Why localize $_ in the map block?

Re your last solution: what do you gain by localizing $_? Or, equivalently, what is the difference between

@my_array_2 = map { local $_ = $_; s/^CHOP_ME_OFF_//; $_ } @my_array;
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and

@my_array_2 = map {s/^CHOP_ME_OFF_//; $_ } @my_array;
[download]

I've used the latter version often and it seemed to work perfectly fine...

pike

Update:OK, to be honest what I really used was

@my_array = map {s/^CHOP_ME_OFF_//; $_ } @my_array;
[download]

i. e. I did the replacements in-place, which explains why I never noticed the effect pointed out by adrianh++.

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Re: Why localize $_ in the map block? by adrianh (Chancellor) on Feb 07, 2003 at 13:59 UTC
Without the local the original array is changed. Since `$_` is aliased to each element the `s//` changes the original array. Try: `my @x = qw(foo bar CHOP_ME_OFF_foo); my @y = @x; my @copies = map { local $_ = $_; s/^CHOP_ME_OFF_//; $_ } @x; my @changes = map {s/^CHOP_ME_OFF_//; $_ } @y; print "x @x\ny @y\ncopies @copies\nchanges @changes\n";` [download]	[reply] [d/l] [select]
Re: Re: Why localize $_ in the map block? by pike (Monk) on Feb 10, 2003 at 16:30 UTC
Ahh, very interesting. I remember that in the Camel book where it explains the use of grep, there is an example along the lines of: `@subset = grep {s/x/y/} @set;` [download] and it points out that this will alter the entries in `@set`, but it never mentions that you can avoid this by localizing `$_`... pike	[reply] [d/l] [select]
Re: Why localize $_ in the map block? by bronto (Priest) on Feb 07, 2003 at 14:26 UTC
adrianh++ It's often (always?) a good practice to localize `$_` whenever you need to use it, like any other global variable: it could be accessed by your program, or used by modules (File::Find is one of them AFAIK) and it's not fair to mess up with it. With localizing it you have a better protection: if something elsewhere in your code relied on `$_`'s value you won't be messing with it. Ciao! `--bronto` The very nature of Perl to be like natural language--inconsistant and full of dwim and special cases--makes it impossible to know it all without simply memorizing the documentation (which is not complete or totally correct anyway). --John M. Dlugosz	[reply] [d/l] [select]