The Free On-line Dictionary of Computing (30 December 2018):

binary search

    A search algorithm which repeatedly divides an
   ordered search space in half according to how the required
   (key) value compares with the middle element.

   The following pseudo-C routine performs a binary search
   return the index of the element of vector "thing[first..last]"
   equal to "target":

    if (target < thing[first] || target > thing[last])
      return NOT_FOUND;
    while (first < last)
    
      mid = (first+last)/2;	/* truncate to integer */
      if (target == thing[mid])
        return mid;
      if (target < thing[mid])
        last = mid-1;
      else
        first = mid+1;
    
    if (target == thing[last])
      return last;
    return NOT_FOUND;

   (2003-01-14)