how to convert string to numeric in python

Question

I would like to convert string (%) to float.but my method didnt work well. the result slightly differ from correct number. for example,

a=pd.Series(data=["0.1%","0.2%"])

0    0.1%
1    0.2%
dtype: object

first, I strip "%"

a.str.rstrip("%")

0    0.1
1    0.2
dtype: object

I tried to convert to numeric, but the result is strange.

I guess this phenomena come from binary digit system...

pd.to_numeric(a.str.rstrip("%"))

0    0.10000000000000000555
1    0.20000000000000001110
dtype: float64

and of course I couldnt convert % to numeric.

pd.to_numeric(a.str.rstrip("%"))/100

0    0.00100000000000000002
1    0.00200000000000000004
dtype: float64

I also tried .astype(float) method. but The result was same..

why this phenomena happen ? and how can I avoid this phenomena

Answer 1

Many rational numbers can't be represented exactly as a floating-point number. In particular, any number that has to have a five as a factor in the denominator, like 1/(2*5), can't be represented exactly. There isn't much you can do about this: either round the displayed number so it looks right, or use an infinite-precision library or a rational-numbers library. Here's a basic way to round the displayed number:

>>> print "%.20f" % 0.1
0.10000000000000000555
>>> print "%.4f" % 0.1
0.1000

Answer 2

As a folow-up to the suggestion by @D-Von, the following python packages can be useful to you: decimal and fractions

Then you can do some things like:

from fractions import Fraction
from decimal import Decimal
f = Fraction(1, 10)
d = Decimal('0.1')
f = f/100
d = d/100
str(d)

And all the time you are not working with floats but with rational numbers. See the documentation for more examples.