Vanesa Piedrahita - 1 year ago 216
Python Question

# Plus and minus sequence in Python

im creating a program which sums the values in a sequence, but it has alternate plus/minus.

I want to make the sum of this sequence 1-(1/3)+(1/5)-(1/7)....How can i use an alternate plus and minus just like the previous example?

Here is the code i have made so far:

``````s=input("Ingrese el numero de terminos a sumar:", )

sum=1.0
num=1.0

for i in xrange(1,s+1):
ident=sum/(i+(i+1.0))
sum=sum+-ident

print "La suma de los",s,"terminos es %.2f"%(sum)
``````

You can raise `-1` to the power corresponding to the index of whatever term you're on:

``````>>> (-1)**0
1
>>> (-1)**1
-1
>>> (-1)**2
1
>>> (-1)**3
-1
>>> (-1)**4
1
``````

Hence, the whole summation can be simplified to

``````result = sum((-1)**i * 1.0/(2*i + 1) for i in xrange(s))
``````

(You shouldn't use `sum` as a variable name, since the name is already taken by the built-in function `sum` I used above.)

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