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+...

• Bram Vanroy: @VasilisG. Why? Default step is 1.
• doctorlove: range(1,n,2)?

Answer 1

A few issues with your current solution:

1. Your exponentiation operator should be ** not ^ which is XOR.

2. You should start range from 0 not 1 (then first multiplier is -1**0 = 1)

3. 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

• M.Papapetros: Thank you very much. Is there a way to solve this problem without the use of "for" or "while"?
• Moses Koledoye: @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.

Answer 2

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