shift operator

molly has asked for the wisdom of the Perl Monks concerning the following question:

Monks:

I have yet another general question. I'm working on debugging a perl program, and have been having to do a lot of reverse-engineering. With my primitive understanding of Perl this has been difficult, but I think I'm almost there. I just need (maybe) one piece of crucial information. Here is the subroutine I'm looking at:

sub SearchPubMed {
    my $InputRow = shift; # temporary! InputRow = SearchTermID
  my $QueryID = shift;
  my $CannedQueriesPurpose = shift;
  my $CannedQueriesQuery = shift;
  my $query;
[download]

What exactly does it mean to define variables as shift? I understand that the shift operator grabs the first element of an array, and that when "shift" appears next to white space, it is assumed that it is grabbing from the @_ special array variable. I don't totally understand where @_ has come from here and what is stored in it. Can someone shed some light onto the @_ special variable for me? Thanks oodles.

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Re: shift operator by GrandFather (Saint) on Jul 23, 2007 at 22:49 UTC
The Perl special variable @_ is the list of parameters passed to the sub. So: `hello ('Hello', 'world'); sub hello { my $hi = shift; my $place = shift; print "$hi $place\n"; }` [download] Prints: `Hello world` [download] Although many people would write the sub as: `sub hello { my ($hi, $place) = @_; print "$hi $place\n"; }` [download] to use list assignment rather then shifting out multiple arguments. An even more sophisticated technique is to use "named" parameters by assigning to a hash instead of the positional technique just shown: `hello (hi => 'Hello', place => 'world'); sub hello { my %params = @_; print "$params{hi} $params{place}\n"; }` [download] Further reading: perlsub, perlop and perlfunc. DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re: shift operator by saintly (Scribe) on Jul 23, 2007 at 22:40 UTC
the '@_' list is your list of input parameters. If someone had said: `SearchPubMed("A","B","C","D");` [download] Then in SearchPubMed, `$InputRow = "A"; $QueryID = "B"; # etc...` [download] It seems a bit squirrely to have to access your input parameters this way, perhaps. Using 'shift' not only accesses the parameter, but it also removes it (meaning that you cannot later say '$firstParam = $_[0]' to get at the original first parameter, a debatable benefit of not using shift). It may be simpler to rewrite several 'shift' lines as a single line: `my($inputRow,$QueryId,...) = @_;` [download] That's typically the way most code is written these days, but there's still some rare cases where repeated use of 'shift' makes for more efficient code.	[reply] [d/l] [select]
Re: shift operator by FunkyMonk (Bishop) on Jul 23, 2007 at 22:41 UTC
In a subroutine `@_` is a list of the arguments to the call. For example... `mysub(1, 2, 3); sub mysub { print "@_\n"; my $x = shift; print "@_\n"; print "$x\n"; } #Outputs: #1 2 3 #2 3 #1` [download] shift within a subroutine grabs the first argument to the call. So in the example above, 1 will be removed from `@_` and assigned to `$x`.	[reply] [d/l] [select]
Re: shift operator by Zaxo (Archbishop) on Jul 24, 2007 at 06:30 UTC
shift is a perl operator which may be applied to any variable with the @ sigil. It strips the array's first position and returns that value. In a sub definition, it is used on @_ which is formed from the argument list. After Compline, Zaxo	[reply]