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looping over multiple arrays

by danger (Priest)
on Jan 06, 2001 at 02:46 UTC ( #50165=snippet: print w/replies, xml ) Need Help??

Just an example of building an iterator with a closure to walk through multiple arrays in parallel (maybe you have a table (2d array) and want to walk the columns). This is just a simple one that runs through the arrays and then resets so it can be used again:

sub parallel_it {
    my @arr_refs = @_;
    my $iter = 0;
    return sub {
        my @return = map{$_->[$iter]}@arr_refs;
        return @return if grep{defined $_}@return;
        $iter = 0;
my @one   = (0, 1, 2, 3, 4, 5, 6);
my @two   = qw/Sun Mon Tue Wed Thu Fri Sat/;
my @three = qw/Zeroday Oneday Twoday Threeday 
               Fourday Fiveday Sixday/;

my $triples = parallel_it( \(@one,@two,@three) );
while( my($a, $b, $c) = $triples->() ){
    print "$a:$b:$c\n";

# update @one and go again
@one = qw/A B C D E F G/;
while( my($a, $b, $c) = $triples->() ){
    print "$a:$b:$c\n";

my @table = ( [ 1, undef, 3, 4],
              [ 6, 7,],
              [qw/a b c d e f/],
my $columns = parallel_it(@table);
while( my @col = $columns->() ){
    print join(',', map{ $_ ? $_ : ' ' } @col),"\n";

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Re: looping over multiple arrays
by chipmunk (Parson) on Jan 06, 2001 at 03:08 UTC
    Very nice. There is one addition that I would suggest; allowing for parallel undefs in the arrays. Otherwise the iterator will reset if all the arrays happen to contain undef at the same index, even if there are more elements later in the arrays. Here's one possible solution:
    sub parallel_it { my @arr_refs = @_; my $iter = 0; return sub { if (not grep $iter < @$_, @arr_refs) { $iter = 0; return; } my @return = map { $_->[$iter] } @arr_refs; $iter++; return @return; }; }

      Oops, I had that taken care of in an alternate version (that worked on copies of the arrays and thus couldn't be reused if arrays were updated), but a few cells back in the visual cortex must have malfunctioned and I didn't see it here. Thanks for pointing it out.

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[Corion]: Maybe doing a double-fork (daemonizing) can make go that information away, but maybe not
[Corion]: But I think my knowledge of unix/Linux datastructures is several decades out of date, so I don't really know what information it keeps on processes
[oiskuu]: The useful bits that relate to your process can be found under /proc/self. What information are you thinking of? Tty name?
[tye]: I just daemonized and getlogin() still knew who I had been.
[tye]: perhaps loginuid ? Not that I concede that something not being in /proc means it is not useful.
[Corion]: tye: That's really interesting, but maybe it is because getlogin() returns the name, or the uid, so if that user has been replaced by another user with the same uid in the meantime, that's no problem to the system...

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