Why Perl's foreach loop can't use an array element as the control variable?

password636 · 07-22-2011, 02:04 AM

Hi,

I'm reading "OReilly Learning Perl 5th Edition", and there are such words:

Code:

You can use an array element like $fred[2] in every place† where you could use any other scalar variable like $fred.

At the bottom of the page, it explains the † like this:

Code:

The most notable exception is that the control variable of a foreach loop, which you’ll see later in this chapter, must be a simple scalar.

Since Perl has the save-and-restore mechanism for the control variable, why an array element can't be used as the control variable?

THANKS!

smallpond · 07-22-2011, 09:58 AM

The control variable is not used as a simple scalar, it is an alias of the current loop variable. For example:

Code:

use Data::Dumper;
@foo=(1, 6, 5); 
foreach $i (@foo) {
  $i++;
}
print Dumper \@foo;

$VAR1 = [
          2,
          7,
          6
        ];

An array element doesn't have the capability to be used as an alias.