I am new to perl and was confused with perl scoping rules after I wrote below code snippet:
#!/usr/bin/perl
my $i = 0;
foreach $i(5..10){
print $i."\n";
}
print "Outside loop i = $i\n";
I expected output to be like :
5
6
7
8
9
10
Outside loop i = 10
But its giving :
5
6
7
8
9
10
Outside loop i = 0
So the variable $i value is not changing after the loop exits. Whats going on in here?
According to the perldoc information regarding foreach loops: here
The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. If the variable is preceded with the keyword my, then it is lexically scoped, and is therefore visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared with my, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localization occurs only in a foreach loop.
If you want to retain the value of $i outside the loop then you can omit $i in the foreach loop call and use perl's special variable $_ an example below:
#!/usr/bin/perl
use strict;
use warnings;
my $i = 0;
foreach (5..10){
print $_."\n";
$i = $_;
}
print "Outside loop i = $i\n";
5 6 7 8 9 10 Outside loop i = 10
foreach as for, the localization occurs. It looks like you'll need a spare variable to get the value of $i in the loop on each iteration.
$_ is generally considered a step backwards for maintainability. Usage of the "C style" for loop or a while loop is probably a better solution to this problem.foreach localize variable to the loop.
use strict;
use warnings;
my $adr;
my $i = 0;
foreach $i(5..10){
$adr = \$i;
print "$i ($adr)\n";
}
$adr = \$i;
print "Outside loop i = $i ($adr)\n";
output
5 (SCALAR(0x9d1e1d8))
6 (SCALAR(0x9d1e1d8))
7 (SCALAR(0x9d1e1d8))
8 (SCALAR(0x9d1e1d8))
9 (SCALAR(0x9d1e1d8))
10 (SCALAR(0x9d1e1d8))
Outside loop i = 0 (SCALAR(0x9d343a0))
From perldoc,
The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. If the variable is preceded with the keyword my, then it is lexically scoped, and is therefore visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared with my, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localization occurs only in a foreach loop.
To preserve value of $i you can use C like for loop,
my $i = 0;
for ($i = 5; $i <= 10; $i++) { .. }
although it's less readable than perl foreach
..), not setting the existing variable to each value in turn.The variable $i is redefined in foreach scope at
foreach $i(5..10){
So the variable outside foreach will not change.
use strict; use warnings;at the top of every Perl script you write. Those two pragmas can save you a lot of painful debugging.