iota to array

[Seb]

On Sunday, 25 February 2018 at 05:24:54 UTC, psychoticRabbit 
wrote:
> Hi. Anyone know whether something like this is possible?
>
> I've tried various conversions/casts, but no luck yet.
>
> Essentially, I want to cast the result set of the iota to an 
> array, during initialisation of the variable.

You can't do that, because iota is a range and only exists as a 
range struct on the stack.
Think about it as a struct with three variables (start, end, 
stepSize) - not as an array.
Of course, iota is a lot smarter than that as things like  
`10.iota[3 .. $]` or `3 in 10.iota` work, but it's important to 
notice that e.g. `iota[1 .. 2]` just returns a new range object. 
No GC allocation happens here and "the array" never actually 
exists in memory.

> no, I don't want to use 'auto'. I want an array object ;-)

This has nothing to do with auto. Auto is just a filler word for 
the compiler that says: "whatever is the type of the declaration, 
use this as the type".
In other words, the compiler does this automatically for you:

---
typeof(10.iota) r = 10.iota;
---

Anyhow it looks like what you want to do is to __allocate__ an 
array. You can do so with std.array.array.

---
import std.array, std.range;
void main()
{
    int[] arr = 10.iota.array;
}
---

https://run.dlang.io/is/kKSzjH

Again, notice that the actual implementation of array is this (in 
the non-optimized case):

---
auto array(Range)(Range r)
{
     auto a = appender!(E[])(); // <- allocation happens in the 
appender
     foreach (e; r)
         a.put(e);
     return a.data;
}
---

(without constraints for simplicity)

Of course, Phobos has a more sophisticated version of 
std.array.array, but it's basically like this:

https://github.com/dlang/phobos/blob/master/std/array.d#L97

> --------------
> module test;
>
> import std.stdio;
> import std.range : iota;
>
> void main()
> {
>     int[] intArr = iota(1, 11); // 1..10
>     double[] doubleArr = iota(1.0, 11.0); // 1.0..10.0
>     char[] charArr = iota('a', '{');  // a..z
> }
> -------------