Re: Arbitrarily Many Nested Loops

David Formosa (aka ? the Platypus) wrote:
On 29 Mar 2006 16:51:14 -0800, Jacob JKW <jacobcdf@xxxxxxxxx> wrote:
This is what I have:

-------------
#!perl

for (my $i = 0; $i<=$n_ra->[0]; $i++) {
for (my $j = 0; $j<=$n_ra->[1]; $j++) {
for (my $k = 0; $k<=$n_ra->[2]; $k++) {
$prob_ra->[$i+$j+$k] += (
$f_raa->[0]->[$i] *
$f_raa->[1]->[$j] *
$f_raa->[2]->[$k] *
);
}
}
-------------
But that's obviously messy and more imprtantly I'd like to be able to
decide at run time to have how nested levels to go (probably be on the
order of 50 or 60). I assume that there's a canonical manner in which
this should be handled (using closures I'd guess) but I can't
sufficiently summarize my issue to make it Google-able.

I quickly worked out one way to do this, no guarties as to effeceny.

sub closefor (&$@) {
my $sub = shift;
my $range = shift;

return sub {
for my $i (0..$range){
$sub->($i,@_)
}
}
}

sub mulitloop (&@) {
my $sub = shift;
for (@_) {
my $oldsub = $sub;
$sub = closefor {$oldsub->(@_)} $_;
}
$sub->();
}

Your code becomes

mulitloop { my $i = shift;
my $j = shift;
my $k = shift;
$prob_ra->[$i+$j+$k] += $f_raa->[0]->[$i] *
$f_raa->[1]->[$j] *
$f_raa->[2]->[$k]
} @$n_ra;

To convert the algorithm into something that does arbitrarily depth
involves a small reworking the insides so that it works with an
arbitrary number of arguments.

mulitloop {
my $sum = 0;
my $product = 1;
for (my $i; $i<=@#_; $i++) {
$sum += $_[$i];
$product *= $f_raa->[$i]->[$_[$i]];
}
$prob_ra->[$sum] += $product;
} @$n_ra;

I made a few synactical changes but it worked out beautifully. Thank
you very, very much for that. :-)
(Thanks to Jim Gibson and John Krahn who both answered my post as well,
for no particularly good reason I didn't get a chance to try out either
of their code samples.)


--
Please excuse my spelling as I suffer from agraphia. See
http://dformosa.zeta.org.au/~dformosa/Spelling.html to find out more.
Do you think if I became agraphic I could become as good a programmer
as you?

.

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