in reply to Variable number of foreach loops

### Non-recursive 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 $k went out-of-bounds, there are no more valid iterations: } while ($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. base-10) 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 right-most 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% --