Item 7: Use for elegance.


Item 7: Use $_ for elegance .

"Dollar underscore ," or $_ : you may love it, or you may hate it. But either way, if you're going to be a proficient Perl programmer, you've got to understand it.

$_ is a default argument for many operators, and also for some control structures. Here are some examples:

$_ as a default argument

 print $_;  print 

print;

Same thing.

 print "found it" if $_ =~ /Rosebud/;  print "found it" if /Rosebud/; 

Matches and substitutions.

Same thing.

 $mod_time = -M $_;  $mod_time = -M; 

Most filehandle tests.

Same thing.

 foreach $_ (@list) { &do_something($_) }  foreach (@list) { &do_something($_) } 

foreach

Same thing.

 while (defined($_ = <STDIN>)) { print $_ }  while (<STDIN>) { print } 

while ; a special case.

Same thing.

The last example illustrates the special case in which using the line input operator < filehandle > alone as the condition of a while loop is a shortcut for reading a line from the file into $_ until the end of file is reached.

This is by no means an exhaustive list. Consult the on-line documentation or your favorite comprehensive Perl reference for more information.

$_ is a normal scalar variablemostly. You can use it, print it, change its value, and so on, just as if it were an ordinary scalar. There are a couple of things to watch out for, though.

$_ and the main package

$_ is always in the package main . This applies even, or especially , if you are in some other package:

 package foo;  $_ = "OK\n";  package main;  print; 

This still means $main::_ .

Prints OK .

In fact, all special variables ( $ -punctuation) have this property. You can use a variable like $foo::_ if you like, but it has no special properties and isn't "the" $_ .

Localizing $_

$_ can only be localized using local . You cannot use my . The following won't work, no matter where or how you try it:

 my $_; 

ILLEGAL

Programming style and $_

Oddly enough, when you use $_ , you may not see much of it:

 while (<>) {    for (split) {      $w5++ if /^\w{5}$/    }  } 

Count all the 5-letter words.

 @small_txt = grep    { /\.txt$/ and (-s) < 5000 }      @files; 

Find files ending in .txt and less than 5,000 bytes long.

Some Perl programmers may feel that $_ is more an aid to obfuscation than elegance. I have a book on my shelf that opines: "Many Perl programmers write programs that have references to $_ running like an invisible thread through their programs. Programs that overuse $_ are hard to read and are easier to break than programs that explicitly reference scalar variables you have named youself." I find this hard to accept. Is:

 while (defined ($line = <STDIN>)) {    print $line if $line =~ /Perl/  } 

really superior to:

 while (<STDIN>) { print if /Perl/ } 

You'll have to decide.



Effective Perl Programming. Writing Better Programs with Perl
Effective Perl Programming: Writing Better Programs with Perl
ISBN: 0201419750
EAN: 2147483647
Year: 1996
Pages: 116

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