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Re^3: Mutable foreach list? (types of for)

by ikegami (Pope)
on Jan 11, 2007 at 18:16 UTC ( #594208=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Mutable foreach list?
in thread Mutable foreach list?

The following are all implemented differently:

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


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