Now I usually only use
$_ in very small
blocks of code, such as in a map or grep block. If the section of code large or is likely to grow then it is much clearer to assign to a named variable and this helps to document the code.
Today I was extending a bit of code that uses $_ absolutely everywhere and there was a point where the behaviour was a bit difficult to understand. I have always thought that you could assume that the magic of $_ made it work so that you could use it as if it were lexically scoped. So when you run this:
use strict;
for (1..3){
print "BEFORE: " . $_;
foo();
print " AFTER: " . $_ . "\n";
}
sub foo{
for(<DATA>){
last;
}
}
__DATA__
a
b
c
the fact that there is a call to
foo() should make no difference to the value printed. And as expected, it doesn't:
BEFORE: 1 AFTER: 1
BEFORE: 2 AFTER: 2
BEFORE: 3 AFTER: 3
========== [C:\users\jake\code\komodo\test2.pl] run finished. ========
+==
So what's the problem? Well if you change foo() to read:
sub foo{
while(<DATA>){
last;
}
}
ie use while instead of for, then the output is:
BEFORE: 1 AFTER: a
BEFORE: 2 AFTER: b
BEFORE: 3 AFTER: c
========== [C:\users\jake\code\komodo\test2.pl] run finished. ========
+==
Now I found this quite surprising. So my questions are:
- Is it supposed to do this?
- Is this behaviour documented?
- Why is there a big difference between for and while?
--
iakobski
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