从给定索引开始的列表中数字的总和，递归

Question

I need to find the solution to this problem. I wrote a code to calculate the sum of given numbers in a list, starting from given index.

def recsumlisti(index, alist):
if len(alist) == 0:
    return 0
elif index >= len(alist):
    return 0
else:
    return alist[index] + recsumlisti(index + 1, alist)

Here's the code that I have. It works perfectly fine when index is positive, but it misbehaves when the index is negative.

For eg. if the parameters are recsumlisti(index= -1, alist=[1,2,3,4]) instead of just giving 4 as the output, the function iterates through all the index until the final index ie index == len(alist) is reached and gives out the sum 4 + 1 + 2 + 3 = 10. Test cases for your reference :

{'index': 2, 'alist': [], 'expected': 0},
{'index': 0, 'alist': [1, 2, 3, 4], 'expected': 10},
{'index': -1, 'alist': [1, 2, 3, 4], 'expected': 4},

I need suggestions to improve this program so that it works for all the indices, positive & negative. I have tried using return alist[index] + recsumlisti(index, alist[(index + 1):]) slicing method but it throws error, too.

Let me know if my assumptions are wrong and the code that I have is alright even for the negative indices. Thank you!

Answer 1

You could just check if the index is negative and, if it is, convert it to the corresponding positive one:

def recsumlisti(index, alist):
    if index < 0:
        index = index + len(alist)
    if len(alist) == 0:
        return 0
    elif index >= len(alist):
        return 0
    else:
        return alist[index] + recsumlisti(index + 1, alist)

The rest of the code is the same. This way the output of recsumListi(-1, [1,2,3,4]) is 4 as expected

Answer 2

You could also add a ternary operator to check to see if index +1 is 0, and set index to len(alist) to terminate the program if it is

def recsumlisti(index, alist):
    if len(alist) == 0:
        return 0
    elif index >= len(alist):
        return 0
    else:
        nextIndex = (index + 1, len(alist))[index + 1 == 0]
        return alist[index] + recsumlisti(nextIndex, alist)

again the rest of the code stays the same