Flatten sparse AoA

FloydATC has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Flatten sparse AoA by moritz (Cardinal) on Jan 22, 2012 at 18:29 UTC
You can save a bit of memory by first grepping for defined elements inside each array, and then combining them: `my @flat = map { grep defined, @$_ } @{$aoa};` [download] That way the "big" list with all the elements, including the undefs, isn't fully created in the first place. Perl 6 - second systems done right	[reply] [d/l]
Re: Flatten sparse AoA by Marshall (Canon) on Jan 22, 2012 at 18:47 UTC
I'm not sure if this is more efficient or not with recent Perl versions, but a thing that is not so obvious is that to return "nothing" from a map, return (); `my @flat = map { (defined @$_) ? @$_ : () )@{$aoa};` [download]	[reply] [d/l]
Re^2: Flatten sparse AoA by JavaFan (Canon) on Jan 22, 2012 at 19:24 UTC
That does something else then what the OP does. You're filtering out the undefined elements of `@$aoa` (that is, top level elements), while the OP wants to remove (according to his code), the undefined elements of the elements of @$aoa. That is, if `$aoa` is `[ [2, undef, 4], [undef, undef, 5], ];` [download] he wants to end with `(2, 4, 5)` [download] not with `(2, undef, 4, undef, undef, 5)` [download] which is what your code ends up with. Note that you probably intended to write `defined $_` instead of `defined @$_`.	[reply] [d/l] [select]
Re^3: Flatten sparse AoA by Marshall (Canon) on Jan 22, 2012 at 19:35 UTC
Yes, thank you. Update: #!/usr/bin/perl -w use strict; use Data::Dumper; $\|++; my @AoA = ( undef, [1, 2], undef, [3, 4]); my @flat = map{defined (@$_) ? @$_ : ()}@AoA; print join (",",@flat),"\n"; # prints: 1,2,3,4 my $AoAref = \@AoA; @flat = map{ defined (@$_) ? @$_ : () }@$AoAref; print join (",",@flat),"\n"; # prints: 1,2,3,4 =so far prints:... 1,2,3,4 1,2,3,4 =cut @AoA = ( undef, [1, undef], undef, [3, 4]); $AoAref = \@AoA; # "undef" is a legal Perl value. # to handle this idea of an undef value within # an defined array (undef is a legal Perl value)! Yes, use grep. # # my map filters out "undefined references to arrays" # within an AoA, I would use grep to filter # that undesired "undef" value out if what is meant # a value within in an array. # as shown before: grep{} map{}. [download] My code is just a special case. The more general case is to flatten the whole thing out and grep out the undefined values provided that if what is meant by "sparse" is that not only entire rows are missing but also columns within defined rows. "undef" is a legal Perl value and requires some kind of "grep" to "get rid of it"	[reply] [d/l]
Re^4: Flatten sparse AoA by JavaFan (Canon) on Jan 22, 2012 at 23:49 UTC
Re: Flatten sparse AoA by jwkrahn (Abbot) on Jan 22, 2012 at 22:15 UTC
is there a more efficient way to eliminate all the undefs right there inside the map() `my @flat = map defined ? @$_ : (), @$aoa;` [download] Or perhaps: `my @flat = map ref eq 'ARRAY' ? @$_ : (), @$aoa;` [download]	[reply] [d/l] [select]
Re: Flatten sparse AoA by educated_foo (Vicar) on Jan 22, 2012 at 21:56 UTC
You (hopefully) meant `@flat = map { @$_ } grep { defined } @{$aoa};` [download] and the answer is "no." If your array is truly sparse, you should implement it with a hash.	[reply] [d/l]
Re: Flatten sparse AoA by JavaFan (Canon) on Jan 23, 2012 at 00:19 UTC
I would write: `@flat = grep {defined} map {@$_} grep {$_} @AoA;` [download] This assumes each element of @AoA is either an arrayref, or an undefined value. I'm not the biggest fan of using `EXPR ? LIST : ()` inside a map just to avoid writing `grep`.	[reply] [d/l] [select]