Clever!
I had missed checking "is-prime" straight away! Checked some large
primes, and my function hung badly. Thank you!
Is grep running on all "2 ..^ $n/2" values, and then picking the first,
or is that somehow lazily evaluated? I assume the former, or you'd have
no need to halve $n. But I don't see any definitive speed increase
either way.
What factors (pun un-intended) lead you to gather/take? I've not used it
much, so I don't have much of a feel for its strengths over other
control flows.
I also need to pay more attention to while's "recursive" abilities, if
that's the proper word for it. Seems to fit well with gather/take. No
explicit returns!
- Rick
On 6/17/22 02:04, Simon Proctor wrote:
Here's my new go to prime-factors function :
sub prime-factors( Int $n is copy where * > 1 ) {
gather {
while ( $n > 1 ) {
if ( $n.is-prime ) {
take $n;
last;
}
my $f = (2..^$n/2).grep($n %% *).first(*.is-prime);
take $f;
$n = $n div $f
}
}
}
On Fri, 17 Jun 2022 at 03:51, Simon Proctor <simon.proc...@gmail.com
<mailto:simon.proc...@gmail.com>> wrote:
This is because if there isn't an explicit return the last statement
evaluated is the implied return value.
So the last call (which returns Nil) was your implicit return.
Generally I try and avoid an implicit return if a function has more
than a couple of lines.
On Thu, 16 Jun 2022, 21:41 Rick Bychowski, <r...@hiranyaloka.com
<mailto:r...@hiranyaloka.com>> wrote:
For example here:
https://github.com/manwar/perlweeklychallenge-club/blob/master/challenge-168/rick-bychowski/raku/ch-2.raku
<https://github.com/manwar/perlweeklychallenge-club/blob/master/challenge-168/rick-bychowski/raku/ch-2.raku>
I called the recursive function at various points in an if/else
statement, but each call was always the very last statement of the
branch/routine. Otherwise, even a trailing "last" or "exit"
silently
"fails".
All good!
- Rick
On 6/16/22 13:21, Rick Bychowski wrote:
> Yep, the "return factors($q, @f)" fixes it. I've not called a
recursion
> from anywhere other than the end of a routine, so haven't run
into this
> issue before. Learned something today!
>
> Thanks to Simon, William and Andinus for your assistance!
>
> - Rick
>
> On 6/16/22 11:07, Simon Proctor wrote:
>> I think, and I don't have my computer to hand to double
check but I
>> think you want a return before your recursive.call to
factors passing
>> in @f.
>>
>> When dealing with recursive functions you need to make sure
what
>> you're returning back up the stack.
>>
>>
>>
>> On Thu, 16 Jun 2022, 18:10 Rick Bychowski,
<r...@hiranyaloka.com <mailto:r...@hiranyaloka.com>
>> <mailto:r...@hiranyaloka.com <mailto:r...@hiranyaloka.com>>>
wrote:
>>
>> Hi Everyone,
>>
>> I've been lurking quite a while, this will be my first
post to perl6
>> users. I've written a lot of short scripts in perl5 for
system admin
>> type stuff at home and work. Lately I'm playing with
Raku, which is a
>> lot of fun. Error reporting is excellent, as is the online
>> documentation.
>>
>> To the point. I recently started the perl weekly
challenge. Lots of
>> math/primes stuff. I wrote an algorithm to list all the
prime
>> factors of
>> an integer, using a recursive subroutine. I'm able to
print the
>> result
>> from the subroutine, but it always returns Nil. What am
I missing?
>>
>> #!/usr/bin/env raku
>>
>> sub MAIN($n = 20) {
>> .say for factors($n); # Nil
>> }
>>
>> sub factors($n, @factors?) {
>> my @f = @factors.elems ?? @factors !! ();
>> my $q;
>> for 2 ..^ $n -> $i {
>> if $n %% $i {
>> $q = Int($n / $i);
>> @f.push($i);
>> if $q.is-prime {
>> @f.push($q);
>> say @f; # [2 2 5]
>> return @f;
>> } else {
>> factors($q, @f);
>> }
>> last;
>> }
>> }
>> }
>>
>> Returns
>> [2 2 5]
>> Nil
>>
>> -- Rick Bychowski
>>
>> The information in this email is confidential and may be
legally
>> privileged. It is intended solely for the addressee(s).
Access to
>> this
>> e-mail by anyone else is unauthorized.
>>
>
--
Rick Bychowski
The information in this email is confidential and may be legally
privileged. It is intended solely for the addressee(s). Access
to this
e-mail by anyone else is unauthorized.
--
Simon Proctor
Cognoscite aliquid novum cotidie
http://www.khanate.co.uk/ <http://www.khanate.co.uk/>
--
Rick Bychowski
The information in this email is confidential and may be legally
privileged. It is intended solely for the addressee(s). Access to this
e-mail by anyone else is unauthorized.
