.. always counts up. I don't know why, but this workaround will do the trick:

for ( 0 .. 10 ) {
   my $i = 10 - $_;
   # do something
}
[download]

Or just

for (reverse 0..10) { ... }
[download]

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

Let's hope the list isn't too long. for (0..x) is evaluated lazily, but I doubt that for (reverse x..0) is as well.

On second thought, that would rarely matter.

Does this create a list all at once so that it can reverse it as opposed ikegami's solution which creates only one value at a time? I ask because I honestly don't know.

Update: Yarr...that's what I get for not refreshing.

thor

Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come

 for my $i ( reverse ( 0 .. 10 ) ) {
    print "$i\n";
}
[download]

If Y is smaller than X, of course, you get an empty list, but that can really be helpful, in some cases.

Actually, its all values greater than OR EQUAL to X, and less than OR EQUAL to Y.

The range operator uses the magic increment operator internally so that it can work on strings (i.e, 'a'..'zzz'). Since there is no corresponding magic decrement operator, it doesn't work in reverse. You could make the argument that for numeric endpoints, you don't need the increment/decrement to be magical, but this is the way it is...

Well, contrariwise, it's useful in its current form if you want to be able to short-circuit out empty lists without having to do an explicit test. For (contrived, but based on a real-world memory) example:

sub contrivance {
    my $min = shift;
    my $max = some_function();
    for ($min .. $max) {
        print "I think that $_ is a valid possibility\n";
    }
}
[download]

Contrived, as I said. But the point is that you can readily find a family of uses for the range operator where you'd want to return an empty list rather than counting backward if the RHS is less than the LHS, just as you can (and indeed, you have) for the opposite. Apparently, Larry thought my use case was more common than yours—lucky me, huh? :-)




---

If God had meant us to fly, he would *never* have given us the railroads.
    --Michael Flanders

this doesn't work as I expected

The key element there being "I".

Perl works as Larry expects. It's up to the rest of us to figure out what that means. (And for some of us to write about it and teach it.)

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

for(my $i=10;$i<=0;$i++) {
  # do something
}
[download]

You can't use .. to count backwards, but try this:

for my $i (reverse 0..10) {
  # do something
}
[download]

for $a(-2,12){for $b(0..7){$c=0;$_?hex substr(
ef7fa1866706caeff02289402844,2*$_+$a,2)&2**(7-$b):0
and $c+=2**(7-$_)for(0..7);print chr $c;}}
[download]

You might prefer

for ( my $i = 10; $i--; ) {
  # do something
}
[download]

I regularly walk array indexes backwards, as you can shave a few cycles off of the standard

for ( my $i = 0; $i < $array_size; $i++ ) {
  # whatever
}
[download]

logic that people love using in those languages that don't have a 'foreach' command.

does anybody know why this behaviour was chozen?

More important, I think, is the question "why doesn't use warnings; warn about this?
I think this is the real question here. I too have been bitten by this behaviour and while it didn't cost me hours to debug, it did slow me down and was annoying to find. I don't know why use warnings; doesn't warn about this. I wish it did :-(
After all, we already get messages such as "Useless use of a constant in void context at...". Why not something like "Useless use of an empty list in loop context at...", or something similiar.

Because that would be another useless warning. I write

    for ($x .. $y) { .. }
[download]

quite often, knowing that it will not do anything if $y < $x.

    @a = 1 .. 3   # 3-element list
    @a = 1 .. 2   # 2-element list
    @a = 1 .. 1   # 1-element list
    @a = 1 .. 0   # 0-element list
[download]

The latter to be a 2 element list would be very surprising (and would require a lot of extra code in my programs).

do{
    $_ = 100 unless$_;--$_;
    # do something
} while $_
[download]

An interval [ a , b ] is defined as (free definition) "Every number greater than a and lesser than b" (definition). So, if a=2 and b=1, there's no number with such condition. That's a valid definition for every set with a ">" operator, so it could work with any set (class) with ">" defined and a valid "generator" (++) (This last statement is only theory, someone with time to test).

for (0..10) {
    $_ = 10 - $_;
    print;
}
[download]

No, since "for" is simply an alias of "foreach", so that your code is exactly quivalent to the original poster's code.