Why does binary search algorithm not work?

Question

I've copied code from the book "grokking algorithms" for finding an item in the list using the binary search algorithm. The code launches great and returns no errors. However, sometimes it just doesn't work for certain numbers, for example if i call it to find "1". What's wrong?

def binary_search(list, item):
    low = 0
    high = len(list) - 1

    while low <= high:
        mid = (low + high)/2
        guess = list[mid]
        if guess == item:
            return mid
        if guess > item:
            high = mid + 1
        else:
            low = mid + 1
    return None

list1 = []

for i in range (1, 101):
    list1.append(i)

print(list1)
print(binary_search(list1, 1))

Answer 1

Two issues:

• Use integer division (so it will work in Python 3): mid = (low + high)//2

• When guess > item you want to exclude the guessed value from the next range, but with high = mid + 1 it still is in the range. Correct to high = mid - 1