Perl - $_ and @_
Perl's a great language for special variables - variables that are set up without the programmer having to intervene and providing information ranging from the number of lines read from the current input file ($.) through the current process ID ($$) and the operating system ($^O). Other special variables effect how certain operations are performed ($| controlling output buffering / flushing, for example), or are fundamental in the operation of certain facilities - no more so than $_ and @_.
Lets clear a misconception. $_ and @_ are different variables. In Perl, you can have a list and a scalar of the same name, and they refer to unrelated pieces of memory.
$_ is known as the "default input and pattern matching space". In other words, if you read in from a file handle at the top of a while loop, or run a foreach loop and don't name a loop variable, $_ is set up for you. Then any regular expression matches, chops (and lcs and many more) without a parameter, and even prints assume you want to work on $_. Thus:
while ($line = <FH>) {
if ($line =~ /Perl/) {
print FHO $line;
}
print uc $line;
}
Shortens to:
while (<FH>) {
/Perl/ and
print FHO ;
print uc;
}
@_ is the list of incoming parameters to a sub. So if you write a sub, you refer to the first parameter in it as $_[0], the second parameter as $_[1] and so on. And you can refer to $_# as the index number of the last parameter:
sub demo {
print "Called with ",$#_+1," params/n";
print "First param was $_[0]/n";
Note that the English module adds in the ability to refer to the special variables by other longer, but easier to remember, names such as @ARG for @_ and $PID for $$. But
use English; can have a detrimental performance effect if you're matching regular expressions against long incoming strings.