Re^2: Trying to understand closures

No, no optimization was involved. Don't confuse my $x with undef $x. my $x does NOT change the value of $x. Aside from returning $x's value, all my $x does is set a flag to clear $x at the end of the current scope (i.e. at the end of the loop pass).

Can you guess what the following snippet prints?

for (1..4) {
   my $x;
   print ++$x;
}

print "\n";

for (1..4) {
   goto SKIP if $_ == 2;
   my $y;
SKIP:
   print ++$y;
}

print "\n";

for (1..4) {
   my $z if $_ != 2;
   print ++$z;
}
[download]

At the end of every pass of the first loop, $x gets cleared, so the output of that loop is 1111.

At the end of every pass *except the second* of the second loop, $y gets cleared, so the output of that loop is 1121.

The third loop is identical to the second one. It also outputs 1121.

Updated: Reworded for clarity. No new content.
Updated: Added code.

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Re^3: Trying to understand closures by superfrink (Curate) on Nov 24, 2006 at 05:51 UTC
`my $x` does actually change the value of `$x` . Update: That's wrong. I don't know why I wrote that. As far as I know `my` creates a new variable so the later references to $x actually refer to a different instance of a variable even though they have the same name. For example this code: `print __LINE__ . ": $x\n"; my $x = 123; print __LINE__ . ": $x\n"; my $x = 456; print __LINE__ . ": $x\n"; my $x; print __LINE__ . ": $x\n";` [download] Prints out: `1: 3: 123 5: 456 7:` [download]	[reply] [d/l] [select]
Re^4: Trying to understand closures by ikegami (Patriarch) on Nov 24, 2006 at 05:54 UTC
Each of those `$x` is a different variable. `"my" variable $x masks earlier declaration in same scope at 585826.pl +line 4. "my" variable $x masks earlier declaration in same scope at 585826.pl +line 6. print __LINE__ . ": " . \$x . " = $x \n"; my $x = 123; print __LINE__ . ": " . \$x . " = $x \n"; my $x = 456; print __LINE__ . ": " . \$x . " = $x \n"; my $x; print __LINE__ . ": " . \$x . " = $x \n";` [download] `1: SCALAR(0x0225fc4) = 3: SCALAR(0x022603c) = 123 5: SCALAR(0x1830b1c) = 456 7: SCALAR(0x1830ba0) =` [download] (Undefined warnings omitted.) Note that `my $x` does not always create a new variable. In my snippet, `$x`, `$y` and `$z` are the same variable every pass through the loop.	[reply] [d/l] [select]
Re^3: Trying to understand closures by gam3 (Curate) on Nov 24, 2006 at 13:01 UTC
Can you guess what the following snippet prints? `for (1..4) { my $x; print ++$x, " ", \$x, "\n"; sub { $y = $x }; }` [download] And this? `for (1..4) { my $x; print ++$x, " ", \$x, "\n"; sub bob { $y = $x }; }` [download] And now the answer: In the first case there is no closure, so only one `$x` is created, but in the second case there is a closure so 2 different `$x`s are created as you can see from the output. for example: 1 SCALAR(0x8160300) 1 SCALAR(0x8160300) 1 SCALAR(0x8160300) 1 SCALAR(0x8160300) 1 SCALAR(0x81649e8) 1 SCALAR(0x814cd38) 1 SCALAR(0x814cd38) 1 SCALAR(0x814cd38) -- gam3 A picture is worth a thousand words, but takes 200K.	[reply] [d/l] [select]