Implementation of C lower_bound

Question

Based on the following definition found here

Returns an iterator pointing to the first element in the sorted range [first,last) which does not com

Answer 1

lower_bound is almost like doing a usual binary search, except:

1. If the element isn't found, you return your current place in the search, rather than returning some null value.
2. If the element is found, you search leftward until you find a non-matching element. Then you return a pointer/iterator to the first matching element.

Yes, it's really that simple. :-)

Answer 2

The lower_bound and upper_bound functions in python would be implemented as follows:

def binLowerBound(a, lo, hi, x):
  if (lo > hi):
    return hi
  mid = (lo + hi) / 2;
  if (a[mid] == x):
    return binLowerBound(a, lo, mid-1, x)
  elif (a[mid] > x):
    return binLowerBound(a, lo, mid-1, x)
  else:
    return binLowerBound(a, mid+1, hi, x)

def binHigherBound(a, lo, hi, x):
  if (lo > hi):
    return lo
  mid = (lo + hi) / 2;
  if (a[mid] == x):
    return binHigherBound(a, mid+1, hi, x)
  elif (a[mid] > x):
    return binHigherBound(a, lo, mid-1, x)
  else:
    return binHigherBound(a, mid+1, hi, x)