Variable "i" can not be read at compile time

[ag0aep6g]

On 24.05.20 18:34, data pulverizer wrote:
> I'm getting the error:
> 
> ```
> Error: variable i cannot be read at compile time
> Error: template instance 
> script.runKernelBenchmarks!(Tuple!(DotProduct!float, Gaussian!float, 
> Polynomial!float, Exponential!float, Log!float, Cauchy!float, 
> Power!float, Wave!float, Sigmoid!float)) error instantiating
> ```
> 
> When I run ...
> 
> ```
> ...
> auto runKernelBenchmarks(KS)(KS kernels, long[] n, bool verbose = true)
> {
>    auto tmp = bench(kernels[0], n, verbose);
>    alias R = typeof(tmp);
>    R[] results = new R[kernels.length];
>    results[0] = tmp;
>    for(size_t i = 1; i < results.length; ++i)
>    {
>      results[i] = bench(kernels[i], n, verbose);
>    }
>    return results;
> }
> 
> void main()
> {
>    alias T = float;
>    auto kernels = tuple(DotProduct!(T)(),   Gaussian!(T)(1), 
> Polynomial!(T)(2.5f, 1),
>                         Exponential!(T)(1), Log!(T)(3), Cauchy!(T)(1),
>                         Power!(T)(2.5f),    Wave!(T)(1), Sigmoid!(T)(1, 
> 1));
>    string[] kernelNames = ["DotProduct",  "Gaussian", "Polynomial",
>                         "Exponential", "Log",      "Cauchy",
>                         "Power",       "Wave",     "Sigmoid"];
>    long[] n = [100L, 500L, 1000L];
>    auto results = runKernelBenchmarks(kernels, n);
>    writeln("Results: ", results);
> }
> ```

Since `kernel` is a `Tuple`, you can only access it with compile-time 
constant indices.

But your loop variable `i` is not a compile-time constant, it's being 
calculated at run time. What's more, it depends on `results.length` 
which is also not a compile-time constant. But `results.length` is the 
same as `kernels.length`. And being the length of a `Tuple`, that one is 
a compile-time constant.

So you can rewrite your loop as a `static foreach` (which is evaluated 
during compile-time) using `kernels.length` instead of `results.length`:

static foreach (i; 1 .. kernels.length)
{
     results[i] = bench(kernels[i], n, verbose);
}