Triangular numbers in python adding a range

Question

I am struggling with what I know should be a really simple loop. I need to add all the numbers from 0 to n. The end result needs to be a positive integer, though I know if it comes out negative I can just get the absolute value to evaluate it to positive.

ex: n = 5 
    sum = 5 + 4 + 3 + 2 + 1
    sum = 15

what I have so far is this

def triangular(n):
    sum_ = 0
    for i in range(n):
        sum_-= n
    return sum_  

Any help would be greatly appreciated.

Answer 1

Wouldn't it just be?

def triangular(n):
    sum_ = 0
    for i in range(n+1):
        sum_+= i
    return sum_ 

This will add all numbers from 0 to n. Though this could be put into one line:

def triangular(n):
    return sum(range(n+1))

Answer 2

The sum of numbers from 0 to N is N(N+1)/2. How about:

return n * (n+1) / 2

rather than using a loop?