text. The function should insert spaces at the beginning of the lines of
text so that if they were printed, the text would be centered, and return the
modified lines.
For example,
should return the list:
because if these lines were printed, they would look like:

The procedure is quite simple, and looks like this:
sub center (@strings) # [1]
{
my $maxlength = @strings>>.chars.max; # [2]
return @strings.map({ .indent(($maxlength  .chars) /2) }); # [3]
}
[1] Pass the strings.
[2] Compute the length of the longest string (by computing all, and taking the largest value).
If you don't like the >>.
syntax, use map
instead:
my $maxlength = @strings.map(*.chars).max;
[3] If we indent
by the max length minus the length of the
actual string, we get right tabulated output. Half the value gives centered output.
See docs.raku.org/routine/indent for more information about «indent».
Here it is, as a program. Specify the arguments on the command line.
File: centersub MAIN (*@strings)
{
.say for center(@strings);
}
sub center (@strings)
{
my $maxlength = @strings>>.chars.max;
return @strings.map({ .Str.indent(($maxlength  .chars) / 2) });
}
Note the coersion to a string with «Str», as «indent» is only implemented for strings. This isn't a problem when getting the values from the command line (as integers will be of the «IntStr» type), but is a problem if used on integers specified in a program.
See my Raku From Zero to 35 or docs.raku.org/type/IntStr for more information about «IntStr».
Running it:
$ raku center "123" "111111111111111111111111111111" "111111" "1"
123
111111111111111111111111111111
111111
1
$ raku center "123" "111111111111111" "111111" "1"
123
111111111111111
111111
1
Note that a string with an even number of characters will be placed wrong in a mathematical sense, as we cannot indent by half a space:
$ raku center "123" "11" "1"
123
11
1
$ raku center "1234" "11" "1"
1234
11
1
sub center (*@strings)
This change makes it possible to use both individual arguments (the first line), or a list (the second line)  or any combinations (but please don't):
say center(1, 2222222222222222222222222222, 33333333333333333333);
say center([1,22222,3]);
And that's it.