Don't understand "if" statements-CodePudding

I don't understand something about Python. This is binary search code and when i put if statement with return first it works, but when I put it below the another if statement something goes wrong. Could someone explain this?

def binary_search(arr, item):
    low = 0
    high = len(arr) - 1
    while low <= high:
        mid = (low high)//2
        guess = arr[mid]
        if guess == item:
            return mid
        if guess > item:
            high = mid - 1

        else:
            low = mid   1

    return None

arr = [1, 12, 16, 22, 24, 45, 54, 67, 89, 121]
print(binary_search(arr, 54))

Output: 6

def binary_search(arr, item):
    low = 0
    high = len(arr) - 1
    while low <= high:
        mid = (low high)//2
        guess = arr[mid]

        if guess > item:
            high = mid - 1
        if guess == item:
            return mid
        else:
            low = mid   1

    return None

arr = [1, 12, 16, 22, 24, 45, 54, 67, 89, 121]
print(binary_search(arr, 54))

Output: None

I thought the operations python does in both cases are the same, but it seems I'm wrong.

CodePudding user response：

Try to iterate the code you have by yourself. The second example would work like this:

0 | low = 0 high = 9 -> mid = 4
1 | low = 5 high = 9 -> mid = 7
2 | low = 8 high = 9 -> stop

If statements behave like this because your condition on guess > item and guess < item are not bound to each other meaning that they could be both checked and, therefore, run. This is what actually happens after third iteration, high should go lower and not the low higher

CodePudding user response：

Here I fixed it for you and it print out as your first code. if,elif and else are binded into one condition. It first check if then, elif and finally else.

def binary_search(arr, item):
    low = 0
    high = len(arr) - 1
    while low <= high:
        mid = (low high)//2
        guess = arr[mid]

        if guess > item:
            high = mid - 1
        elif guess == item:
            return mid
        else:
            low = mid   1

    return None

arr = [1, 12, 16, 22, 24, 45, 54, 67, 89, 121]
print(binary_search(arr, 54))