Re: compact my wordlist generator
- From: philip.g.potter@xxxxxxxxx (Philip Potter)
- Date: Mon, 26 Oct 2009 21:23:12 +0000
2009/10/26 Michael Alipio <daem0nb0y@xxxxxxxxx>:
Then as suggested (see far below), a recursive function will do what I want.. After some googling I found this:
permutate(0,2);
sub permutate($$){
my ($cur,$max) = @_;
if ($cur>=$max){
print "$result\n";
return;
}
for(@word){
substr($result,$cur,1)=$_;
perm($cur+1,$max);
}
}
Can anyone tell me how the code above works?
Yes, not very well:
Undefined subroutine &main::perm called at foo.pl line 14.
ie you defined 'sub permutate' but you called 'perm()'. Please,
please, post *working* code if you want us to help you. I know what's
wrong and I can fix it myself, but you do yourself no favours by
making those who would help you work harder.
My original program must deal with arbitrary length and generate all the possible combinations (even repeating) of a particular set. What could take me gazillions of for loops for that, somebody just came up with less than ten lines.
I can just copy and paste the code I found above but I want to learn how it works. How could someone write this code so easily but when I tried even writing just a pseudocode for it, my head almost exploded.
I want to be a good programmer, but at this rate, I don't think I can be one if I will just keep copying and pasting someone else's code or downloading modules from CPAN.
Can anyone teach me how my own code (the one with gazillion for loops), can be converted into a pseudocode then eventually into a working sub procedure?
The code you found will teach you bad habits. Among other things:
* it uses the package variable $result to communicate between
recursive calls, rather than (in my version posted to the list a few
days ago) in an extra argument. This is a Bad Thing.
* it uses prototypes. Don't use prototypes; long story
http://xrl.us/bffo9k ; short story: prototypes are not like C
prototypes. They don't give you type safety. They don't force you to
use a scalar where it expects a scalar argument - you can still
provide an array or hash. And when you do, it will give you confusing,
hard-to-track-down bugs.
I would recommend you look instead at the code I posted to the list,
and I'll be happy to explain it to you.
As for how to "think in recursion", I find the best way to think is this way:
* How would I do it for a trivial case?
* Given I can do it for a case of size N-1, how would I do it for a
case of size N?
* How do I express this in perl?
As an example, the factorial function, the three steps become:
* How do I calculate 0!?
Answer: it is 1 by definition.
* Given that I can calculate (N-1)!, how do I calculate N! ?
Answer: I can calculate it thus: N * (N-1)!
* How do I express this in perl?
sub fact {
my ($n) = @_;
die "Can't calculate factorial of a negative or fractional number" if $n<0;
if ($n == 0) { # Base case
return 1;
}
# Solve fact($n) given we can already do fact($n-1)
return $n * fact ($n-1);
}
If you know your maths, this process is very similar to "proof by induction".
Philip
.
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