Passing references to array slices

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

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Re: Passing references to array slices by broquaint (Abbot) on Mar 17, 2004 at 17:11 UTC
Since you can't create a reference to an array slice as such, since it returns a list, it's best just to wrap it in an anonymous array e.g `CalcMinAvgMaxSum( [ @{ $values[1] }[ $start .. $end ] ] );` [download] See. tye's References quick reference and `perlreftut` for good starters on referencing + dereferencing in perl. HTH `_________ broquaint`	[reply] [d/l]
Re: Passing references to array slices by halley (Prior) on Mar 17, 2004 at 17:11 UTC
As far as I know, you can't grab a reference to a slice. References can only refer to first-class datatypes (such as the things which can inhabit fields of a typeglob). A slice is not a first-class datatype, but a syntactic shortcut which implies certain simple logic, like, "by the way, please iterate over these elements." -- `[ e d @ h a l l e y . c c ]`	[reply]
Re: Passing references to array slices by kvale (Monsignor) on Mar 17, 2004 at 17:21 UTC
The second element in @values is already an array reference. In the first line of your sub, you are assigning the number of elements in @_, not the first element itself. Your code can be simplified as follows: `my @values=([some,things] , [1,2,3,4,5,6]); my ($a, $b, $c, $d) = CalcMinAvgMaxSum( $values[1]); sub CalcMinAvgMaxSum() { my $ArrayOfValues_REF = shift; for my $val (@{$ArrayOfValues_REF}) { print "$val\n"; } }` [download] -Mark	[reply] [d/l]
Re: Passing references to array slices by davido (Cardinal) on Mar 17, 2004 at 18:00 UTC
An array slice is not a datastructure, it is simply a list of elements from the array. If you recall from the docs, when you do something like this: `my $ref = \ (1, 2, 3); print ${$ref}, "\n";` [download] The output will be '3'. You've just created a reference to an anonymous scalar value, which happened to be the last value of the list. If you had said, `my @ref = \ ( 1, 2, 3 ); print ${$_}, "\n" for @ref;` [download] Your output would be '123'. That's because the \ operator is distributive to each element in the list; and @ref receives as values three references, each to one of the anonymous scalar values in the list. Dave	[reply] [d/l] [select]
Re: Passing references to array slices by RMGir (Prior) on Mar 17, 2004 at 17:48 UTC
You can't pass a ref to an array slice, but you CAN pass a list of refs to the slice members. This may or may not let you do what you need to do; I hope it helps. `#!/usr/bin/perl -w use strict; my @x=0..10; sub dbl { foreach(@_){ $$_*=2 } } dbl(\(@x[1,2,3,5,6,7])); print join ",",@x; print "\n";` [download] Mike	[reply] [d/l]
Re: Passing references to array slices by bageler (Hermit) on Mar 17, 2004 at 18:14 UTC
your code is passing an arrayref, which is a scalar. `$ArrayOfValues_REF` is picking up @_, an array. `$_[0]` is your arrayref that you are passing, not @_. `$ArrayOfValues_REF = shift` should work.	[reply] [d/l] [select]
Re: Passing references to array slices by calin (Deacon) on Mar 17, 2004 at 19:23 UTC
I don't know what you're really trying to do, but I foresee two possible scenarios: Pass a reference to a new array containing copies of the scalars in the slice. It's simple and clear: `$ perl my @a = qw/a b c d e/; sub printthese { local $, = ', '; print @{$_[0]}; print "\n"; } printthese( [ @a[0, 2, 4] ]); ^D a, c, e` [download] Pass a reference to a new array containing aliases to the scalars in the slice, so you can modify them `$ perl my @a = qw/1 2 3 4 5/; sub zerothese { $_ = 0 for @{$_[0]}; } zerothese( &{sub {\@_}} (@a[0, 2, 4]) ); print join ', ', @a; print "\n"; ^D 0, 2, 0, 4, 0` [download]	[reply] [d/l] [select]
Re: Re: Passing references to array slices by Anonymous Monk on Mar 18, 2004 at 10:52 UTC
Thankyou to you all for the help; I discovered the problem was with this line of code; `$ArrayOfValues_REF=@_;` [download] it should have read `($ArrayOfValues_REF)=@_;` [download] Peace and Love, Matt.	[reply] [d/l] [select]