函数返回None而不是index

Question

This code should return index of element, which is surrounded with equal sums of elements

def find_even_index(arr):
    for elem in arr:
        i = arr.index(elem)
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i
        else:
            pass
print find_even_index([1,2,3,4,3,2,1])
print find_even_index([20,10,30,10,10,15,35])

and it returns

3

None

instead of None should return 3 but it doesn't. Why??

Answer 1

Your problem is arr.index(elem) returns only the first index of elem in list arr .

To demonstrate:

>>> lst = [1, 2, 3, 1, 2]                                   
>>> lst.index(1)                                            
0                                                           
>>> lst.index(2)                                            
1                                                           
>>> lst.index(3)                                            
2                                           

Use enumerate instead to keep track of index while iterating through list, like so:

def find_even_index(arr):
    for i, _ in enumerate(arr):
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i

print(find_even_index([1,2,3,4,3,2,1]))  # 3
print(find_even_index([20,10,30,10,10,15,35]))  # 3

Answer 2

try this out. it tends to work, just that I am using recursion instead of looping

    def func(array, i):
        if sum(array[:i]) == sum(array[i+1:]):
            return i
        else:
            a = i + 1
            return func(array, a)

    print(func([20,10,30,10,10,15,35], 0)) #produces 3 as the answer

Answer 3

The problem with the second case is when you get to the fourth iteration and your script executes i = arr.index(10) . You expect this to return 3 , but list.index() finds the first instance of 10 in the list, which is in position 1. In this case, you don't actually want to iterate over the elements of the list, but over their indices. Try this:

def find_even_index(arr):
    for i in range(len(arr)):
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i

Answer 4

The problem in your code is that arr.index(elem) always returns 1 (first index of elem in arr ) if elem equals 10, in your second call of find_even_index() . So it can't work.

I suggest you the following solution, iterating directly on the indexes instead of on the values of the list:

def find_even_index(arr):
    for i in range(len(arr)):
        if sum(arr[0:i]) == sum(arr[i+1:]):
            return i
    return None

print(find_even_index([1,2,3,4,3,2,1]))          # 3
print(find_even_index([20,10,30,10,10,15,35]))   # 3