Re^2: adding items, but with interim results

my @y = map { state $z = 0; $z += $_ } @x;
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The use of the state scalar variable $z in the map BLOCK in this statement is neat, but it presents a stumbling block to which it is perhaps unwise to expose a novice lest he or she be precipitated, however inadvertently, from the True Path.

A state variable, of course, maintains its state from one access to the next regardless of whether or not the lexical scope enclosing the variable is exited and re-entered. This is fine if the statement containing the variable is executed once and only once in the execution of a program; if it is not, the results may be surprising. (In the example below the source array @w is repeatedly dumped just to show that it never changes.)

>perl -wMstrict -MData::Dump -lE
"sub summer { return map { state $z = 0;  $z += $_; } @_; }
 ;;
 my @w = qw(0 1 3 2);
 my @x = summer(@w);
 my @y;  @y = map { state $z = 0;  $z += $_; } @w for 1 .. 3;
 my @z = map { state $z = 0;  $z += $_; } @w;
 ;;
 dd \@w;
 dd \@x;
 dd \@y;
 dd \@z;
 ;;
 @x = summer(@w);
 @y = map { state $z = 0;  $z += $_; } @w for 1 .. 3;
 @z = map { state $z = 0;  $z += $_; } @w;
 ;;
 print '----------';
 dd \@w;
 dd \@x;
 dd \@y;
 dd \@z;
 ;;
 print '----------';
 dd \@w;
"
[0, 1, 3, 2]

[0, 1, 4, 6]

[12, 13, 16, 18]

[0, 1, 4, 6]

----------
[0, 1, 3, 2]

[6, 7, 10, 12]

[12, 13, 16, 18]

[0, 1, 4, 6]

----------
[0, 1, 3, 2]
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Re^3: adding items, but with interim results by LanX (Saint) on Mar 31, 2013 at 00:24 UTC
> but it presents a stumbling block to which it is perhaps unwise to expose a novice I second this! Especially because `state` has some implementation issues besides needing to be activated with `use feature`. a simple `my $z` outside the `map` has the same effect and is backwards compatible. `DB<104> @x=1..4 => (1, 2, 3, 4) DB<105> use feature state; my @y = map { state $z = 0; $z += $_ } @x +; => (1, 3, 6, 10) DB<106> my $z; my @y = map { $z += $_ } @x; => (1, 3, 6, 10)` [download] and if the scope of $z is of importance, use blocks to limit it. `DB<107> my @y; { my $z; @y = map { $z += $_ } @x; } => (1, 3, 6, 10) DB<109> my @y = do { my $z; map { $z += $_ } @x } => (1, 3, 6, 10)` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l] [select]
Re^3: adding items, but with interim results by LanX (Saint) on Oct 17, 2013 at 23:30 UTC
IMHO it's not state which is buggy but rather map. see this `DB<168> sub tst (&@) { my $cr=shift; print $cr->($_)," " for @_ } DB<169> use feature 'state'; tst {state $i++} 1..3 for 1..3 => "" 0 1 2 0 1 2 0 1 2` [download] The blocks of map and grep are no anonymous subroutines as Tobyink pointed out recently. IMHO thats why scoping fails in your tests. `DB<173> use feature 'state'; map {state $i++; print "$i "} 1..3 for +1..3 => "" 1 2 3 4 5 6 7 8 9` [download] (at least with 5.10) udate also while conditions `DB<195> use feature 'state'; for (1..3) { $y=0;while (++(my $x)) { p +rint " $x .";last if $y++>3 } } => "" 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . 1 . DB<196> use feature 'state'; for (1..3) { $y=0;while (++(state $x)) +{ print " $x .";last if $y++>3 } } => "" 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 10 . 11 . 12 . 13 . 14 . 15 .` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l] [select]

udate