algorithm's .filter!() by range key

[Steven Schveighoffer via Digitalmars-d-learn]

On 2/9/16 3:40 PM, Charles wrote:
> This seems to be true of any range function really... is there a way to
> access the key within my range?
>
> Example of what I want to do:
>
> auto x = [1,2,3,4,5];
> x.filter( x_key % 2 == 1 ).sum(); // sum odd elements in array

An array is not an indexed range. It only works with foreach by key 
because of special foreach behavior.

What you want is std.range.enumerate:

import std.range;
import std.algorithm;
import std.stdio;

void main()
{
     auto x = [1, 2, 3, 4, 5];
     writeln(x.enumerate.filter!(a => a[0] % 2 == 1).map!(a => 
a[1]).sum); // 6
}

-Steve