How would I print the sum of my range for my code?

Question

Code for printing a range where odd numbers are negative and even numbers are positive. Need help with adding the sum of the range.

def evenoddsequence(i):
  for i in range(i):
    if i%2 != 0:
      i = i*-1
      print(i, end=" ")
    elif i == None:
      return i
    else: 
      print(i, end=" ")
  print()
result = evenoddsequence(7)
print("Sum of terms in the sequence:", result)

Answer 1

I think you are overcomplicating things. What you want to do is to change the sign for every other element in the sequence:

x->(-1)**(x%2)*x

i.e. 0->0, 1->-1, 2->2, 3->-3. So you can just apply this function to every element in the sequence and get a new sequence:

[(-1)**(x%2)*x for x in range(7)] -> [0, -1, 2, -3, 4, -5, 6]

Given that your function is named evenoddsequence I would suggest returning that and do the sum outside:

def evenoddsequence(i):
    return [(-1)**(x%2)*x for x in range(i)]

print(sum(evenoddsequence(7)))

or rename it to say sum_of_evenoddsequence:

def sum_of_evenoddsequence(i):
    return sum([(-1)**(x%2)*x for x in range(i)])

print(sum_of_evenoddsequence(7))

FWIW, you may notice that you add -1 to the sum for every other element:

 0-1, 2-3, 4-5, ... 

if there are an odd number of elems the sum will be positive, otherwise negative. The absolute value of the sum will therefor be half the length of the sequence:

(len(range(i))//2)*(-1)**len(range(i-1))

A less cryptic version:

def g(i):
    if i%2==0:
        return -1*(i//2)
    else:
        return i//2