perlish way for ( 0 .. $n) , but downcounting

[hofer]

Hi,

I know I can live without it or with a self written iterator
or . . . , but would still like to know
other perlish ways


creating an array with numbers from 0 to 9 i would use
@a = (0...9);
for the reverse
@a = reverse(0..9);

cycling trough 0 to n (n verly large)
I'd do
$n=100000;
foreach $v (0..$n){ dowith($v) }
# If I understood well the (0..$n) would NOT create a list, but act as
iterator

for the reverse I'd hesitate to use reverse() as it creates probably
the entire list first
so I'd do either
foreach $v_up (0..$n) { $v_down = $n-$v; dowith($v); }

or foreach( $v=$n;$v>=0;$v--){ dowith($v); }



Does anybody have a 'nicer' way ?

 

[Justin C]

Hi,

I know I can live without it or with a self written iterator
or . . . , but would still like to know
other perlish ways


creating an array with numbers from 0 to 9 i would use
@a = (0...9);
for the reverse
@a = reverse(0..9);

cycling trough 0 to n (n verly large)
I'd do
$n=100000;
foreach $v (0..$n){ dowith($v) }
# If I understood well the (0..$n) would NOT create a list, but act as
iterator

[snip]

Not pretty, but works without warnings:

my $i = 1000

for ( -$i .. 0 ) {
print "$i\n";
$i--;
}

Justin.

 

[Willem]

Justin C wrote:
) Not pretty, but works without warnings:
)
) my $i = 1000
)
) for ( -$i .. 0 ) {
) print "$i\n";
) $i--;
) }

Why not go all the way then:

for (-1000 .. 0) { dowith(-$_) }


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

 

[Jim Gibson]

Hi,

I know I can live without it or with a self written iterator
or . . . , but would still like to know
other perlish ways


creating an array with numbers from 0 to 9 i would use
@a = (0...9);
for the reverse
@a = reverse(0..9);

cycling trough 0 to n (n verly large)
I'd do
$n=100000;
foreach $v (0..$n){ dowith($v) }
# If I understood well the (0..$n) would NOT create a list, but act as
iterator

for the reverse I'd hesitate to use reverse() as it creates probably
the entire list first
so I'd do either
foreach $v_up (0..$n) { $v_down = $n-$v; dowith($v); }

or foreach( $v=$n;$v>=0;$v--){ dowith($v); }



Does anybody have a 'nicer' way ?

Not any nicer, but different:

my $v = $n;
while( $v-- >= 0 ) {
dowith($v);
}

 

[Michael Carman]

Appending C<print scalar @x> in each case is left as an exercise for
courious reader (BTW, how to say that abreviated?)

print 0+@x;

-mjc

 

[xhoster]

Don't guess, benchmark first!

But benchmark something that makes sense. None of the benchmarks
you posted make any sense in this context.

You need to use an example large enough to actually make a noticable dent
in memory, and you need to use the construct in question, which was
the range operators used directly in a foreach, with or without a reverse,
not a array being created first and then used in a foreach.

perl -le 'foreach (1..1e6) {}; print +(`ps -p $$ -o rss `)[1];'
1340

perl -le 'foreach (reverse 1..1e6) {}; print +(`ps -p $$ -o rss `)[1];'
72072

Yes, the reverse does build the whole list first.

Xho

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[Eric Pozharski]

But benchmark something that makes sense. None of the benchmarks you
posted make any sense in this context.

Yes, I've seen the light. My fault. I was misguided by

You need to use an example large enough to actually make a noticable
dent in memory, and you need to use the construct in question, which
was the range operators used directly in a foreach, with or without a
reverse, not a array being created first and then used in a foreach.

I should slightly disagree with that. The impact was noticable enough
comparing to void (it was at the bottom).

However, I've made some experiments yet. And found that if a list would
have been created explicitly, then C<foreach (reverse @l)> doesn't
create reversed copy.

*SKIP*

perl -le 'foreach (reverse 1..1e6) {}; print +(`ps -p $$ -o rss `)[1];'
72072

I've got 11K pages (44Kb) for that. 64bit? Then, I think, I've lost
that ball. Pity on me.

*CUT*

 

[Ben Morrow]

However, I've made some experiments yet. And found that if a list would
have been created explicitly, then C<foreach (reverse @l)> doesn't
create reversed copy.

That is correct. Examining the code (pp_enteriter in pp_ctl.c) shows
that

1. if the argument to for() is just a range, it iterates without
creating the whole range;

2. if the argument is reverse(LIST), it expands LIST and then
iterates over it backwards;

3. otherwise, it expands the whole list.

Smells like an opportunity for a minor optimization patch...

Ben

 

[sheinrich]

print 0+@x;

-mjc

LEXTOR ?