How can I delete elements from a Perl array?

Question

I have a pretty big array and I want to delete the 2nd, 8th, 14th etc. element from an array. My array currently is like this:

Element1 x A B C
Element 2 y A B C
Element 3 z A B C

Broadly, I want to delete the x, y and z (just as an example, my array is slighly more complex). And pull up the rest. As in, I don't want to have a blank space in their positions. I want to get:

Element 1 A B C
Element 2 A B C
Element 3 A B C

I tried to give this a try with my array "todelete":

print "#Before Deleting"; print
$todelete[0]; print "\n"; print
$todelete[2]; print "\n"; print
$todelete[3];

for ($count=2; $count<@todelete;
$count=$count+6) {  delete
$todelete[$count]; }

print "#After Deleting"; print
$todelete[0]; print "\n"; print
$todelete[2]; print "\n"; print
$todelete[3];$todelete[3];

But, currently, I think it just unitializes my value, because when I print the result, it tells me:

Use of uninitialized value in print

Suggestions?

Porculus · Accepted Answer · 2010-11-04 21:35:37Z

5

The function you want is splice.

answered Nov 4, 2010 at 21:35

Porculus

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5 Comments

Ether Over a year ago

@poutine: someone is amazing because they have read the basic documentation that comes with every installation of perl? Isn't that setting the bar a little low?

poutine Over a year ago

Ah ... but now I face another problem, since the index has been shifted. The element, I need to delete is no more at position 8. Its now at position 7. Ah .. and the same for the rest. I think a better alternative would be to just delete the element, then use a filter and then splice those out? Ether ...your right. For a brief moment, when I thought it worked, I was amazed .. since I spent a lot of time trying to think about it in terms of a for loop.

cjm Over a year ago

@poutine, the traditional solution to that is to delete backwards. Instead of 2, 8, 14, ... go 14, 8, 2. That way, the indexes don't change before you reach the element.

poutine Over a year ago

Thanks cjm, but I used a work-around decrementing the index. A little more work, but I was able to get it going ..

ishnid Over a year ago

Decrementing an index is tremendously ugly. Instead, I'd use a 'while' loop in which I either remove an element OR increment the counter... ... or remove them backwards.

mob · Accepted Answer · 2010-11-04 22:26:04Z

2

delete $array[$index] is the same as calling $array[$index] = undef; it leaves a blank space in your array. For your specific problem, how about something like

@array = @array[ grep { $_ % 6 != 2 } 0 .. $#array ];

answered Nov 4, 2010 at 22:26

mob

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1 Comment

daotoad Over a year ago

delete on arrays is deprecated--another good reason not to use it.

hexcoder · Accepted Answer · 2010-11-04 22:57:59Z

2

You can also use grep as a filter:

my $cnt = 0; @todelete = grep { ++$cnt % 6 != 2 } @todelete;

answered Nov 4, 2010 at 22:57

hexcoder

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