Hi,

The following works as expected:

auto range = [1, 2, 3, 4, 5];
foreach (i, el; range) {
	writeln(i, ": ", el);
}

but this slight modification doesn't:

auto range = iota(5);
foreach (i, el; range) {
	writeln(i, ": ", el);
}

DMD 2.078.3 says:
Error: cannot infer argument types, expected 1 argument, not 2

The error message is not helpful either, because indicating the types, as in:

foreach (int i, int el; range) { ... }

throws the same error.
What's going on here?

Arredondo

On 02/03/2018 11:21 PM, Arredondo wrote:
> Hi,
> 
> The following works as expected:
> 
> auto range = [1, 2, 3, 4, 5];
> foreach (i, el; range) {
>      writeln(i, ": ", el);
> }

s/range/array/

Arrays have a different foreach syntax than ranges do.

On Friday, 2 March 2018 at 10:21:39 UTC, Arredondo wrote:
> Hi,
>
> The following works as expected:
>
> auto range = [1, 2, 3, 4, 5];
> foreach (i, el; range) {
> 	writeln(i, ": ", el);
> }
>
> but this slight modification doesn't:
>
> auto range = iota(5);
> foreach (i, el; range) {
> 	writeln(i, ": ", el);
> }
>
> DMD 2.078.3 says:
> Error: cannot infer argument types, expected 1 argument, not 2
>
> The error message is not helpful either, because indicating the types, as in:
>
> foreach (int i, int el; range) { ... }
>
> throws the same error.
> What's going on here?
>
> Arredondo

try
https://dlang.org/phobos/std_range.html#enumerate

> foreach (i, el; enumerate(range)) {
> 	writeln(i, ": ", el);
> }

On Friday, 2 March 2018 at 10:27:27 UTC, Nicholas Wilson wrote:
> try
> https://dlang.org/phobos/std_range.html#enumerate

This worked. Thank you!

On Friday, March 02, 2018 10:21:39 Arredondo via Digitalmars-d-learn wrote:
> Hi,
>
> The following works as expected:
>
> auto range = [1, 2, 3, 4, 5];
> foreach (i, el; range) {
>   writeln(i, ": ", el);
> }
>
> but this slight modification doesn't:
>
> auto range = iota(5);
> foreach (i, el; range) {
>   writeln(i, ": ", el);
> }
>
> DMD 2.078.3 says:
> Error: cannot infer argument types, expected 1 argument, not 2
>
> The error message is not helpful either, because indicating the types, as in:
>
> foreach (int i, int el; range) { ... }
>
> throws the same error.
> What's going on here?

foreach does not support indices for ranges, only arrays. When you have

foreach(e; range)

it gets lowered to

foreach(auto __range = range; !__range.empty; __range.popFront())
{
    auto e = __range.front;
}

There are no indices involved there, and if a range isn't a random-access range, it doesn't support any kind of indices anyway. The compiler would have to add a variable to count the elements, and it doesn't support that.

Your best options are either lockstep with iota or enumerate:

https://dlang.org/phobos/std_range.html#lockstep https://dlang.org/phobos/std_range.html#enumerate

- Jonathan M Davis

On Friday, 2 March 2018 at 10:21:39 UTC, Arredondo wrote:
> Hi,
>
> The following works as expected:
>
> auto range = [1, 2, 3, 4, 5];
> foreach (i, el; range) {
> 	writeln(i, ": ", el);
> }
>
> but this slight modification doesn't:
>
> auto range = iota(5);
> foreach (i, el; range) {
> 	writeln(i, ": ", el);
> }
>
> DMD 2.078.3 says:
> Error: cannot infer argument types, expected 1 argument, not 2
>
> The error message is not helpful either, because indicating the types, as in:
>
> foreach (int i, int el; range) { ... }
>
> throws the same error.
> What's going on here?
>
> Arredondo

What you want to do is call "enumerate" from "std.range".

    auto range = iota(5).enumerate;

    foreach (i, el; range) {
        writeln(i, ": ", el);
    }

You can also call "array" from "std.array".

    auto range = iota(5).array;

    foreach (i, el; range) {
        writeln(i, ": ", el);
    }

On Friday, 2 March 2018 at 10:32:08 UTC, Jonathan M Davis wrote:
> foreach does not support indices for ranges, only arrays. When you have
>
> foreach(e; range)
>
> it gets lowered to
>
> foreach(auto __range = range; !__range.empty; __range.popFront())
> {
>     auto e = __range.front;
> }
>
> There are no indices involved there, and if a range isn't a random-access range, it doesn't support any kind of indices anyway. The compiler would have to add a variable to count the elements, and it doesn't support that.

I understand. I guess I was expecting the compiler to automatically do something along the lines of what enumerate does. Although, a nicer error message would have saved the day just as well.

Arredondo

On Friday, 2 March 2018 at 10:34:31 UTC, bauss wrote:
> You can also call "array" from "std.array".
>
>     auto range = iota(5).array;
>
>     foreach (i, el; range) {
>         writeln(i, ": ", el);
>     }

Thank you. That's how I had it in my original code, I was just trying to avoid gratuitous memory allocation.

Arredondo