ValueError: could not convert string to float: pi

Question

i'm making a program (w/python 2.7) to approximate sin(x) with taylor series, here's the code:

from math import pi
from math import sin
from math import factorial
x=float(raw_input("Degree(in radian, example: 5*pi/4): "))
n=input("n: ")
Sum=0
for i in range(1,n+1):
    Sum=Sum+(pow(-1,(i+1))*pow(x,(2*i-1))/factorial(2*i-1))
error=math.fabs(sin(x)-Sum)
print "Using Taylor series for sin(%s) with n = %d returns %f,with error= %f"(x,n,Sum,error)

(sorry for the from math import mess up there, not exactly good with this)

however, when run with x = 5*pi/4, the program returns

ValueError: invalid literal for float(): 5*pi/4

what am I doing wrong here? I think that python reads x as a string and fails to float that, but what do I know

any help would be appreciated!

Python's float() function doesn't understand '5*pi/4' any more than it understands 'airspeed of an unladen swallow plus two'. You have to pass it a floating-point number or its string representation, like '4' or '2.5883'. — TigerhawkT3
– TigerhawkT3, Commented Mar 27, 2016 at 8:22
Don't bother learning Python 2, rather switch to Python 3 directly. The raw_input() equivalent is called input() there and print is a function. The rest should be mostly the same. That said, you can hardcode the input value to get closer to the required minimal example. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Mar 27, 2016 at 8:30

dotdeb · Accepted Answer · 2016-03-27 08:14:06Z

1

You are right, python does read x as a string:

y = raw_input("Degree(in radian, example: 5*pi/4): ")
print y => 5*pi/4

You would have to precompute the value in radian and pass it to your program:

(5 * math.pi)/2 = 7.853981633974483

This would be the value you give as input to your program.

answered Mar 27, 2016 at 8:14

dotdeb

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Ulrich Eckhardt · Accepted Answer · 2016-03-27 08:38:24Z

1

There may be a solution for achieving what you want. Read the documentation of the Python 2 input() function before using this solution though. The documentation also explains the reason it was removed in Python 3!

Here's the code:

from math import pi
x = input()
print x

If you run this code and input 3 * pi, it will print 9.42477796076938.

answered Mar 27, 2016 at 8:38

Ulrich Eckhardt

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jhagk · Accepted Answer · 2016-03-27 08:43:55Z

1

If you want to take values in the a*pi/b format from the command line, you can use the following.

x=(raw_input("Degree(in radian, example: 5*pi/4): "))
numerator =  float (x.split('*')[0])
denominator =float (x.split('/')[1]) 
x = numerator* pi /denominator

answered Mar 27, 2016 at 8:43

jhagk

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