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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 function alternate : int list -> int that takes a list of
numbers and adds them with alternating sign. For example alternate
[1,2,3,4] = 1 - 2 + 3 - 4 = -2."

Full disclosure, the question was written with Standard ML in mind but I have been attempting to learn Python and came across the question. I'm imagining it involves some combination of:

splitting the list,

if [i] % 2 == 0:

and then concatenating the alternate plus and minus signs.

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()

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
>>>

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

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)

You could try this list comprehension:

sum([-e if c%2 else e for c,e in enumerate(yourlistylist)])

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