Take the average of two signed numbers in C

Let us say we have x and y and both are signed integers in C, how do we find the most accurate mean value between the two?

I would prefer a solution that does not take advantage of any machine/compiler/toolchain specific workings.

The best I have come up with is:(a / 2) + (b / 2) + !!(a % 2) * !!(b %2) Is there a solution that is more accurate? Faster? Simpler?

What if we know if one is larger than the other a priori?

Thanks.

Editor's Note: Please note that the OP expects answers that are not subject to integer overflow when input values are close to the maximum absolute bounds of the C int type. This was not stated in the original question, but is important when giving an answer.

c optimization average numerics

Edit Report

1 Answer

Edit: version fixed by @chux - Reinstate Monica:

if ((a < 0) == (b < 0)) {  // a,b same sign
  return a/2 + b/2 + (a%2 + b%2)/2;
} else {
  return (a+b)/2;
}

Original answer (I'd have deleted it if it hadn't been accepted).

a/2 + b/2 + (a%2 + b%2)/2

Seems the simplest one fitting the bill of no assumption on implementation characteristics (it has a dependency on C99 which specifying the result of / as "truncated toward 0" while it was implementation dependent for C90).

It has the advantage of having no test (and thus no costly jumps) and all divisions/remainder are by 2 so the use of bit twiddling techniques by the compiler is possible.

answered 2011-04-18T09:29:15.620

Take the average of two signed numbers in C

1 Answer

Your Answer