in reply to matching values in arrays

Easiest looking up if you keep track of which array each number goes to: 
my @which;
$which[$_] = "array1" for @array1;
$which[$_] = "array2" for @array2;
$which[$_] = "array3" for @array3;

my @pairs = @array4;
while (my ($one,$two) = splice(@pairs,0,2)) {
   print "$one and $two are in $which[$one] and $which[$two]\n";
}

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If the numbers are negative or non-integral or too big, use a hash %which instead of the array @which.

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Re^2: matching values in arrays
by Excalibor (Pilgrim) on Dec 14, 2004 at 11:25 UTC

    This solution is nice, and the hash is probably the way to go with it, but if the arrays contain repeated numbers, (i.e. their intersection is not void) it will probably fail (as it will always mark the last array, instead of the first one, or all of them)...

    Expanding your solution a bit will yield this: 

    
my %which;
do { push @{ $which{$_} } => "array1" } for @array1;
do { push @{ $which{$_} } => "array2" } for @array2;
do { push @{ $which{$_} } => "array3" } for @array3;
 
my @pairs = @array4;
while (my ($one,$two) = splice(@pairs,0,2)) {
     print "$one and $two are in (@{$which{$one}}) and (@{$which{$two}
+}) respectively\n";
}

    [download]

    which is more generic and works even if the numbers are shared by any of the arrays...

    Great idea, anyway!

    --
    our $Perl6 is Fantastic;

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      Yeah, but it uses a lot more memory...

      Another approach would be to use grep, loop the fourth array and in each iteration use grep on the other arrays...

[reply]

        It uses pretty little space (not just in this particular example, but in general...) considering what's doing... Think of it as an index on the joins of the three arrays...

        For really big arrays using greps on every iteration would make the operation really cumbersome and memory would be used anyway for the temporaries for grep on each of the arrays... This approach allows logarithmic access to the key once the index is done, yours would be at least of order O(N^2)...

        Example: 

        # some are shared by others...
@array1 = (1 .. 100001);
@array2 = (100001 .. 200001);
@array3 = (200001 .. 300000);

for $i ( 1 .. 100000, 1 .. 10000 ) 
{
    $a = int( rand( 3 ) ) + 1;
    # dark magic, beware...
    push @array4, ${*{"array$a"}}[$i];
}

my %which; 
do { push @{ $which{$_} } => "array1" } for @array1;
do { push @{ $which{$_} } => "array2" } for @array2;
do { push @{ $which{$_} } => "array3" } for @array3;

my @pairs = @array4;
while (my ($one,$two) = splice(@pairs,0,2)) {
   print "." if "$one and $two are in (@{$which{$one}}) and (@{$which{
+$two}}) respectively\n";
}

__END__

This renders (on my box):
4.05user 0.15system 0:04.31elapsed 97%CPU (0avgtext+0avgdata 0maxresid
+ent)k
0inputs+0outputs (0major+22995minor)pagefaults 0swaps

        [download]

        with bigger arrays it will grow bigger, of course, but if you have that many data you will have a lot of RAM... memory is cheap nowadays, cheaper than CPU cycles, I think...

        best regards

        --
        our $Perl6 is Fantastic;

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