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Re: Mutable foreach list?

by almut (Canon)
on Jan 11, 2007 at 15:35 UTC ( #594181=note: print w/replies, xml ) Need Help??


in reply to Mutable foreach list?

"aliasing the loop variable" means that the $_ (in your case) will be aliased to (i.e. not be a copy of) the individual elements of the list. So, modifying $_ will modify the contents of the array. This has nothing to do with pushing stuff onto the array @headers, though...

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Re^2: Mutable foreach list?
by zigdon (Deacon) on Jan 11, 2007 at 15:38 UTC
    Oh, I totally agree. I'm just trying to understand the foreach portion - does the list given get evaluated at the first encounter? On each iteration? I guess this might really be a perlguts kind of question.

    -- zigdon

      The following are all implemented differently:

      • for (EXPR; EXPR; EXPR) ("C-style for loop", an augmented while loop.)
      • for (EXPRX..EXPRY) (A range and nothing else.)
      • for (reverse CONSTX..CONSTY) (A constant range, preceded by reverse.)
      • for (reverse EXPRX..EXPRY) (A variable range, preceded by reverse.)
      • for (@ARRAY) (An array and nothing else.)
      • for (reverse @ARRAY) (Reverse of an array and nothing else.)
      • for (reverse LIST) (Reverse of any list that doesn't fit the above patterns.)
      • for (LIST) (Any list that doesn't fit the above patterns.)

      You might find the difference between foreach (@ARRAY) and foreach (LIST) interesting.

      { my $x = 1; my $y = 4; # The initial values are saved. my @a; foreach ($x..$y) { push @a, $_; $y++; } print("@a\n"); # 1 2 3 4 } { my $x = 1; my $y = 4; # The initial values are saved. my @a; foreach (reverse $x..$y) { push @a, $_; $x--; } print("@a\n"); # 4 3 2 1 } { my @a = (1, 2, 3, 4); my $i = 5; # Loops "while (pass_num < @a)". # In this case, that means loop forever. foreach (@a) { # Loops "while (pass_num < @a)" push(@a, $i++); if (@a == 20) { push(@a, '...'); last; } # Avoid infinite loop. } print("@a\n"); # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ... } { my @a = (1, 2); my @b = (3, 4); my $i = 5; # Creates a list at the start of the # loop and iterates over that list. # In this case, elements are added to @b, # but not to the list on the stack, so # it loops 4 times. foreach (@a, @b) { push(@b, $i++); } print("@a @b\n"); # 1 2 3 4 5 6 7 8 }

      The difference between
      foreach (reverse CONSTX..CONSTY)
      and
      foreach (reverse EXPRX..EXPRY)
      is that the list in built at compile time in the former.

      >perl -le "for (1..2) { for (reverse 1..3) { print; $_=5; } }" 3 2 1 5 5 5 >perl -le "for (1..2) { for (reverse 1..($x=3)) { print; $_=5; } }" 3 2 1 3 2 1

      Empirical evidence suggests ;) the list given to foreach (...) is not being re-evaluated upon every iteration -- though I wouldn't want to rely on it... As Fletch already said, just "don't muck with arrays you're looping over".

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