Finding the Remaining of Array by Slice

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

Dear Masters,
I have to arrays. One is larger than the other, and the smaller one is the subset.
Now what I want to do is to remove the elements in the larger array based on that smaller one.
Something like:

perl -MData::Dumper -e '
@ar1 = qw( a b c d e ); 
@sl =  qw( b d ); 
@ar2 = @ar1[@sl]; 
print Dumper \@ar2;'
[download]

Now from that code I expect it to return

$VAR1 = [
          'a',
          'c',
          'e'
        ];
[download]

But it doesn't. What's wrong with it? And is there a quicker/better way to do it?

---
neversaint and everlastingly indebted.......

Comment on Finding the Remaining of Array by Slice - A quick way? Select or Download Code

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Re: Finding the Remaining of Array by Slice - A quick way? by Zaxo (Archbishop) on Aug 29, 2005 at 09:21 UTC
`my %sl; @sl{@sl} = (); my @ar2 = grep {!exists $sl{$_}} @ar1; print Dumper \@ar2;` [download] After Compline, Zaxo	[reply] [d/l]
Re: Finding the Remaining of Array by Slice - A quick way? by davidrw (Prior) on Aug 29, 2005 at 11:47 UTC
The above hash/grep solutions and the link to the Q&A node How can I find the union/difference/intersection of two arrays? are both good, so i won't repeat the solution, but i will answer the other part about why your solution didn't work .. `@ar2 = @ar1[@sl];` [download] Is an array slice setting elements of `@ar2` to elements of `@ar1` using the values* of `@s1` as indices. To write it out longhand, it is doing: `$ar2[0] = $ar1[ $s1[0] ]; $ar2[1] = $ar1[ $s1[1] ];` [download] Now substitute in the values of @s1 elements: `$ar2[0] = $ar1[ 'b' ]; $ar2[1] = $ar1[ 'd' ];` [download] And we see why it failed -- the characters `'b'` and `'d'` cannot be used as the index of an array. Apparently from the output, perl was cast the chars to 0 and that's why both elements are `'a'`. The last lesson here is always use strict and warnings by default* .. Adding `use warnings` (or `perl -w`) generates these messages: `Argument "b" isn't numeric in array slice at /tmp/r line 10. Argument "d" isn't numeric in array slice at /tmp/r line 10.` [download]	[reply] [d/l] [select]
Re: Finding the Remaining of Array by Slice - A quick way? by Fang (Pilgrim) on Aug 29, 2005 at 09:25 UTC
Unless you absolutely need to use a slice, this node offers other viable solutions.	[reply]
Re: Finding the Remaining of Array by Slice - A quick way? by kprasanna_79 (Hermit) on Aug 29, 2005 at 10:56 UTC
`my @list = qw/1 2 3/; %temp=(); my @stoplist = qw/1 2/; @temp{@list} = (); delete @temp{@stoplist}; print keys %temp;` [download] -prasanna.k	[reply] [d/l]