Find square root of number using binary search

Find square root of number using binary search :

double sqrt(double a)
{
 //firstly check if a is non-negative value
 if(a<0) return -1;
 //also check if a==0 or a==1 because in these two cases sqrt(a) = a
 if(a==0 || a==1) return a;
 
 //now start the core part of the code
 double precision = 0.00001;//define the precision, so we stop when precision is achieved
 double start = 0;
 double end = a;
 //we define these two start/end values because usually 0<sqrt(a)<a
 //however, if a<1; then 0<a<sqrt(a)
 if(a<1)
  end = 1;
 
 //define a loop to continue if the precision is not yet achieved
 while(end-start>precision)
 {
  double mid = (start+end)/2;
  double midSqr = mid*mid;
  if(midSqr==a) return mid;//we find the exact sqrt value!
  else if(midSqr<a) start = mid;//we shift our focus to bigger half
  else end = mid;//shift focus to smaller half
 }
 
 //if we did not find exact sqrt value, we return the approxiated value with the defined precision
 return (start+end)/2;
}

int main()
{
printf("%lf",sqrt(120));
return 0;
}