Question Regarding remove elements from a list

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

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Re: Question Regarding remove elements from a list by si_lence (Deacon) on Aug 26, 2004 at 12:44 UTC
You could use grep to remove the elements you do not want from the list: `my @results = grep $expression, @input_list;` [download] The expression will be evaluated for each element in the @imput_list (put into $_) and only the element where the experssion evaluates to true end up in @result. hth si_lence	[reply] [d/l]
Re: Question Regarding remove elements from a list by hardburn (Abbot) on Aug 26, 2004 at 12:46 UTC
Hard to say without seeing your actual data, but it sounds like you want to use `grep` to filter out the data before the foreach loop: `my @new_data = grep { $_ eq 'good data' } @old_data;` [download] Just change the block to run the tests on each element of the data. See the grep documentation for details. "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.	[reply] [d/l] [select]
Re: Question Regarding remove elements from a list by keszler (Priest) on Aug 26, 2004 at 15:20 UTC
If each existing foreach removes more elements than it keeps, the order is not sorted, and memory is not an issue, then `my @tmp; while (my $elem = pop @array) { # or s/pop/shift/ # do stuff if ($keep) { push @tmp, $elem; } } @array = @tmp;` [download] Another method is to use an array index instead of foreach, and splice: `my $i=0; while ($i < @array) { if (keep($array[$i]) { print "keeping $array[$i]\n"; $i++; } else { my $x = splice(@array,$i,1); print "discarding $x\n"; } }` [download]	[reply] [d/l] [select]
Re: Question Regarding remove elements from a list by johnnywang (Priest) on Aug 26, 2004 at 17:44 UTC
If your logic for examning each element is a little complicated, I'll turn the array into a hash, and delete the key in each pass, something like: `my %hash = map{$_=>undef} @inputs; foreach my $k (keys %hash){ # do something delete $hash{$k} if $do_not_keep; } # then do this loop again...` [download]	[reply] [d/l]
Re: Question Regarding remove elements from a list by Forsaken (Friar) on Aug 27, 2004 at 07:57 UTC
perhaps a less simple solution than using ie grep, but how about embedding the foreach in a for loop? during the foreach keep track of all items that are to be removed at the end of that foreach loop, ie in a separate array, and before the next instance of the for loop simply cut out the slices using (number of slice - number of items already removed in that instance of the for loop).	[reply]