Getting an Array Reference Into an Array

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

I have to produce code that is readable and easy to follow for our support group. Sometimes I have a slighly complex datastructure like this:

$myHash{"uno"} = "one";
$myHash{"dos"} = "two";
$myHash{"tres"}->[0] = "three-point-oh";
$myHash{"tres"}->[1] = "three-point-one";
$myHash{"tres"}->[2] = "three-point-two";
$myHash{"quatro"} = "four";
[download]

I want to manipulate the array in $myHash{"tres"}, and in order to prevent deferencing it all the time, I'll copy it into an actual array:

@myArray = @{$myHash{"tres"}};   #DeRef & Copy to @myArray

$myArray[3] = "three-point-three";
$myArray[4] = "three-point-four";
[download]

Once I'm done manipulating the data, I want to put it back into the original data structure. I can do either one of these:

@{$myHash{"tres"}} =  @myArray;    #Copies Array to Hash
$myHash{"tres"} =  \@myArray;      #Copies Ref to Hash
[download]

For very large arrays, the second statement is faster than the first one.

Of course, the problem is my initial copy to @myArray is not very efficient either. What I'd really love to do is something like this:

\@myArray = $myHash{"tres"};
[download]

Which would allow me to refer to $myHash{"tres"} as @myArray in my code without copying the whole array in the first place. (Then, I wouldn't even worry about putting the array back into my original data structure).

I guess what I'm really interested in is creating an alias of $myHash{"tres"} as @myArray. Is there anyway to do that in Perl?

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Re: Getting an Array Reference Into an Array by Joost (Canon) on Jun 07, 2006 at 19:51 UTC
Fletch has already answered your question, but... I want to manipulate the array in $myHash{"tres"}, and in order to prevent deferencing it all the time, I'll copy it into an actual array Personally, i prefer to just copy the reference. `my $aref = $myHash{"tres"}; $aref->[0] = 'bla'; # etc.` [download] This also avoids copying the arrays. Dereferencing is a tiny bit slower than using an alias, but my guess is that it just won't be noticable, and using local() unless you really have to is just ugly IMO. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l]
Re: Getting an Array Reference Into an Array by Fletch (Bishop) on Jun 07, 2006 at 19:37 UTC
You want to make a local alias. `{ local *myArray = $myHash{ "tres" }; $myArray[ 99 ] = "luftballons"; }` [download]	[reply] [d/l]
Re^2: Getting an Array Reference Into an Array by ikegami (Patriarch) on Jun 07, 2006 at 19:48 UTC
Adding `our @myArray` allows the snippet to be used with `use strict 'vars';`. `{ our @myArray; local *myArray = $myHash{ "tres" }; $myArray[ 99 ] = "luftballons"; }` [download]	[reply] [d/l] [select]
Re^3: Getting an Array Reference Into an Array by qazwart (Scribe) on Jun 07, 2006 at 20:45 UTC
Adding `our @myArray` allows the snippet to be used with `use strict 'vars';`. Yeah, I figured that I needed the `our @myArray` when I added the `use strict;` to the program. However, I tried the program with and without the `local` and it seems to work either way. Does the `our @myArray` remove the need for `local`? I've got the entire program below. use strict; use warnings; my %myHash; $myHash{"uno"} = "one"; $myHash{"dos"} = "two"; $myHash{"tres"}->[0] = "three-point-oh"; $myHash{"tres"}->[1] = "three-point-one"; $myHash{"tres"}->[2] = "three-point-two"; $myHash{"quatro"} = "four"; our @myArray; *myArray = $myHash{"tres"}; $myArray[3] = "three-point-three"; $myArray[4] = "three-point-four"; $myArray[5] = "three-point-five"; foreach my $key (keys(%myHash)) { if (ref($myHash{$key}) eq "ARRAY") { for (my $item = 0; $item <= $#{$myHash{$key}}; $item++) { print qq($key->).qq([$item] = "$myHash{$key}->[$item]"\n); } } else { print qq($key = "$myHash{$key}"\n); } } [download]	[reply] [d/l] [select]
Re^4: Getting an Array Reference Into an Array by Fletch (Bishop) on Jun 07, 2006 at 20:50 UTC
Re: Getting an Array Reference Into an Array by NetWallah (Canon) on Jun 07, 2006 at 19:57 UTC
There are several other ways to achieve your objective - you could pass the array ref into a sub, and use a local alias to an arayref, as in `sub HandleTres{ my $tresRef = shift; $tresref->[0]= "Something else"; }` [download] or, use a reference assignment in your mainline code: `my $TresRef = $myHash{tres}; $TresRef->[1] = "Tres punto uno";` [download] "For every complex problem, there is a simple answer ... and it is wrong." --H.L. Mencken	[reply] [d/l] [select]
Re: Getting an Array Reference Into an Array by debiandude (Scribe) on Jun 07, 2006 at 19:51 UTC
Why don't you do this: `my @myArray = qw/three-point-oh three-point-one three-point-two/; $myHash{"tres"} = \@myArray; # then do everything onto @myArray; push @myArray, "three-point-three", "three-point-four"; # And $myHash is automatically updated print join(' ', @{$myHash{"tres"}}), "\n";` [download]	[reply] [d/l]
Re^2: Getting an Array Reference Into an Array by qazwart (Scribe) on Jun 07, 2006 at 20:34 UTC
Why don't you do this: `my @myArray = qw/three-point-oh three-point-one three-point-two/; $myHash{"tres"} = \@myArray; # then do everything onto @myArray;` [download] Because in my actual program, I'm not creating the array from scratch. The whole datastructure is given to me, and I have to manipulate it and return it back to the calling program. In the actual program, it is an array of a hash of a hash, and the array itself could contain thousands of members. That's why I don't want to use dereferencing and that's why I didn't want to copy it. I posted a simple program to demonstrate that concept.	[reply] [d/l]
Re: Getting an Array Reference Into an Array by qazwart (Scribe) on Jun 08, 2006 at 14:52 UTC
Okay: Using the typeglob for myArray allows me to point `@myArray` to the same memory location as `$myHash{"tres"}`. This is probably the best solution because it allows me to treat the data in `$myHash{"tres"}` in such functions as `join`, `split`, and other array specific functions. Using `our` is needed for getting around the `use strict` Perl pragma. Wrapping up everything in a block and using `local` keeps the scope of my redefinition of `@myArray` to that area of the program. Otherwise, if I use `@myArray` again somewhere else in the program, I could be changing $myHash{"tres"} without meaning to. Thanks everybody for your help.	[reply] [d/l] [select]