seeking different ways to slice a 2-d array

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

I've got an object method which digs into a hash of arrays of arrays and pulls out the last element of each stored array. Here's my code:

sub whatever
{
  my $self = shift;

  # ... do stuff to set ar_data...

  # Slice out the last element of each stored array into a new array
  my @last_elements;
  push @last_elements, $_->[$#_] for @{$ar_data};
  return \@last_elements;
}
[download]

After grabbing some water, I walked back and realized that @last_elements is (uh-oh) a transitory variable. (and wouldn'tcha know it, I try to look up on the web why transitory variables are bad, and I can't find anything. Dang; I'm sure dominus wrote about this somewhere. Or maybe this is hard water I'm drinking.) So, with a vague idea that some variables are "good" and others are "bad", I wanted to re-write my method to just return the list of @last_elements without actually storing into a list. (Sidenote: or is this foolish? Am I just inviting bogarts into my code?)

After a bit of toying, I realized that there are other ways to iterate over a list than by using for. Enter sort, map, and grep.

#!/usr/bin/perl

use strict;
use warnings;

# fill the 2-d array
my @data;
for ( 0..2 ) {
  push @data, [qw 'a bc def' ];
}

# try extracting last element from each stored array
my @for_data = slice_with_for ( @data );
my @map_data = slice_with_map ( @data );
my @sort_data = slice_with_sort ( @data );
my @grep_data = slice_with_grep ( @data );

# show the y-axis slices
print "for :  @for_data\n";
print "map :  @map_data\n";
print "sort:  @sort_data\n";
print "grep:  @grep_data\n";

exit;


sub slice_with_for
{
  my @slice_me = @_;
  my @last_fields;
  push @last_fields, $_->[2] for @slice_me;
  @last_fields;
}


sub slice_with_map
{
  my @slice_me = @_;
  return map { $_->[2] ? $_->[2] : () } @slice_me;
}


sub slice_with_sort
{
  my @slice_me = @_;
  sort { $a->[2] cmp $b->[2] } @slice_me;
}

sub slice_with_grep
{
  my @slice_me = @data;
  grep { $_->[2] } @slice_me;
}

exit;
[download]

Prints:
for : def def def
map : def def def
sort: ARRAY(0x80fad5c) ARRAY(0x810c844) ARRAY(0x810c7c0)
grep: ARRAY(0x80fad5c) ARRAY(0x810c844) ARRAY(0x810c7c0)

My questions:
1. What other ways are there to iterate over the list besides for, sort, map, and grep?
2. Why don't my slice_with_grep and slice_with_sort functions return what I'm expecting?
3. Am I drunk, or is the water? That is, have I invented this whole notion of a "transitory variable", or is it really something that exists (albeit momentarily) and should be avoided?

blyman
setenv EXINIT 'set noai ts=2'

Comment on seeking different ways to slice a 2-d array Select or Download Code

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Re: seeking different ways to slice a 2-d array by stephen (Priest) on Apr 25, 2002 at 03:34 UTC
A transitory variable is a variable that isn't used for anything other than to schlep data from one line to another. (I'm not sure the term is right, but I think it's what you mean. I call 'em "useless variables", myself.) In other words, if I'm going to say: `my $foo = get_foo(); do_something($foo);` [download] Then `$foo` is a transitory variable. It would be just as simple, and less error-prone, to say: `do_something( get_foo() );` [download] Why? Because variables can and should vary. It doesn't seem like such a big deal when the assignment and the use of the variable are right next to each other, but what if they get separated? `my $foo = get_foo(); if ($snargle = 3 && $foo = 2) { whatever($snargle, $foo); # lots of other stuff here... } do_something($foo);` [download] Oops, on line 2 up there we changed the value of `$foo`. Also, it makes refactoring harder-- what if you wanted to pull out the stuff inside the if statement into its own subroutine? It'd be simpler if you didn't have `$foo` there at all, and just called `get_foo()` when you needed a foo-value. In your code, `@for_data`, `@map_data`, `@sort_data`, and `@grep_data` are all transitory variables. Ironically, `@last_elements` is not... you're using it to collect elements! Nothing wrong with that. As for why `slce_with_sort()` and `slice_with_grep()` aren't doing what you expect, it's because sort and grep don't do what you apparently think they do. :) The block argument to grep is a test block, not a filter block. So the statement `grep { $_->[2] } @slice_me;` [download] means, "If the item at index 2 of `@$_` evaluates to true, return `$_`." The last bit is important-- it doesn't return the item at index 2, it returns the whole thing. A similar thing is happening with sort. You're sorting each row based on the value of the last column, and returning a list of the rows. Since the values are identical, it doesn't even change their order. :) So where did you get that water? Sounds like powerful stuff. :) stephen	[reply] [d/l] [select]
Re: seeking different ways to slice a 2-d array by Fletch (Bishop) on Apr 25, 2002 at 02:18 UTC
ITYM `lexically scoped variable', not transitory. And you're probably thinking of mjd's Coping with Scoping.	[reply]
Re: Re: seeking different ways to slice a 2-d array by belden (Friar) on Apr 25, 2002 at 16:58 UTC
As I learn more about Perl and programming in general, I find myself re-reading articles and saying, "Ohhh.... so that's what it means...". Case in point: Coping with Scoping. Thanks for the pointer. Dropping off to sleep last night I realized that the article I was thinking of was a Red Flags article on perl.com. The actual term mjd used was synthetic variables although in a different Red Flags article he talks about synthetic code blyman setenv EXINIT 'set noai ts=2'	[reply]
Re: Re: seeking different ways to slice a 2-d array by samtregar (Abbot) on Apr 25, 2002 at 08:06 UTC
I'd like to read that again when it's updated to describe 'our'. I use 'our' all the time but the cross-over between the previously totally unrelated realms of lexical and package variables is still a bit vague in my mind. -sam	[reply]
(jeffa) 3Re: seeking different ways to slice a 2-d array by jeffa (Bishop) on Apr 25, 2002 at 14:42 UTC
I am very curious to know why you use our so much. I have never had a reason to use it myself because i tend to use OO a lot. I recently had a big revelation at (jeffa) 3Re: Whether to use local(), hopefully this post will remove the vagueness from your mind as well. UPDATE: Oh $#!^ - you are Sam Tregar. Scratch that last sentance! :O jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply]
Re: (jeffa) 3Re: seeking different ways to slice a 2-d array by samtregar (Abbot) on Apr 25, 2002 at 17:35 UTC
Re: Re: (jeffa) 3Re: seeking different ways to slice a 2-d array by Anonymous Monk on May 14, 2002 at 20:46 UTC
(jeffa) Re: seeking different ways to slice a 2-d array by jeffa (Bishop) on Apr 25, 2002 at 14:19 UTC
I always found splice to be a good way to slice an array. Here is an example that you can play with. I am assuming that you always want the last element from each inner array. `use strict; use Data::Dumper; my @arry = ( [qw(one two three)], [qw(foo bar baz)], ); my @new; push @new, splice(@$_,$#$_,1) for @arry; print Dumper \@new;` [download] The third argument of '1' for splice is not necessary in this case, but i recommend keeping it in case you decide to remove only one element from any other position. ;) Hope this works for you. jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re: (jeffa) Re: seeking different ways to slice a 2-d array by belden (Friar) on Apr 25, 2002 at 17:01 UTC
I am assuming that you always want the last element from each inner array. Correct! Here is an example that you can play with....Hope this works for you. Yes, it does - thanks for the new technique blyman setenv EXINIT 'set noai ts=2'	[reply]


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