Loops

A for loop walks through a sequence:

for x in [1, 2, 3]:
    print(x)

range(n) gives you the integers 0, 1, 2, ..., n - 1:

for i in range(5):
    print(i)        # prints 0 then 1 then 2 then 3 then 4

A while loop runs as long as a condition stays true.

Your task

Write a function sum_evens(n) that returns the sum of all even numbers from 0 up to (and including) n.

sum_evens(0)   # 0
sum_evens(2)   # 0 + 2 = 2
sum_evens(10)  # 0 + 2 + 4 + 6 + 8 + 10 = 30

If n is negative, return 0.