Re: Updating 1 array affects the other array

the trap of cross-reference happens like this:
firstly you have an aoa:

@array1=(
    [1,2,3],
    [4,5,6],
    [7,8,9]
);
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then if you make this:

$array2[1]=$array1[1]
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and change value in $array1

$array1[1][0]='something';
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you will get

print $array2[1][0]; # shows 'something'
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the $array21 is virtually have the same memory space with corresponding value in @array1, because the thing stored in $array1 is actually a reference to the anonymous list 4,5,6. So, what you give to array2 is the same reference, but not the values.

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Re^2: Updating 1 array affects the other array by iphony (Acolyte) on Sep 02, 2008 at 12:24 UTC
Hi, thanks for the explanation. I understand your explanation, but I've checked thru my codes but ca'nt see such mistake being committed. The only (write) operations I've done to the arrays (prior to discovering the problem) is shown below. What I am trying to do is to remove duplicate entries (based on certain column combinations) and store them in a new array (those with the "unique" prefix). The "cross-referencing" seems to have been committed here. `my %seen_values; my @unique_terminal_id = grep { !($seen_values{$_->[4]}++) } @temp_ope +n_single; my %seen_values; my @unique_manufacturer_model = grep { !($seen_values{$_->[9]}{$_->[10 +]}++) } @unique_terminal_id; my %seen_values; my @unique_manufacturer_model_version = grep { !($seen_values{$_->[9]} +{$_->[10]}{$_->[11]}++) } @unique_terminal_id;` [download]	[reply] [d/l]
Re^3: Updating 1 array affects the other array by JadeNB (Chaplain) on Sep 02, 2008 at 14:29 UTC
As other people have mentioned, what you're copying are references (as indicated by the de-referencing arrows that you're using in the `grep`s). Namely, your array `@temp_open_single` is, presumably, an array of arrayrefs. Let's say that the first arrayref is pointing to the array at `0x18045c0`, and that that array passes the test in the `grep` for `@unique_terminal_id`. Then `@unique_terminal_id` also contains a reference to the array at `0x18045c0`. Let's suppose further that that array also passes the test in the `grep` for `@unique_manufacturer_model`. Then `@unique_manufacturer_model` also contains a reference to the—exact same!—array at `0x18045c0`. It is copying the reference, not the referent. That means that, when you refer to `$unique_terminal_id[0][0]` or to `$unique_manufacturer_model[0][0]`, in either case you're just looking at the entry at index `0` of the single array at `0x18045c0`. After you change that single array in one place, the changes will still be visible even if you look at it from another location. As others have mentioned, you need something like Storable's `dclone` function (which makes a 'deep clone') ~~will allow you~~ to avoid this difficulty.	[reply] [d/l] [select]