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Perl - $_ and @_

In our Wilts training roomPerl'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.


Illustration - a delegate at Well House Manor on a Perl Course
(written 2006-12-07, updated 2010-06-22)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P209 - Subroutines in Perl
  [3833] Learning to use existing classes in Perl - (2012-08-10)
  [3574] Perl functions such as chop change their input parameters - (2012-01-10)
  [3066] Separating groups of variables into namespaces - (2010-11-24)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2069] Efficient calls to subs in Perl - avoid duplication, gain speed - (2009-03-07)
  [1921] Romeo and Julie - (2008-12-04)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1850] Daisy the Cow and a Pint of Ginger Beer - (2008-10-21)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1784] Global - Tcl, PHP, Python - (2008-09-03)
  [1782] Calling procs in Tcl and how it compares to Perl - (2008-09-02)
  [1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [775] Do not duplicate your code - (2006-06-23)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [531] Packages in packages in Perl - (2005-12-16)
  [357] Where do Perl modules load from - (2005-06-24)
  [308] Call by name v call by value - (2005-05-11)
  [96] Variable Scope - (2004-10-22)

P210 - Perl - Topicalization and Special Variables
  [4301] Perl - still a very effective language indeed for extracting and reporting - (2014-09-20)
  [3449] Apache Internal Dummy Connection - what is it and what should I do with it? - (2011-09-19)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [1922] Flurinci knows Raby Lae PHP and Jeve - (2008-12-04)
  [1829] Dont bother to write a Perl program - (2008-10-10)
  [1728] A short Perl example - (2008-07-30)
  [1705] Environment variables in Perl / use Env - (2008-07-11)
  [1704] Finding operating system settings in Perl - (2008-07-10)
  [1508] How not to write Perl? - (2008-01-15)
  [1444] Using English can slow you right down! - (2007-11-25)
  [1289] Pure Perl - (2007-08-03)
  [1232] Bathtub example - (2007-06-14)
  [1221] Bathtubs and pecking birds - (2007-06-07)
  [1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP - (2007-04-06)
  [639] Progress bars and other dynamic reports - (2006-03-09)
  [493] Running a Perl script within a PHP page - (2005-11-12)


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Perl - $_ and @_
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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Comment: "Emil, thanks for your comment - but I do not agree. In ..."
Visitor Ranking 4.5 (5=excellent, 1=poor)

Comment by Graham (published 2010-05-24)
Emil, thanks for your comment - but I do not agree. In most code, $_[0] and @_[0] will work equally well - but @_[0] is a list slice of one element and $_[0] is the specific scalar member so using @ is a little less efficient and also - on occasions - can lead to a different context being used and the results changing to the NUMBER of elements in a slice ... "1" !! [#3604]

Comment by Emil Stanchev (published 2010-05-24)
In the second example, $_ should be @_. Like

$_[0] -> @_[0]
$_[1] -> $_[1]. [#3594]

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