Python function-unexpected output

Question

I have this Python function:

def main(n,x):
   g=0
   for i in range(1,n):
       g+=((-1)^i)*(x^(2*i+1))/(2*i+1)
   return g

print main(3,2)

and the output is -6, when I think it should be 86/15. Where is my mistake? I want to find the n-value of x-(x^3)/3+(x^5)/5+...

@VasilisG. Why? Default step is 1.

Bram Vanroy
– Bram Vanroy

2018-07-21 15:05:10 +00:00
Commented Jul 21, 2018 at 15:05 — Bram Vanroy
– Bram Vanroy, Commented Jul 21, 2018 at 15:05
range(1,n,2)?

doctorlove
– doctorlove

2018-07-21 15:06:46 +00:00
Commented Jul 21, 2018 at 15:06 — doctorlove
– doctorlove, Commented Jul 21, 2018 at 15:06

Moses Koledoye · Accepted Answer · 2018-07-21 15:19:04Z

3

A few issues with your current solution:

Your exponentiation operator should be ** not ^ which is XOR.
You should start range from 0 not 1 (then first multiplier is -1**0 = 1)
Change one of the numbers in the division to float, to avoid integer division in Python 2.

def main(n, x):
   g = 0
   for i in range(n):
       g += ((-1)**i) * (x**(2*i+1))/float(2*i+1)
   return g

edited Jul 21, 2018 at 15:19

answered Jul 21, 2018 at 15:13

Moses Koledoye

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2 Comments

M.Papapetros Over a year ago

Thank you very much. Is there a way to solve this problem without the use of "for" or "while"?

Moses Koledoye Over a year ago

@M.Papapetros My (power) series maths is a bit rusty. Try asking on math.stackexchange.com if there's an arithmetic reduction for your series.

Bal Krishna Jha · Accepted Answer · 2018-07-21 15:22:11Z

0

If you want your answer in fraction you can use:

from fractions import Fraction
def main(n,x):
   g=0
   for i in range(n):
       g+=Fraction(((-1)**i)*(x**(2*i+1)),(2*i+1))
   return g

print main(3,2)

It gives output:

86/15

edited Jul 21, 2018 at 15:22

answered Jul 21, 2018 at 15:11

Bal Krishna Jha

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