my and local??

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

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RE: my and local?? by davorg (Chancellor) on Jul 28, 2000 at 13:40 UTC
Mark-Jason Dominus wrote some great articles on this subject for The Perl Journal. You can read them on his web site. Coping with Scoping Seven Useful Uses of `local` -- <http://www.dave.org.uk> European Perl Conference - Sept 22/24 2000, ICA, London <http://www.yapc.org/Europe/>	[reply]
more fun with my and local?? by jlistf (Monk) on Jul 28, 2000 at 17:46 UTC
some repeat of previous posts, but: my - creates a lexical variable. this variable is a temporary variable, only visible within the block in which it was declared. also provides deep-binding which is used to create closures. local - creates a dynamic variable. this variable replaces the value of a package global variable until the end of the block. sub-blocks and subroutines called from this block retain this local value. does not provide deep-binding and so cannot be used to create closures.	[reply]
Re: my and local?? by monk (Scribe) on Jul 28, 2000 at 16:32 UTC
The basic difference is that, when you declare $foo = 3; and then you go into a block or sub and say local $foo = 5; the value in $foo inside the blocks or sub is 5, but outside is 3 here is some code: `#!/usr/bin/perl -w #didn't use strict, to make the example clearer. $foo = 3; #default value of $foo { local $foo = 5; #now the value of $foo is 5 while inside the braces print "$foo\n"; # prints 5. } print "$foo\n"; # prints 3.` [download] With my is a bit different code examples: `#!/usr/bin/perl -w use strict; #<- very important. my $foo = 3; # $foo has a value of 3, and is seen by the compiler. { my $bar = 5; # $ bar life is limited to the block. print "$bar\n"; # you get 5. print "$foo\n"; # you get 3. } print "$bar\n"; # <- compiler gives error.$bar does not exsist (kind o +f). print "$foo\n"; # you get 3.` [download] I hope this clear, things up a bit, and i hope i'm not completely wrong if i am i'm sure the other monks, will point it out. monk Update: `s/compiler/interpreter/ig;` [download] monk	[reply] [d/l] [select]
Re: my and local?? by merlyn (Sage) on Jul 28, 2000 at 16:36 UTC
The simplest explanation is something like: `my` creates a temporary variable that is not in any package `local` creates a temporary value for a global (package) variable -- Randal L. Schwartz, Perl hacker	[reply]
Re: my and local?? by toadi (Chaplain) on Jul 28, 2000 at 13:27 UTC
</CODE> WARNING: In general, you should be using `my' instead of `local', because it's faster and safer. Exceptions to this include the global punctuation variables, filehandles and formats, and direct manipulation of the Perl symbol table itself. Format variables often use `local' though, as do other variables whose current value must be visible to called subroutines. </CODE> This is what is written in man perlsub. I think this is kind of straightforward? -- My opinions may have changed, but not the fact that I am right	[reply]
RE: my and local?? by Adam (Vicar) on Jul 28, 2000 at 22:10 UTC
Read merlyn's and jlistf's posts above. The only thing I wanted to add was to make it clear that `local` restores the old value after the localized variable goes out of scope. This has both good and bad repercutions. Good in that it allows for things like a recursive function that uses a global variable, bad in that the variable could be created inside a block, and when the block goes out of scope you are holding `undef`. Also, the best text explanation I've seen on all this is Sriam Srinivasan's "Advanced Perl Programming" available from O'Reilly. I think that's a black panther on the cover. (My HS mascot)	[reply]
RE: RE: my and local?? by KM (Priest) on Jul 28, 2000 at 22:16 UTC
A good place to read on uses of local is MJDs Seven Useful Uses of local article. Sort of an aside, but a good article. Cheers, KM	[reply]
Welcome Back KM!! by Adam (Vicar) on Jul 28, 2000 at 22:24 UTC
Welcome Back KM!!	[reply]