Question

For example suppose I have a method

double roundDown(double unrounded, double precision) {
}

roundDown(137.42973, 0.01) would return 137.42
roundDown(137.42973, 0.05) would return 137.4
roundDown(137.42973, 0.25) would return 137.25

Are there libraries to do something like this otherwise is the algorithm reasonably simple?

Answer 1

Divide the original number by the precision, round down to the nearest integer, and then multiply by the precision again:

double roundDown(double unrounded, double precision) {
    return precision * Math.floor(unrounded / precision);
}

Note that inherent floating point inaccuracies mean that your number may not be exactly the answer you’re looking for – e.g. you might get 137.399999 instead of 137.4