Python - round a float to 2 digits

Question

I would need to have a float variable rounded to 2 significant digits and store the result into a new variable (or the same of before, it doesn't matter) but this is what happens:

>>> a    
981.32000000000005    
>>> b= round(a,2)    
>>> b
981.32000000000005

I would need this result, but into a variable that cannot be a string since I need to insert it as a float...

>>> print b    
981.32

Actually truncate would also work I don't need extreme precision in this case.

Answer 1

What you are trying to do is in fact impossible. That's because 981.32 is not exactly representable as a binary floating point value. The closest double precision binary floating point value is:

981.3200000000000500222085975110530853271484375

I suspect that this may come as something of a shock to you. If so, then I suggest that you read What Every Computer Scientist Should Know About Floating-Point Arithmetic.

You might choose to tackle your problem in one of the following ways:

1. Accept that binary floating point numbers cannot represent such values exactly, and continue to use them. Don't do any rounding at all, and keep the full value. When you wish to display the value as text, format it so that only two decimal places are emitted.
2. Use a data type that can represent your number exactly. That means a decimal rather than binary type. In Python you would use decimal.

Answer 2

Try this :

Round = lambda x, n: eval('"%.' + str(int(n)) + 'f" % ' + repr(x))

print Round(0.1, 2)
0.10
print Round(0.1, 4)
0.1000
print Round(981,32000000000005, 2)
981,32

Just indicate the number of digits you want as a second kwarg

Answer 3

I wrote a solution of this problem. Plz try

from decimal import *
from autorounddecimal.core import adround,decimal_round_digit


decimal_round_digit(Decimal("981.32000000000005")) #=> Decimal("981.32")
adround(981.32000000000005) # just wrap decimal_round_digit

More detail can be found in https://github.com/niitsuma/autorounddecimal

Answer 4

There is a difference between the way Python prints floats and the way it stores floats. For example:

>>> a = 1.0/5.0
>>> a
0.20000000000000001
>>> print a
0.2

It's not actually possible to store an exact representation of many floats, as David Heffernan points out. It can be done if, looking at the float as a fraction, the denominator is a power of 2 (such as 1/4, 3/8, 5/64). Otherwise, due to the inherent limitations of binary, it has to make do with an approximation.

Python recognizes this, and when you use the print function, it will use the nicer representation seen above. This may make you think that Python is storing the float exactly, when in fact it is not, because it's not possible with the IEEE standard float representation. The difference in calculation is pretty insignificant, though, so for most practical purposes it isn't a problem. If you really really need those significant digits, though, use the decimal package.