[tialaramex]

Because the exponent varies, it matters what order we add floats together, if we add them in one order we get a very different answer to another. If we expected a consistent result that's very surprising.

Suppose you have a billion, and also you have a million ones, all as 32-bit floating point numbers. If we begin with the billion, and add each one, the additions do nothing, because in the representation used for the billion a single one is too small to matter, so our answer is just one billion again each time -- but if we begin with the ones, adding the billion last, we've reached a million when we add the billion, that's not too small to matter and we get 1.001 billion.

[skratky]

Use the Kahan summation algorithm.

[tialaramex]

Kahan solves my examples but in general cannot be relied on to deliver any specific benefit - it's never worse but it may not be meaningfully better either.

Pairwise algorithms can promise better results but aren't applicable unless we're OK with the idea of separately storing the numbers somewhere while we run the algorithm or we provide this hash table with a random access mechanism it may otherwise have no use for.