Re^5: Unobvious Pathological Code Snippets

See Juerd's original thread: Non-destructive array processing for some explanation/discussion.

How it works, as I understand, and anyone knows better, please correct me.

When you pass an array to a sub, each of the elements of @_ is alias to the elements of the original array. As the destructive manipulations are applied to @_ rather than the original array, the original array and it's contents remain untouched by the process.

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Comment on Re^5: Unobvious Pathological Code Snippets

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Re^6: Unobvious Pathological Code Snippets by xdg (Monsignor) on Mar 28, 2006 at 23:16 UTC
It's really no different than a `shift`. If I understand perlsub, the subtlety here is that `@_` itself is not an alias to the original array/list of arguments -- rather, `@_` is a local array with each element an alias to the corresponding element in the argument list -- which is why you can `shift`, `pop` or (as in this case) `splice` `@_` at will. However, assigning to `@_` or even using `splice` to overwrite part of an array, destroys that aliasing for whatever is overwritten. Thus: `use strict; use warnings; sub splice_insert { splice( @_, 2, 2, ('a' .. 'c') ); print "@_ # after splice\n"; $_ = '.' for @_; print "@_ # after for loop\n"; } my @a = ( 0 .. 6 ); print "@a # before subroutine\n"; splice_insert(@a); print "@a # after subroutine\n";` [download] Prints: `0 1 2 3 4 5 6 # before subroutine 0 1 a b c 4 5 6 # after splice . . . . . . . . # after for loop . . 2 3 . . . # after subroutine` [download] -xdg Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.	[reply] [d/l] [select]

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Re^6: Unobvious Pathological Code Snippets
by xdg (Monsignor) on Mar 28, 2006 at 23:16 UTC

It's really no different than a shift.

If I understand perlsub, the subtlety here is that @_ itself is not an alias to the original array/list of arguments -- rather, @_ is a local array with each element an alias to the corresponding element in the argument list -- which is why you can shift, pop or (as in this case) splice @_ at will.

However, assigning to @_ or even using splice to overwrite part of an array, destroys that aliasing for whatever is overwritten. Thus:

use strict;
use warnings;

sub splice_insert {
    splice( @_, 2, 2, ('a' .. 'c') );
    print "@_ # after splice\n";
    $_ = '.' for @_;
    print "@_ # after for loop\n";
}

my @a = ( 0 .. 6 );

print "@a   # before subroutine\n";

splice_insert(@a);

print "@a   # after subroutine\n";
[download]

Prints:

0 1 2 3 4 5 6   # before subroutine
0 1 a b c 4 5 6 # after splice
. . . . . . . . # after for loop
. . 2 3 . . .   # after subroutine
[download]

-xdg

Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

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