Higher Order Function Python (never ending?)

Question

I have a code that works when it's input with an increasing value like '123' but not when the next character is smaller than the previous character (eg. '132') and if there's a same number, the recursion never stops (eg. '122')

Please kindly point out the issue in my code and a solution would be appreciated greatly.

def sum(term, a, next, b):
    if (a > b):
        return 0
    else:
        print (term(a), a, next(a), b)
        return term(a) + sum(term, next(a), next, b)

def knocked_down(game):

    t1 = lambda x:int(game[x])
    t2 = 0
    t3 = lambda x: int(game[x])
    t4 = len(game)-1
    return sum(t1, t2, t3, t4)

results=knocked_down("123")
print('---')
print(results)

Answer 1

In sum function, the parameters might mean

• term : Function to get value by index
• a : Start index
• next : Function to get next index
• b : Last index

And OP said:

The sum() function wasn't written by me. I was tasked to use it and I'm not allowed to change the code.

So I didn't touch sum function.
You can change t3 into lambda x: x + 1 .

def sum(term, a, next, b):
    if (a > b):
        return 0
    else:
        print(term(a), a, next(a), b)
        return term(a) + sum(term, next(a), next, b)


def knocked_down(game):
    t1 = lambda x: int(game[x])
    t2 = 0
    t3 = lambda x: x + 1
    t4 = len(game) - 1
    return sum(t1, t2, t3, t4)


results = knocked_down("132")
print('---')
print(results)

output:

1 0 1 2
3 1 2 2
2 2 3 2
---
6