cheap replacement for sqrt

[TheHitchhiker]

I am working at one very interesting thing which requires many square rooting, which is too slow. Is there a cheap replacement for sqrt from math.h ? Accuracy is NOT priority at all.

[digfarenough]

if the range of numbers you need the square roots of is relatively small, you could just precalculate the square roots of the numbers at some interval and put them in a look up table and use linear interpolation to estimate the actual square root

Code: Select all

// to initialize:
int maxsqrt = 1024;
float sqrts[maxsqrt];
for (int i=0; i<maxsqrt; i++)
  sqrts[i] = sqrt(i);

// to use:
float mysqrt(float x)
{
  if(x>maxsqrt) // if we haven't precalculated it, do it the slow way:
    return sqrt(x);
  else
  {
  int roundedx = int(x); // rounds down to next lowest integer
  return sqrts[roundedx] + (x-roundedx)*(sqrts[roundedx+1]-sqrts[roundedx]);
  }
}

untested code, but it should work.. not perfectly accurate, but you said you don't mind

[Guest]

thank for replay, but I have heared about some mathematical equations which can aproximate sqrt pretty nice. Does anyone knows anything like that?

[digfarenough]

here, pick your favorite: http://www.azillionmonkeys.com/qed/sqroot.html

[arras]

http://www.flipcode.com/articles/articl ... ance.shtml

[Guest]

GREAT