Hi
What am I doing wrong here?

import std.stdio, std.range, std.algorithm, std.uni, std.utf, std.conv, std.typecons, std.array;

auto SE(A, B)(in A a, in B b){
    return (a-b)^^2;
}

void main(){
    auto a = [1, 2, 3], b = [1, 1, 1];
    lockstep(a, b, StoppingPolicy.requireSameLength).each!((a, b){ writeln(SE(a, b)); });
    lockstep(a, b, StoppingPolicy.requireSameLength).map !((a, b){ return  SE(a, b) ; }).each!writeln;  <- error here
}

The error:
map(Range)(Range r)
  with Range = Lockstep!(int[], int[])
  must satisfy the following constraint:
       isInputRange!(Unqual!Range)

Why it works with each (or foreach), but not with map? o.O

I just wanted to make a Sum of squared errors function.

Thanks in advance!

On 2021-03-05 19:49, realhet wrote:

> Why it works with each (or foreach), but not with map? o.O

`lockstep` is specifically designed to work with `foreach`. I think `each` has a special case to work with `lockstep`. If you want to use other range functions, you should use `zip` instead of `lockstep`.

-- 
/Jacob Carlborg

On Friday, 5 March 2021 at 19:26:38 UTC, Jacob Carlborg wrote:
> On 2021-03-05 19:49, realhet wrote:
>
>> Why it works with each (or foreach), but not with map? o.O
>
> `lockstep` is specifically designed to work with `foreach`. I think `each` has a special case to work with `lockstep`. If you want to use other range functions, you should use `zip` instead of `lockstep`.

It works now:

  zip(StoppingPolicy.requireSameLength, a, b).map!(a => SE(a[])).sum / float(a.length);

I had a misconception (lazyness of learning) that zip is making a simple array, not a tuple array like I guessed lockstep does.

Also in zip() the StoppingPolicy is the first parameter and in lockstep() it's the last.

Thank you very much!