Can someome please explain why the result here is 6 ?

Question

def func(x):
res = 0
for i in range(x):
res += i
return res

print(func(4))

and why if I change the number 4 with 5 I will get 10.
thank you

Answer 1

Because i is being incremented each loop so. 1 + 2 + 3 + 4 + 5 ...

Answer 2

thank you very much, now I understand