"""
Binary search - efficient searching in a sorted array
"""

def binary_search(a, value) :
    """Return the index in which the given value appears in the
    sorted array a
    """
    left = 0
    right = len(a) - 1
    while left <= right :
        mid = (left+right)/2
        if value == a[mid]:
            return mid
        else:
            if value < a[mid]:
                right = mid-1
            else:
                left = mid+1
    return -1

print binary_search(range(100), 20)    
print binary_search(range(1, 100, 2), 20)