Iteratively delete elements from array

Question

I have an array, @cuts, of indices of the elements that I would like to remove from @Data. Is this an appropriate way to do so?

foreach (@cuts){

    $Data[$_] = "NULL";

}


for my $i (0 .. $#Data){
    if ($Data[$i] eq "NULL"){
        splice(@Data,$i,1);
        } 
}

ikegami · Accepted Answer · 2014-07-17 17:31:32Z

3

You don't need to use a sentinel value ("NULL").

my %cuts = map { $_ => 1 } @cuts;
my @keeps = grep !$cuts{$_}, 0..$#Data;
@Data = @Data[@keeps];

This can surely be simplified by merging it with the preceding code.

answered Jul 17, 2014 at 17:31

ikegami

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1 Comment

Samuel Weisenthal Over a year ago

I am very confused as to why this works, but this is the best solution. My subroutine was working on a global array (stackoverflow.com/questions/24812456/…), and this was the only one that removed elements not only in the scope of the sub but also in the main scope my @newData = grep { !/^NULL\z/ } @Data; removes them in the sub, but then they get replaced by blanks in the main program...

RobEarl · Accepted Answer · 2014-07-17 16:03:12Z

2

Combining @toolic and @user2752322:

delete @Data[@cuts];
my @newData = grep { defined } @Data;

answered Jul 17, 2014 at 16:03

RobEarl

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3 Comments

Slade Over a year ago

undef @Data[@cuts] or @Data[@cuts] = () would be preferable to delete.

Slade Over a year ago

@ikegami The documentation for delete warns that its use on array values is deprecated (although I'm admittedly unaware of the current attitude toward it on p5p--the warning may be outdated, it's been around since at least 5.12). Plus it conveys intent better to undef the values you don't want, then grep the remaining defined values.

ikegami Over a year ago

@Slade, Oops, yeah, was confused because there's no actual harm in using delete on array elements. Using undef on variables, on the other hand, is usually a bad idea unless you specifically intend to defy the optimization it defies. Doesn't matter for a slice, though. Assigning undef (as you unclearly did in your second snippet) is always fine, except it prevents undef from naturally existing in @Data.

ikegami · Accepted Answer · 2014-07-17 17:28:17Z

2

my @newData = grep { !/^NULL\z/ } @Data;

edited Jul 17, 2014 at 17:28

ikegami

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answered Jul 17, 2014 at 15:26

user2752322

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