Why does my Python binary search algorithm not work for certain items?

Question

My binary search algorithm manages to find some items but does not find others:

myList = [1,3,4,7,12,13,14,16,19,20,28,29,40,45,48,50,67,89,91,94]

item = 67
found = False
lowerBound = 0
upperBound = len(myList) - 1
index = 0

while not found and lowerBound != upperBound:
    index = (upperBound+lowerBound) // 2
    if item == myList[index]:
        found=True
    if item > myList[index]:
        lowerBound = index+1
    if item < myList[index]:
        upperBound = index-1


if found:
    print('Item found')
else:
    print('Item not found')

For example, it manages to find 91, 89, 50, 48, 40, 29, etc., but doesn't find 94, 67, 45, or 28.

There seems to be a pattern in the numbers it finds and those it doesn't. Did I make a mistake somewhere in the code?

Answer 1

The problem is caused by the statement in the while loop. You wrote while lowerBound != upperBound, but that is wrong because if the value at that index is the right value, then you just exit the loop without checking. That can be fixed by using a different condition which is lowerBound <= upperBound. That is a safer and more correct condition to use in this case. The code with the correction made is bellow:

myList = [1,3,4,7,12,13,14,16,19,20,28,29,40,45,48,50,67,89,91,94]

item = 94
found = False
lowerBound = 0
upperBound = len(myList) - 1
index = 0

while not found and lowerBound <= upperBound:
    index = (upperBound+lowerBound) // 2
    if item == myList[index]:
        found=True
    if item > myList[index]:
        lowerBound = index+1
    if item < myList[index]:
        upperBound = index-1


if found:
    print('Item found')
else:
    print('Item not found')