在while循环外部还是内部变量？

Question

Why doesn't my function work when the variables mid and mid_val are defined outside the while loop, just like low and high ?

def binary_search(arr, x):
    low = 0
    high = len(arr) - 1 

    while low <= high:

        mid = (low + high) // 2 
        mid_val = arr[mid] 

        if x == mid_val: 
            return mid 
        elif x > mid_val: 
            low = mid + 1
        else: 
            high = mid - 1
    
    return "Out of range"


array = [2,4,5,6]
x = 6

print(binary_search(array, x))

Answer 1

When you set the value of a variable with mid = (low + high) // 2 , the value is calculated using the current values of other variables. This is a one-and-done operation. mid will not automatically update if the value of low or high changes. So you need this assignment in the while loop to update mid .