Hi so i'm trying to make a function where I subtract the first number with the second and then add the third then subtract the fourth ie. x1-x2+x3-x4+x5-x6...
So far I have this, I can only add two variables, x and y. Was thinking of doing
>>> reduce(lambda x,y: (x-y) +x, [2,5,8,10]
Still not getting it
pretty simple stuff just confused..
In this very case it would be easier to use sums:
a = [2,5,8,10]
sum(a[::2])-sum(a[1::2])
-5
Use a multiplier that you revert to +- after each addition.
result = 0
mult = 1
for i in [2,5,8,10]:
result += i*mult
mult *= -1
You can keep track of the position (and thus whether to do + or -) with enumerate, and you could use the fact that -12n is +1 and -12n+1 is -1. Use this as a factor and sum all the terms.
>>> sum(e * (-1)**i for i, e in enumerate([2,5,8,10]))
-5
If you really want to use reduce, for some reason, you could do something like this:
class plusminus(object):
def __init__(self):
self._plus = True
def __call__(self, a, b):
self._plus ^= True
if self._plus:
return a+b
return a-b
reduce(plusminus(), [2,5,8,10]) # output: -5
Or just using sum and a generator:
In [18]: xs
Out[18]: [1, 2, 3, 4, 5]
In [19]: def plusminus(iterable):
....: for i, x in enumerate(iterable):
....: if i%2 == 0:
....: yield x
....: else:
....: yield -x
....:
In [20]: sum(plusminus(xs))
Out[20]: 3
Which could also be expressed as:
sum(map(lambda x: operator.mul(*x), zip(xs, itertools.cycle([1, -1]))))
reduceto solve this. How about a loop?lambdas in Python or just how to write a function to computex1-x2+x3-x4+x5-x6?