boom.roasted has asked for the wisdom of the Perl Monks concerning the following question:

I'm learning perl, and I'm trying to write a subroutine that will shuffle a deck of card, but not getting the whole deck back, why?

#!/usr/bin/perl

require 'obj13-lib.pl';

@deck = ("A H","2 H","3 H","4 H","5 H","6 H","7 H","8 H",
     "9 H","10 H","J H","Q H","K H",
     "A D","2 D","3 D","4 D","5 D","6 D","7 D","8 D",
     "9 D","10 D","J D","Q D","K D",
     "A C","2 C","3 C","4 C","5 C","6 C","7 C","8 C",
     "9 C","10 C","J C","Q C","K C",
     "A S","2 S","3 S","4 S","5 S","6 S","7 S","8 S",
     "9 S","10 S","J S","Q S","K S");

{
for(@deck){
s/H/Hearts/;
s/D/Diamonds/;
s/C/Clubs/;
s/S/Spades/;
}
print"original deck:\n";
print"@deck\n";
@hand = shuf(@deck);
print"shuffled deck:\n";
print"@hand\n";
}

sub shuf
{                                                                     
+                                                                     
+                                  
foreach $_ (@deck){
  $lastcard = pop(@deck);
  $firstcard = shift(@deck);
  push(@newdeck,$firstcard,$lastcard);}

@answer = @newdeck;
return @answer;                                                       
+                                                                     
+                                  
}

[download]

Replies are listed 'Best First'.
Re: array shuffle
by ikegami (Patriarch) on Aug 18, 2011 at 03:25 UTC

    Don't modify an array over which you are iterating using foreach. You actually want while (@deck) here anyway. It will produce a really bad shuffle, though. To shuffle a list or array, one normally uses List::Util's shuffle.

    Use use strict; use warnings;!!! You forgot to limit the scope many variables.

[reply]
[d/l]
[select]

      I'm trying to test my skills without using the shuffle function. I was def using the wrong loop.

      Here's my problem now though, I want to shuffle the "deck" twice.

      using the  while (@deck) I can get the deck shuffled once, but when I use:

      @hand = shuf(@deck);

print "deck shuffled once:\n";
print"@hand\n";

@deck = @hand;
@newhand = shuf(@deck);

print"deck shuffled twice:\n";
print"@newhand\n";

      [download]

      I don't get the result I'm hoping for, what am I doing wrong now?

[reply]
[d/l]
[select]

        Assuming that shuffling is simple is a mistake, and yet once you see an unbiased algorithm on paper (or screen) you will probably say, "Oooh, I get it." But done wrong, shuffling can introduce bias, and it's easy to do wrong.

        There is a nice article on Wikipedia: Fisher-Yates Shuffle. Implemented as a computer algorithm, it is sometimes known as the Knuth Shuffle. Also looking at the Pure Perl version of List::Util (here: Source for List::Util::PP) you will see a Perlish implementation of Fisher-Yates.

        Another post here pointed you to perlfaq4, which shows another Fisher-Yates implementation that is easier to read than the List::Util::PP version.

        For the masochistic or exceptionally curious: The Art of Computer Programming vol. 2 (3rd ed.), pg 145-146.


        Dave

[reply]
[reply]
        You're using a global variable which you empty the first time around instead of passing the deck to your function. You need to properly scope your variables.
[reply]
Re: array shuffle
by johngg (Canon) on Aug 18, 2011 at 08:54 UTC

    As an aside, you can save yourself a lot of fiddly typing by using maps.

    $ perl -E '
> my @suits  = qw{ Hearts Diamonds Clubs Spades };
> my @values = qw{ A 2 3 4 5 6 7 8 9 10 J Q K };
> my @deck   =
>    map { my $suit = $_; map { qq{$_ $suit} } @values } @suits;
> say for @deck;'
A Hearts
2 Hearts
3 Hearts
4 Hearts
5 Hearts
6 Hearts
7 Hearts
8 Hearts
9 Hearts
10 Hearts
J Hearts
Q Hearts
K Hearts
A Diamonds
2 Diamonds
3 Diamonds
4 Diamonds
5 Diamonds
6 Diamonds
7 Diamonds
8 Diamonds
9 Diamonds
10 Diamonds
J Diamonds
Q Diamonds
K Diamonds
A Clubs
2 Clubs
3 Clubs
4 Clubs
5 Clubs
6 Clubs
7 Clubs
8 Clubs
9 Clubs
10 Clubs
J Clubs
Q Clubs
K Clubs
A Spades
2 Spades
3 Spades
4 Spades
5 Spades
6 Spades
7 Spades
8 Spades
9 Spades
10 Spades
J Spades
Q Spades
K Spades

    [download]

    I hope this is helpful.

    Cheers,

    JohnGG

[reply]
[d/l]
Re: array shuffle
by Anonymous Monk on Aug 18, 2011 at 11:29 UTC

    The only thing random about manually shuffling a deck of cards is that we humans do it imperfectly. The ideal shuffle splits a deck in half and then interleaves the two halves. This is the basis of many "magic" card tricks: good shuffling.

    I realize this is just a programming exercise but you still need to decide if you're trying to randomize the deck or modeling an ideal (non-random) shuffle.

    There's a lot of theory surrounding shuffling and random but I fail to see the need to do an in place shuffle for a deck of cards unless you do it a LOT. Same goes for random, perl's would seem to be good enough for your needs but I wouldn't start a real money blackjack web site without something better.

[reply]

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