undefined | Better HN

0 pointsdnautics6y ago0 comments

it's not just that they are expensive, it's that there is a nondetermistic compute time.

Let's say we need to do a comparison. Set

    a = 34241432415/344425151233

and

    b = 45034983295/453218433828

Which is greater?

Or even more feindish, Set

    a = 14488683657616/14488641242046

and

    b = 10733594563328/10733563140768

which is greater?

By what algorithm would you do the computation, and could you guarantee me the same compute time as comparing 2/3 and 4/5?

0 comments

WaldoDude6y ago

I’m not sure I follow. Isn’t it just two integer multiplications followed by a comparison.

a/b > x/y is the same as ay > xb

Assuming you don’t overflow your integer type.

atq21196y ago

> Assuming you don’t overflow your integer type.

There's your answer :)

It's far too easy to overflow your integer type by simply adding a bunch of rationals whose denominators happen to be coprime, or just by multiplying rationals. For this reason, the vast majority of rational implementations use arbitrary precision integers, and of course arithmetic on those isn't constant time.

_bxg16y ago

One approach would be to hold on to rationals for as long as possible, to eliminate drift, and then dump them out to the nearest floating-point at the very last moment

j / k navigate · click thread line to collapse