Re: what does shift do?

shift applies to an array, as in, shift @a or, shift(@a). It removes the first (i.e., 0th) element from the array, shifting the remaining elements back by one (hence the name, I suppose), and returns the first element as its value.

So, if you have an array, initialized like this:

my @a = ( 'a', 'b', 'c');

my $x = shift @a;
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then after this code executes, $x will contain the scalar value, 'a', and @a will contain, ( 'b', 'c' ).

In lexical scope (i.e., within a subroutine), shift by itself, without any arguments, implicitly acts upon the variable @_, which contains the subroutine's arguments.

At file scope (i.e., outside of any subroutine), it implicitly operates instead upon @ARGV (also known as ::@ARGV, or main::@ARGV), which contains the command line parameters passed to the script.

See the docs, "shift".

dmm

There are no stupid questions -- just stupid-making responses

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Re: Re: what does shift do? by dev2000 (Sexton) on May 19, 2002 at 20:36 UTC
So, why exactly was it needed at all in my example? Wouldn't: `send_http_header('text/plain'); print("mod_perl rules!\n");` [download] be identical? (guess I should try this, huh?) It didn't work... Undefined subroutine ! P.S. the original example was the ENTIRE program, not within a subroutine, that's why I was so puzzled as to what was shifting (and why).	[reply] [d/l]
Re: Re: Re: what does shift do? by Juerd (Abbot) on May 19, 2002 at 20:48 UTC
`sub greet { my $name = shift; print "Hello, $name\n"; } greet('dev2000');` [download] In mod_perl, with Apache::Registry, your script is cached in a sub, and that sub gets the Apache object as its argument. You could of course write this: `$_[0]->send_http_header('text/plain'); $_[0]->print("mod_perl rules!\n");` [download] But it's easier to have a named variable for that, especially when you use other subs and your `@_` gets overwritten. Back to the point, your script: `my $r = shift; ...` [download] is wrapped in a sub so it can be cached: `package Some::URL::Based::Package::Name;use Apache qw(exit);sub handle +r { #line 1 script.pl my $r = shift; ... }` [download] And suddenly, it all makes sense. - Yes, I reinvent wheels. - Spam: Visit eurotraQ.	[reply] [d/l] [select]
Re(2): what does shift do? by dmmiller2k (Chaplain) on May 19, 2002 at 20:41 UTC
Because `$r` is being used as an object, by calling its `send_http_header()` and `print()` methods. Without the shift (I presume this is inside a subroutine), the object represented by the reference `$r` would never be referenced and the code would not work. P.S., without seeing the context in which your example lives, it I do not exactly know what kind of object `$r` is , but from its method names, I suspect it's one of the Http::XXX'en. dmm If you GIVE a man a fish you feed him for a day But, TEACH him to fish and you feed him for a lifetime	[reply] [d/l] [select]
Re: Re(2): what does shift do? by Juerd (Abbot) on May 19, 2002 at 20:52 UTC
P.S., without seeing the context in which your example lives, it I do not exactly know what kind of object $r is , but from its method names, I suspect it's one of the Http::XXX'en. The context is mod_perl, and probably Apache::Registry, which wraps scripts in subs for caching. - Yes, I reinvent wheels. - Spam: Visit eurotraQ.	[reply]


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