Initializing multidimensional arrays

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

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Re: Initializing multidimensional arrays by Zaxo (Archbishop) on Jun 01, 2005 at 08:16 UTC
`my @two_d = ( [(undef) x 5], [(undef) x 5], [(undef) x 5], [(undef) x 5], [(undef) x 5], );` [download] or `my @two_d; push @two_d, [(undef) x 5] for 1 .. 5;` [download] It's tempting to try the `x 5` trick a second time to fill the top level, but that gets you five references to the same row. Replace `(undef)` with `(0)` if that's what you prefer. After Compline, Zaxo	[reply] [d/l] [select]
Re: Initializing multidimensional arrays by holli (Abbot) on Jun 01, 2005 at 08:18 UTC
`my @matrix; push @matrix, [(0)x5] for (0..4);` [download] holli, /regexed monk/	[reply] [d/l]
Re^2: Initializing multidimensional arrays by blazar (Canon) on Jun 01, 2005 at 08:23 UTC
`my @matrix=map [(0)x5], 0..4;` [download]	[reply] [d/l]
Re: Initializing multidimensional arrays by Cody Pendant (Prior) on Jun 01, 2005 at 10:22 UTC
Do we have to initialise undef'd items one to four, when creating item five does that anyway? `$matrix[$_][4]=undef for(0..4);` [download] Which leads to the next logical question, why do we need to do this at all? The creation of the highest-index item in the array creates all lower-index undefs anyway, and if you try to get item and it's not there, the array's going to return an undef anyway. So the quickest way to do it is not do it at all. ($_='kkvvttuu bbooppuuiiffss qqffssmm iibbddllffss') =~y~b-v~a-z~s; print	[reply] [d/l]
Re^2: Initializing multidimensional arrays by cyocum (Curate) on Jun 01, 2005 at 12:37 UTC
Cody is right on this. Just declare the array and the treat it as if it were a 5x5 matrix because as long as you treat it like one, it will be one. I guess you could do this: `use strict; use warnings; my @matrix; $matrix[4][4];` [download] Perl will fill in the array with undef's for you. update: monarch is correct. It should be 4 because Perl defaults to an index of 0 for arrays unless you set the special variable `$[ = 1`.	[reply] [d/l] [select]
Re^3: Initializing multidimensional arrays by monarch (Priest) on Jun 01, 2005 at 13:25 UTC
Just a question.. given that Perl arrays are 0 indexed would it be more correct to state: `$matrix[4][4];` [download] . Also can't you do something like: `$#matrix = 4;` [download] (not sure how you'd do the second dimension).	[reply] [d/l] [select]
Re^3: Initializing multidimensional arrays by ihb (Deacon) on Jun 03, 2005 at 19:57 UTC
Note that `; $matrix[4][4];` only fills out the first level with five elements and populates the last with an array reference. This is because the last subscript doesn't do anything. Just like `if ($foo[100000] == 1) { ... }` [download] doesn't extend `@foo` to be 100001 elements. This is also why it gives a "useless use of array element in void context" warning. The first level is extended though, but only filled with `undef`s except for the fifth element which becomes an array reference, which will be empty because of the reason above, looking like `[ undef, undef, undef, undef, [ undef, undef, undef, undef , undef ], ]` [download] Using the element in some way, not just using the value, like `$matrix[4][4] = undef` [download] would fill out the array reference at index four in the first level, but the other levels would still be untouched, so you'd have a "half-expanded" 2D array, looking like `[ undef, undef, undef, undef, [ undef, undef, undef, undef , undef ], ]` [download] This is why Cody Pendant uses the `for` loop. `ihb` See perltoc if you don't know which perldoc to read!	[reply] [d/l] [select]
Re^2: Initializing multidimensional arrays by Nkuvu (Priest) on Jun 01, 2005 at 16:06 UTC
So the quickest way to do it is not do it at all. Or, if you are trying to reinitialize a 5x5 matrix that may have stuff in it: `@matrix = ()`	[reply] [d/l]