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Alias std.numeric.FPTemporary

Defines the fastest type to use when storing temporaries of a calculation intended to ultimately yield a result of type F (where F must be one of float, double, or real). When doing a multi-step computation, you may want to store intermediate results as FPTemporary!F.

alias FPTemporary(F) = real;

The necessity of FPTemporary stems from the optimized floating-point operations and registers present in virtually all processors. When adding numbers in the example above, the addition may in fact be done in real precision internally. In that case, storing the intermediate result in double format is not only less precise, it is also (surprisingly) slower, because a conversion from real to double is performed every pass through the loop. This being a lose-lose situation, FPTemporary!F has been defined as the fastest type to use for calculations at precision F. There is no need to define a type for the most accurate calculations, as that is always real.

Finally, there is no guarantee that using FPTemporary!F will always be fastest, as the speed of floating-point calculations depends on very many factors.


import std.math : approxEqual;

// Average numbers in an array
double avg(in double[] a)
    if (a.length == 0) return 0;
    FPTemporary!double result = 0;
    foreach (e; a) result += e;
    return result / a.length;

auto a = [1.0, 2.0, 3.0];
assert(approxEqual(avg(a), 2));


Andrei Alexandrescu, Don Clugston, Robert Jacques, Ilya Yaroshenko


Boost License 1.0.