in reply to Variable number of foreach loops
Nonrecursive solution
For completeness, here's a solution that does not use a recursive subroutine:
sub nested_foreach(&@) { my $code = shift; my @indices = map { 0 } @_; # First set of indices is all zeroes my @sizes = map { scalar @$_ } @_; # Cache array sizes (optional) my $k; do { # Determine the array elements corresponding to the current set # of indices, and pass them to the closure: $code>( map { $_[$_][$indices[$_]] } 0..$#_ ); # Determine the next set of indices: for ($k = $#_; $k >= 0; $k) { $indices[$k]++; if ($indices[$k] < $sizes[$k]) { last; } else { $indices[$k] = 0; } } # If $k went outofbounds, there are no more valid iterations: } while ($k >= 0); } my @a = ...; my @b = ...; my @c = ...; nested_foreach { say join ' ', @_ } \@a, \@b, \@c;
The "Determine the next set of indices" step may seem a little complicated at first sight, but it becomes more intuitive if you think of the @indices array as an integer number (with each element representing a digit), and imagine that we want to "increment" that "number" by 1. It's not a decimal (i.e. base10) number, but rather one where each digit can have a different base (i.e. the sizes of the input arrays)  but that doesn't really change anything.
Incrementing the "number" by 1 works just like the integer addition (here with an addend of 1) that you were taught back in primary school: Start with the rightmost digit; increment it; if it's still within the valid range of digits then you're done; if instead it went above the limit then wrap it around to zero, "carry the one", and repeat the same steps with the next digit to the left.
Update:
Performance comparison
Interestingly, my iterative solution seems to be significantly slower than BrowserUK's recursive solution, at least when running on my PC and with various different numbers/sizes of input arrays I tried:
sub nested_foreach(&@) { ... # see above } sub nForX(&@) { ... # see BrowserUK's post } # my @size = (500, 900); # my @size = (5, 5, 5, 5, 5, 5, 5, 5); my @size = (100, 4, 75, 23); my @AoA = map { [map { chr($_+64) x int(rand(10)) } 1 .. $_] } @size; cmpthese 10, { iterative => sub { nested_foreach { join("", @_) } @AoA }, recursive => sub { nForX { join("", @_) } scalar @AoA, @AoA }, };
s/iter iterative recursive iterative 1.86  71% recursive 0.532 249% 


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Re^2: Variable number of foreach loops (nonrecursive solution)
by abhay180 (Sexton) on Nov 29, 2013 at 09:48 UTC  
by BrowserUk (Patriarch) on Nov 29, 2013 at 11:23 UTC 