Why is for() less efficient than foreach?

[Stefan Koch via Digitalmars-d-learn]

On Friday, 10 February 2017 at 12:39:50 UTC, Bastiaan Veelo wrote:
> Benchmarking for() against foreach():
>
> /////////
> enum size_t maxarray = 500_000;
>
> double[maxarray] a, b, c, d;
>
> void main()
> {
>     import std.stdio;
>     import std.datetime;
>
>     import std.random;
>     for (int n = 0; n < maxarray; n++)
>     {
>         a[n] = uniform01;
>         b[n] = uniform01;
>         c[n] = uniform01 + 0.1;
>     }
>
>     void overhead() {}
>
>     void foreach_loop()
>     {
>         foreach(n, elem; d[])
>             elem = a[n] * b[n] / c[n];
>     }
>
>     void for_loop()
>     {
>         for (int n = 0; n < maxarray; n++)
>             d[n] = a[n] * b[n] / c[n];
>     }
>
>     auto r = benchmark!(overhead,
>                         foreach_loop,
>                         for_loop)(10_000);
>
>     import std.conv : to;
>     foreach (i, d; r)
>         writeln("Function ", i, " took: ", d.to!Duration);
> }
> /////////
>
>
> Depending on the machine this is run on, for() performs a 
> factor 3-8 slower than foreach(). Can someone explain this to 
> me? Or, taking for() as the norm, how can foreach() be so 
> blazingly fast?
> Thanks!

The foreach loop behaves differently.
It does not modify d.

If you want it to modify the array you have to use a ref elem.
If you do you will see that foreach is a little slower.