Use accumulate over reduce when necessary

[bernhardmgruber]

std::reduce can only be used over std::accumulate when the binary operation is commutative and associative. This does not hold for the exercises computing squared sums, so std::reduce is a bug there.

[hageboeck]

I'm puzzled by the motivation for the change. The op should be commutative, but it's likely not associative (i.e. different groupings lead to a different end result). I wouldn't say is wrong, though. It's not deterministic. Did I overlook something?

[hageboeck]

BTW: The same would hold for the simple sum.

[bernhardmgruber]

The op should be commutative, but it's likely not associative (i.e. different groupings lead to a different end result).

We are both right. The documenation says for the binary function:

The behavior is non-deterministic if binary_op is not associative or not commutative.

auto op = [](const T& s, const T& a) { return s + a * a; } is definitely neither commutative nor associative. op(1, 2) == 5 and op(2, 1) == 3. and op(1, op(2, 3)) == 122 and op(op(1, 2), 3) == 14.

I wouldn't say is wrong, though. It's not deterministic. Did I overlook something?

For computing the sum of squares, this is definitely wrong IMO. For the plain sum, your may get different rouding errors, which are fine IMO.

[hageboeck]

Oh, I didn't see that the op takes only one square ... It's better to change this, indeed.