map confusion

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

Hello. I'm confused about why map is acting differently with ++ and * :

use 5.016;
my @array = 0..5;
my @incremented = map { $_++ } @array;
say "@array";         # 1 2 3 4 5 6
say "@incremented";   # 0 1 2 3 4 5

my @array = 0..5;
my @doubled = map {$_ * 2} @array;
say "@array";         # 0 1 2 3 4 5
say "@doubled";       # 0 2 4 6 8 10
[download]

Why is @incremented not incremented but @doubled is doubled? And why is @array affected in the first case by manipulating $_ but not the second? I'm reading perldoc -f map to see a reason but I'm not grokking it so far. I know to use foreach to modify the org array if I had to - just wondering why map is acting like this. Thanks for any insight.

Comment on map confusion Download Code

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Re: map confusion by GrandFather (Saint) on Dec 15, 2015 at 23:48 UTC
`$_++` is a post increment. The value used is the value $_ contained before being incremented. Unless there is a compelling reason to use the post increment operator you should always use the pre increment operator: `++$_`. Oh, and it's nothing to do with map! Premature optimization is the root of all job security	[reply] [d/l] [select]
Re^2: map confusion by Anonymous Monk on Dec 15, 2015 at 23:53 UTC
Thank-you. However even with ++$_, @array is still affected in the first case, but not with *. Any thoughts on why?	[reply]
Re^3: map confusion by FreeBeerReekingMonk (Deacon) on Dec 16, 2015 at 00:04 UTC
Let's say you have a variable `$A` and you do this: `$A = 5; $B = $A++; print $A;` # prints out 6. $B is 5 `$A = 5; $B = ++$A; print $A;` # prints out 6. $B is 6 `$A = 5; $B = $A*2; print $A;` # prints out 5. $B is 10 `$A = 5; $B = $A+1; print $A;` # prints out 5. $B is 6 Because ++ changes the original AFTER the operation, and mutiplication just returns the result, without modifying the original.	[reply] [d/l] [select]
Re: map confusion by Cristoforo (Curate) on Dec 16, 2015 at 00:12 UTC
As GrandFather said, $_ gets passed to @incremented before its incremented. From the docs for map: Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. That explains why the original elements in @array change. $_++ modifies the element $_ and $_ * 2 doesn't modify $_.	[reply]
Re^2: map confusion by Anonymous Monk on Dec 16, 2015 at 00:43 UTC
Thanks Monks. Got it.	[reply]