Write a recursive function addElements that returns the sum of the elements in a list. For example, addElements([2,1,3]) gives 6.
def addElements(s):
if s == []:
return 0
else:
s[0] + addElements(s[1:])
return s
Error:
TypeError: unsupported operand type(s) for +: 'int' and 'list'
getting this error, Any help would be good thanks :)
The problem is in these lines.
s[0] + addElements(s[1:])
return s
You are finding the sum of two elements, ignoring them and returning the list. When you simply return s, previous addElements(s[1:]) call will get the s[1:] and you will be trying to
s[0] + s[1:]
where s[0] would be the first element in the list, and s[1:] would be rest of the list. That is why you are getting that error.
What you should have done is
return s[0] + addElements(s[1:])
So, your recursion will become like this
addElements([2, 1, 3])
||
2 + (addElements([1, 3]))
||
2 + (1 + (addElements([3])))
||
2 + (1 + (3 + (addElements([]))))
||
2 + (1 + (3 + (0)))
||
2 + (1 + (3))
||
2 + (4)
||
6 # Your answer
This has nothing to do with recursion. The first element of s is obviously an integer and you are trying to add a list to an int as the error stated.
Maybe you meant:
def addElements(s):
if not s: return 0
return s[0] + addElements(s[1:])
You should see the different exit points of the function and see what types are the values you return.
If you see this alternative code:
def addElements(s):
return 0 if s == [] else s[0] + addElements(s[1:])
you can see the two alternatives being 0 or s[0] (which should yield an integer in your flat list) + addElements(s[1:]) (which should yield another call to itself (having the cut condition an integer as return value), or an integer).
Also I'd suggest you use sum(s) instead of your custom function, since sum() is native and compiled, and will be quicker.
sum(s).