Applying foreach to multidimensional arrays. Simple program included.

[SparceMatrix]

The following program is taken from this post:

http://www.tek-tips.com/viewthread.cfm?qid=833207

@foo = (
         ['e11', 'e12' ],
         ['e21', 'e22' ],
       );

print "\$#foo = $#foo\n";

for($i=0; $i<=$#foo; $i++)
{
  for($j=0; $j<2; $j++)
  {
    print "\$i = $i, \$j = $j, $foo[$i][$j]\n";
  }
}

The output is
=============
$#foo = 1
$i = 0, $j = 0, e11
$i = 0, $j = 1, e12
$i = 1, $j = 0, e21
$i = 1, $j = 1, e22

Using "for" to loop through a multidimensional array is pretty transparent, but what should be even more transparent is to apply the "foreach" loop:

@foo = (
         ['e11', 'e12' ],
         ['e21', 'e22' ],
       );

print "\$#foo = $#foo\n";

foreach $fooArray (@foo)
{	$i=0; $i++;
  foreach($fooCell (@fooArray)
  {		$j=0; $j++;
    print "\$i = $i, \$j = $j, $foo[$i][$j]\n";
  }
}

This does not work, but I have experimented several ways to suggest to me that $fooArray does exist in some way as an array.

How do you generalize the common PERL language of arrays to manipulate multidimensional arrays? What about in this particular case using "foreach"?

 

[TonyGroves]

You have $i=0; and $j=0; inside their respective loops, so $i and $j never get past 1. They should be outside the loops.

 

[SparceMatrix]

Yes, of course. But since the basics don't work, there is no way to debug that out of the program. Those are errors that are not relly relevent to the problem.

I stumbled on the solution which is buried in the badly documented subject of referencing and dereferencing.

This works:

#!/usr/bin/perl

@foo = (
         ['e11', 'e12' ],
         ['e21', 'e22' ],
       );

print "\$#foo = $#foo\n";

$i=0;
foreach $fooArray (@foo)
{	$j=0;
  foreach $fooCell (@$fooArray)  
  {		
    print "\$i = $i, \$j = $j, $foo[$i][$j]\n";
	$j++;
  }  $i++;
}

Pretty simple and I'll bet that you can nest foreach's for as many dimensions as you have in your array.

 

[KevinADC]

a bit more perlish:

@foo = (
         ['e11', 'e12' ],
         ['e21', 'e22' ],
       );

print "\$#foo = $#foo\n";

for my $i (0..$#foo) {
   for my $j (0..$#{$foo[$i]}) {        
      print "\$i = $i, \$j = $j, $foo[$i][$j]\n";
   }
}

 

[KevinADC]

>> Pretty simple and I'll bet that you can nest foreach's for as many dimensions as you have in your array.

I believe there is a limit. I think you will get warning about "deep recursions" after so many nested loops.

 

[SparceMatrix]

I'll keep the presence of a limit in mind and thank you for your perlish addition.

 

[SparceMatrix]

I said, ...

I stumbled on the solution which is buried in the badly documented subject of referencing and dereferencing.

Actually this solution and also the perlish solution and all the various brackets and their relationship with arrays is simply and elegantly explain with excellent examples in:

Learning Perl Objects, References & Modules, Chapter 3, Introduction to References

This is included in the Perl Bookshelf and except for the "Mastering Regular Expressions" in a PDF file is another good justification for picking it up.

 

[KevinADC]

Yea, once you get the hang of reading the brackets in context it is simple and fairly elegant.