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Hey, I have written a function that maybe others will find useful to play
with. It takes a list and then calls a specified function on that list. I'm
wondering if anyone knows of anything to optimise the code, as there are a
few list copies going on that allocate/deallocate many times. Is this a
problem, or is Perl pretty smart about it?
(to be clearer, given a list ('a','b','c') it will call the function many
times, each time giving one of: ('a','b','c'), ('a','c','b), ('b','a','c'),
('b','c','a'), ('c','a','b') and ('c','b','a'). It also prints progress on
stderr, but that is more for my own benefit)
It can be called with something like:
permute { push @result, [EMAIL PROTECTED] } @inputList;
if you want to build a list of all the results in @result.
The function is:
# This function takes a list of values, and calls a referenced
# function on each permutation of the list. The first argument is the
# function, the second is the list (there is a third argument used
# internally only.) Note that there are going to be a lot of
# combinations, n!, where n is the number of items in the list.
sub permute(&\@;\@) {
my $func = shift();
my @srcList = @{ shift() };
my @dstList;
if (@_) {
@dstList = @{ shift() };
}
if (@srcList) {
# Copy the srcList, pop an entry off the copy, recurse with that
# appended to the dstList.
my @srcCopy = @srcList;
while (@srcCopy) {
if ([EMAIL PROTECTED]) {
print STDERR @srcCopy."/"[EMAIL PROTECTED]"\n";
}
my $value = pop(@srcCopy);
# Make another copy of srcList, and remove only the one that
# we are moving. This copy will be passed on to the next
# iteration.
my @srcCopy2 = @srcList;
splice(@srcCopy2, scalar @srcCopy, 1);
my @tList = (@dstList, $value);
permute(\&$func, @srcCopy2, @tList);
}
} else {
# base case
&$func(@dstList);
}
}
- --
Robin <[EMAIL PROTECTED]> JabberID: <[EMAIL PROTECTED]>
Hostes alienigeni me abduxerunt. Qui annus est?
PGP Key 0x776DB663 = DD10 5C62 1E29 A385 9866 0853 CD38 E07A 776D B663
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