Writing a function that alternates plus and minus signs between list indices

Question

In a homework set I\'m working on, I\'ve come across the following question, which I am having trouble answering in a Python-3 function:

\"Write a fun

Answer 1

Not using fancy modules or operators since you are learning Python.

>>> mylist = range(2,20,3)
>>> mylist
[2, 5, 8, 11, 14, 17]
>>> sum(item if i%2 else -1*item for i,item in enumerate(mylist, 1))
-9
>>>

How it works?

>>> mylist = range(2,20,3)
>>> mylist
[2, 5, 8, 11, 14, 17]

enumerate(mylist, 1) - returns each item in the list and its index in the list starting from 1

If the index is odd, then add the item. If the index is even add the negative of the item.

if i%2:
  return item
else:
  return -1*item

Add everything using sum bulitin.

>>> sum(item if i%2 else -1*item for i,item in enumerate(mylist, 1))
-9
>>>

Answer 2

def alternate(l):
  return sum(l[::2]) - sum(l[1::2])

Take the sum of all the even indexed elements and subtract the sum of all the odd indexed elements. Empty lists sum to 0 so it coincidently handles lists of length 0 or 1 without code specifically for those cases.

References:

• list slice examples
• sum()

Answer 3

Here is one way using operator module:

In [21]: from operator import pos, neg

In [23]: ops = (pos, neg)

In [24]: sum(ops[ind%2](value) for ind, value in enumerate(lst))
Out[24]: -2

Answer 4

my_list = range(3, 20, 2)
sum(item * ((-1)**index) for index, item in enumerate(my_list))

sum = 11 (result of 3-5+7-9+11-13+15-17+19)

Answer 5

Although this already has an accepted answer I felt it would be better to also provide a solution that isn't a one-liner.

def alt_sum(lst):
    total = 0
    for i, value in enumerate(lst):
        # checks if current index is odd or even
        # if even then add, if odd then subtract
        if i % 2 == 0:
            total += value
        else:
            total -= value
    return total

>>> alt_sum([1, 2, 3, 4])
-2

Answer 6

You can use cycle from itertools to alternate +/-

>>> from itertools import cycle
>>> data = [*range(1, 5)]
>>> sum(i * s for i, s in zip(data, cycle([1, -1])))
-2