caciqueman has asked for the wisdom of the Perl Monks concerning the following question:

I am "trying" to get this to work, but I can't seem to find an answer in either the cookbook or programming perl. What I would like to do is randomize an entire array of jpgs in a database of about 50, then take 20 or so and print them out to a template page. What happens is that it puts ONE picture on the page, 20 times, instead of 20 thumbnails once each. Could you please help a newkid out ? Here is what I sometimes refer to as my "code". 
#!/usr/bin/perl -w

print "Content-type: text/html\n\n";
$dir = "/home/usr/public/images/";
$view = "view.html";
$number_to_show = 15;
$title = "Your Title Could Go Here";


srand( time() ^ ($$ + ($$ << 15)) );

opendir DIR, "$dir";
my @arr = grep -f, grep /\.jpg$/i, map "$dir/$_", readdir DIR;
$n = @arr;

foreach (reverse 0..$n) {
my $pic = splice(@arr, rand $n, 1);
push @arr, $pic;
$#arr = $number_to_show;
}

&print_it();

sub print_it {
open (PAGE, "$dir$view") || die ("I am unable to open the template $vi
+ew");
while (<PAGE>) {
s/%%title%%/$title/g;
s/%%images%%/@arr/g;
print $_;
}
close (PAGE);
 }

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Replies are listed 'Best First'.
Re: array problem
by gav^ (Curate) on Mar 15, 2002 at 02:03 UTC
    A quick perldoc -q shuffle array gives:
    How do I shuffle an array randomly? 
    Use this:

    # fisher_yates_shuffle( \@array ) :
    # generate a random permutation of @array in place
    sub fisher_yates_shuffle {
        my $array = shift;
        my $i;
        for ($i = @$array; --$i; ) {
            my $j = int rand ($i+1);
            next if $i == $j;
            @$array[$i,$j] = @$array[$j,$i];
        }
    }

    fisher_yates_shuffle( \@array );    # permutes @array in place

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    Other points to consider:
    • What on earth are you trying to do to $#arr?
    • You probably don't want to use @arr in the subsitution
    • You probably don't want to reinvent the wheel when there are perfectly good templating modules such as HTML::Template and Text::Template
    • splice wants an integer offset, rand returns a fractional number
    • print $_ is the same as print
    I'm sure I've missed a few things. Sorry to sound overly harsh but I'm trying to be constructive :)

    gav^

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Re: array problem
by Trimbach (Curate) on Mar 15, 2002 at 02:18 UTC
    A couple more suggestions. Instead of 
    opendir DIR, "$dir";
my @arr = grep -f, grep /\.jpg$/i, map "$dir/$_", readdir DIR;

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    you can do the same much more simply by just 
    my @arr = glob("*.jpg");

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    See "perldoc -f glob" for more information.

    Also, when you call a subroutine you're gonna wanna do a &print_it or print_it(), but not a &print_it(). They are different, but you only have to tell Perl it's a subroutine once. :-D

    Gary Blackburn
    Trained Killer

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Re: array problem
by dws (Chancellor) on Mar 15, 2002 at 02:44 UTC
    Another approach is to shuffle a set of indices using a Schwartzian transform, and then use a subset of the randomized indices to select a subset of the images. 
    my @indices = map { $_->[0] }
              sort { $a->[1] <=> $b->[1] }
              map { [$_, rand()] }
              0 .. $#arr;

my @subset = @arr[@indices[0..$number_to_show - 1]];

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Re: array problem
by particle (Vicar) on Mar 15, 2002 at 05:17 UTC
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Re: array problem
by YuckFoo (Abbot) on Mar 15, 2002 at 14:18 UTC
    Your immediate problem is here:

    s/%%images%%/@arr/g;

    Try:

    s/%%images%%/shift(@arr)/eg;

    This will remove and use the first element of @arr as a replacement. Note the addition of the 'e' modifier, to evaluate the replacement expression.

    Not sure why using @arr in s// yields a single item. The item it picks seems to be arbitrary as well.

    Also, shoot for 'use strict;' on all your programs and quit using global variables.

    YuckFoo

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Re: array problem
by grummerX (Pilgrim) on Mar 15, 2002 at 18:19 UTC
    You could also check out the shuffle method in List::Util. List::Util, as the name implies, is basically just a handy set of subroutines for dealing with lists. Most of the routines are fairly simple and you could write your own versions, but it's nice to be able to drop a tried-and-true routine into your code and be done with it.

    -- grummerX

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