Sum of a List using Recursion

Question

I don't understand why "head" is undefined in the following code, I'm trying to use a Accumulate pattern to get the some of a list of numbers.

def sum(items):
    if (items == None): #base case
        return 0
    else:
        return head(items) + sum(tail(items))
def main():
    items = (input("give me a list"))
    print ("the sum of the list is", sum(items))

main()

Answer 1

Since head and tail are being called by passing arguments to them (items is the argument to both), you should declare them as functions:

def head(items):
    # ...
    return ...

def tail(items):
    # ...
    return ...

Note:

• If this is a homework, then the problem statement should specify what are those functions supposed to do.

Answer 2

def sum(items):
    if not items:
        return 0
    else:
        head = lambda x: x[0]
        tail = lambda x: x[1:]
        return head(items) + sum(tail(items))