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% --
-
Are you posting in the right place? Check out Where do I post X? to know for sure.
-
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
<u> <ul>
-
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).
-
Want more info? How to link
or How to display code and escape characters
are good places to start.