Hi,
Chris Brown 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 $_ contains...
> can anyone clear this up for me? Thanks
Interesting question, which can be answered in a multitude of ways. The
long and the short of it is that the perl variable "$_" is THE implicit
variable. It's the root of all evil, and the saviour of the language
depending on how you look at it.
The documentation says, and I quote (perldoc perlvar):
---Documentation--
$_
$ARG
The default input and pattern-searching space. The following pairs are
equivalent:
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
[snip]
--Documentation--
What the documentation tries to explain is that $_ is the default
variable in much of perl. The implicit use of $_ allows a person to
write very very compact code which seems to "magically" do alot of
things. The use of $_ is rather often leads to code which doesn't (or
does) reveal it's true purpose upon first read. The use of avoidance of
$_ is fairly core to perl's TAMWTDI.
Take the following example:
my @array = (0,1,2,3,4,5,6,7,8,9,10);
foreach(@array) { # iterates through the array, placing the next value
# into $_
print; # prints the implicit variable
# equivaltent to print $_;
}
In essense, $_ is the hidden variable in the above loop. It can be used to
"un-hide" the default.
Here's another example:
# this example avoids using the "implicit" $_ variable.
open(FILE,"afile");
while(chomp(my $line = <FILE>)){
$line =~ s/replacethis/withthis/g;
if ($line =~ m/matchthis/) {
do 'this';
}
print $line;
}
close(FILE);
# this example uses the "implicit" $_ variable, accomplishing the same
# thing as above
open(FILE,"afile");
while(<FILE>){
chomp;
s/replacethis/withthis/g;
do 'this' if m/matchthis/;
print;
}
close(<FILE>);
In the second example, $_ stands in for "the default input", and what
the loop is iterating over.
Hope that helps,
--Andy
