$_ and list flattening with map()

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

Aristotle's answer to Switching on internal UTF-8 flaq on DBI result from database made me find the following phenomenon:

$a = [[0]];
foreach (@$a) { $_++ foreach @$_ }
print $a->[0][0],$/;
$_++ for map @$_, @{$a};
print $a->[0][0],$/;
__END__
1
1
[download]

The expected output is:

1
2
[download]

The strange thing is that the extra list flattening with map() doesn't work. I don't see any reason why it shouldn't. I guess the question is: is this a bug or not?

Liz

Update:
Not a bug, just copying of the list with map(). Thanks to !1, blokhead and holo for the enlightenment.

Comment on $_ and list flattening with map() Select or Download Code

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Re: $_ and list flattening with map() by !1 (Hermit) on Dec 14, 2003 at 21:18 UTC
I always thought map just returns copies of the values? `my @a = qw(1 2); my @b = \(@a); my @c = \(map $_,@a); print "Addresses do not match$/" unless "@b" eq "@c"; __END__ Addresses do not match` [download] Of course, changing your map to: `${$_}++ for map \(@$_),@{$a};` [download] Will make it work as expected.	[reply] [d/l] [select]
Re: $_ and list flattening with map() by blokhead (Monsignor) on Dec 14, 2003 at 21:23 UTC
`$_++ for map @$_, @{$a};` [download] The difference is that you do the increment outside of the scope in which `map` aliases `$_`. This foreach is iterating over the return value of map, which is a list of copies (not aliases back into `@$a`). foreach now aliases `$_` to this anonymous list of copies, so your modifications to `$_` are lost. If you use map as a control structure (making it more of a direct translation of the first example), and do the increment inside map's block, you'll get what you expected originally: `map { map { $_++ } @$_ } @$a;` [download] blokhead	[reply] [d/l] [select]
Re: $_ and list flattening with map() by holo (Monk) on Dec 14, 2003 at 21:44 UTC
The line: `$_++ for map $_, @a;` generates an anonymous list with `map`, then iterates through it with `for` incrementing all its values. Once `for` is done, the list is thrown away (expensive no-op). The line: `$_++ for my @b = map $_, @a;` generates a list with `map` and throws it in `@b`, then iterates through it with `for` incrementing all values as before. This time, the list is not anonymous and may be accessed by using `@b` and it's not thrown away. Once `map` is evaluated, values in `@b` are totaly independent from values in `@a`.	[reply] [d/l] [select]
Re: $_ and list flattening with map() by pg (Canon) on Dec 14, 2003 at 22:05 UTC
This tells why: `use strict; use warnings; use Data::Dumper; my $a = [[0,1,2], [10,11,12]]; print \$a->[0][0], "\n"; print \$a->[0][1], "\n"; print \$a->[0][2], "\n"; print \$a->[1][0], "\n"; print \$a->[1][1], "\n"; print \$a->[1][2], "\n"; print "=======\n"; print \$_, "\n" for map @$_, @{$a};#one of your way print "=======\n"; foreach (@$a) { print \$_, "\n" foreach @$_ }#your other way` [download] Result, see how the 3rd set is the same as the 1st one, but the 2nd set stands alone: `SCALAR(0x15551a4) SCALAR(0x15551b0) SCALAR(0x15551c8) SCALAR(0x155abd4) SCALAR(0x155500c) SCALAR(0x1571284) ======= SCALAR(0x15712c0) SCALAR(0x1571290) SCALAR(0x15551d4) SCALAR(0x15712e4) SCALAR(0x15712d8) SCALAR(0x15712cc) ======= SCALAR(0x15551a4) SCALAR(0x15551b0) SCALAR(0x15551c8) SCALAR(0x155abd4) SCALAR(0x155500c) SCALAR(0x1571284)` [download]	[reply] [d/l] [select]