digitalmars.D - foreach and retro
- Joseph Rushton Wakeling (17/17) Jul 02 2012 Hello all,
- Timon Gehr (2/20) Jul 02 2012 What would be your expected output?
- Tobias Pankrath (3/4) Jul 02 2012 From the inner to the outer expression:
- Joseph Rushton Wakeling (9/10) Jul 02 2012 I'd expect to see
- Namespace (12/23) Jul 02 2012 Use the .array() property:
- kenji hara (18/30) Jul 02 2012 D does not provide index for the range iteration.
- Joseph Rushton Wakeling (2/4) Jul 04 2012 Oh, cool. That's a nice feature that I can think of other uses for ... ...
- bearophile (12/15) Jul 02 2012 It's not a bug, it's caused by how ranges like retro work. retro
- travert phare.normalesup.org (Christophe Travert) (4/6) Jul 02 2012 But you can overload OpApply.
- Joseph Rushton Wakeling (5/12) Jul 04 2012 Looks like a nice idea!
Hello all,
A problem with the retro function from std.range -- although it apparently
operates on a bidirectional range, it fails when used with foreach requesting
both value and index. Running this code:
////////////////////////////////////////////////
import std.range, std.stdio;
void main()
{
double[] a = [ 0, 1, 2, 3, 4, 5 ];
foreach(i, x; retro(a))
writeln(i, "\t", x);
}
////////////////////////////////////////////////
... results in an error: "cannot infer argument types".
Is there any reason why this should be so, or is it (as it seems to me) just a
bug?
Thanks & best wishes,
-- Joe
Jul 02 2012
On 07/02/2012 05:36 PM, Joseph Rushton Wakeling wrote:
Hello all,
A problem with the retro function from std.range -- although it
apparently operates on a bidirectional range, it fails when used with
foreach requesting both value and index. Running this code:
////////////////////////////////////////////////
import std.range, std.stdio;
void main()
{
double[] a = [ 0, 1, 2, 3, 4, 5 ];
foreach(i, x; retro(a))
writeln(i, "\t", x);
}
////////////////////////////////////////////////
... results in an error: "cannot infer argument types".
Is there any reason why this should be so, or is it (as it seems to me)
just a bug?
Thanks & best wishes,
-- Joe
What would be your expected output?
Jul 02 2012
"Tobias Pankrath" <tobias pankrath.net> What would be your expected output?From the inner to the outer expression: First the range is reversed and then the elements of this range are enumerated.
Jul 02 2012
On 02/07/12 17:48, Timon Gehr wrote:What would be your expected output?I'd expect to see 0 5 1 4 2 3 3 2 4 1 5 0 i.e. as if I was foreach-ing over an array with the same values in inverted order.
Jul 02 2012
On Monday, 2 July 2012 at 16:49:06 UTC, Joseph Rushton Wakeling wrote:On 02/07/12 17:48, Timon Gehr wrote:Use the .array() property: auto Range(alias func, Args...)(Args args) { return func(args).array(); } void main() {} double[] ad = [ 0, 1, 2, 3, 4, 5 ]; foreach(index, x; Range!(retro)(ad)) { writeln(index, "\t", x); } }What would be your expected output?I'd expect to see 0 5 1 4 2 3 3 2 4 1 5 0 i.e. as if I was foreach-ing over an array with the same values in inverted order.
Jul 02 2012
D does not provide index for the range iteration.
Instead, you can create 'zipped' range.
void main() {
auto intr = sequence!"n"(); // 0, 1, 2, ...
double[] a = [ 0, 1, 2, 3, 4, 5 ];
foreach(i, x; zip(intr, retro(a)))
writeln(i, "\t", x);
}
zip(intr, retro(a)) is a range of Tuple!(size_t, double), and foreach
automatically expand the tow fields of zip front into i and x like
follows.
foreach(__e; zip(intr, retro(a))) {
auto i = __elem[0], x = __elem[1]; // inserted by the compiler
writeln(i, "\t", x);
}
After all, you can get 'index' for range iteration.
Kenji Hara
2012/7/3 Joseph Rushton Wakeling <joseph.wakeling webdrake.net>:
On 02/07/12 17:48, Timon Gehr wrote:
What would be your expected output?
I'd expect to see
0 5
1 4
2 3
3 2
4 1
5 0
i.e. as if I was foreach-ing over an array with the same values in inverted
order.
Jul 02 2012
On 02/07/12 18:57, kenji hara wrote:D does not provide index for the range iteration. Instead, you can create 'zipped' range.Oh, cool. That's a nice feature that I can think of other uses for ... :-)
Jul 04 2012
Joseph Rushton Wakeling:
double[] a = [ 0, 1, 2, 3, 4, 5 ];
foreach(i, x; retro(a))
writeln(i, "\t", x);
It's not a bug, it's caused by how ranges like retro work. retro
yields a single item. In D you can't overload on return values,
so foreach can't try to call a second retro.front overload that
yields an (index,item) tuple (that later foreach is able to
unpack on the fly) instead of the retro.front that just yields
the item.
To solve that problem this is what I have suggested to add to
Phobos:
http://d.puremagic.com/issues/show_bug.cgi?id=5550
Bye,
bearophile
Jul 02 2012
"bearophile" , dans le message (digitalmars.D:171013), a écrit :It's not a bug, it's caused by how ranges like retro work. retro yields a single item. In D you can't overload on return values,But you can overload OpApply. -- Christophe
Jul 02 2012
On 02/07/12 18:43, bearophile wrote:It's not a bug, it's caused by how ranges like retro work. retro yields a single item. In D you can't overload on return values, so foreach can't try to call a second retro.front overload that yields an (index,item) tuple (that later foreach is able to unpack on the fly) instead of the retro.front that just yields the item.Ahh, OK. Clear, just a little surprising.To solve that problem this is what I have suggested to add to Phobos: http://d.puremagic.com/issues/show_bug.cgi?id=5550Looks like a nice idea! The reason I was interested was because of a case where a foreach_reverse was called for across an array, but where it was useful to flip the indices as well.
Jul 04 2012