Binary search algorithm in python

Question

I am trying to implement the binary search in python and have written it as follows. However, I can't make it stop whenever needle_element is larger than the largest element in the array.

Can you help? Thanks.

def binary_search(array, needle_element):
    mid = (len(array)) / 2
    if not len(array):
        raise "Error"
    if needle_element == array[mid]:
        return mid
    elif needle_element > array[mid]:
        return mid + binary_search(array[mid:],needle_element)
    elif needle_element < array[mid]:
        return binary_search(array[:mid],needle_element)
    else:
        raise "Error"

Answer 1

It would be much better to work with a lower and upper indexes as Lasse V. Karlsen was suggesting in a comment to the question.

This would be the code:

def binary_search(array, target):
    lower = 0
    upper = len(array)
    while lower < upper:   # use < instead of <=
        x = lower + (upper - lower) // 2
        val = array[x]
        if target == val:
            return x
        elif target > val:
            if lower == x:   # these two are the actual lines
                break        # you're looking for
            lower = x
        elif target < val:
            upper = x

• lower < upper will stop once you have reached the smaller number (from the left side)
• if lower == x: break will stop once you've reached the higher number (from the right side)

Example:

>>> binary_search([1,5,8,10], 5)   # return 1
1
>>> binary_search([1,5,8,10], 0)   # return None
>>> binary_search([1,5,8,10], 15)  # return None