At 02:47 PM 4/22/2001, you wrote:
>i have read about it in 3 books and even used it in scripts i have made
>but i still dont truly know how to be sure what $_ contains...can anyone
>clear this up for me? Thanks
>Chris Brown
I think the best way to talk about $_ is to speak about it
linguistically. What $_ really is is a pronoun. Think of it as 'it'. For
example:
while (<FILE>) {
print;
}
In this case, you are using $_ without even mentioning it. Perl looks at
this and says "My programmer wants me to read a line out of a file and
print (it) while there are still lines left to be read in the file
FILE." Perl's pretty smart like that.
We do this kind of thing all the time in English, talking about things
without mentioning them. For example the directions on the back of a
bottle of shampoo.
Lather, rinse, repeat.
An assembly programmer wouldn't understand this unless you told them the
following, because assembler languages have few ways to add context to
statements.
Lather the shampoo into your hair, rinse the shampoo out of your hair, and
then later the shampoo into your hair and rinse it out of your hair again,
until your hair is clean, or you don't feel like doing it anymore.
You have to keep in mind that Perl was developed by a linguist, not a
computer scientist. Larry wanted to make sure that Perl had a lot of
features that natural languages do. And one of these features is context.
You'll notice as you look through perlfunc that a lot of functions will
work on $_ if you provide no argument to them. This is how Perl lets you
work in context. If you don't tell it otherwise, Perl will work on 'it'.
(or them, which is @_.)
A note about it's usage, though. "Root of all evil"-type shouting matches
aside, it's a useful construct, but there is nothing that says you MUST use
it. (Aside from the grep and map functions, and perhaps a few other
exceptions.) So, if you really don't like it, don't use it. But I think
you are missing out.
So whenever you find yourself doing something like this:
foreach my $line (@lines) {
next if $line =~ /^#/;
chomp $line;
$line =~ s/stuff/things/;
print $line;
}
consider saying this:
foreach (@lines) {
next if /^#/;
chomp;
s/stuff/things/;
print;
}
Also note, that in both instance next is using an implied default
argument. To fully qualify it, I should have labeled the loop with
something like LINE: and say:
next LINE if $line =~ /^#/;
Context is cool. : )
Thank you for your time,
Sean.
