Easy way of finding decimal places

Question

Is there an easy way or integrated function to find out the decimal places of a floating point number?

The number is parsed from a string, so one way is to count the digits after the '.' sign, but that looks quite clumsy to me. Is there a possibility to get the information needed out of a float or Decimal object?

SOLUTION (one of them, of course :) )

I chose to use the python decimal.Decimal class to help me with my problem:

e = abs(Decimal(string_value).as_tuple().exponent) 

NOTE: this only works when the parameter from which the Decimal is constructed is a string and not a float (which would lead to floating point inaccuracies).

Thanks a lot for all other contributions.

Answer 1

To repeat what others have said (because I had already typed it out!), I'm not even sure such a value would be meaningful in the case of a floating point number, because of the difference between the decimal and binary representation; often a number representable by a finite number of decimal digits will have only an infinite-digit representation in binary.

In the case of a decimal.Decimal object, you can retrieve the exponent using the as_tuple method, which returns a namedtuple with sign, digits, and exponent attributes:

>>> d = decimal.Decimal('56.4325') >>> d.as_tuple().exponent -4 >>> d = decimal.Decimal('56.43256436') >>> d.as_tuple().exponent -8 

The negation of the exponent is the number of digits after the decimal point, unless the exponent is greater than 0.

Answer 2

"the number of decimal places" is not a property a floating point number has, because of the way they are stored and handled internally.

You can get as many decimal places as you like from a floating point number. The question is how much accuracy you want. When converting a floating point number to a string, part of the process is deciding on the accuracy.

Try for instance:

1.1 - int(1.1) 

And you will see that the answer is:

0.10000000000000009 

So, for this case, the number of decimals is 17. This is probably not the number you are looking for.

You can, however, round the number to a certain number of decimals with "round":

round(3.1415 - int(3.1415), 3) 

For this case, the number of decimals is cut to 3.

You can't get "the number of decimals from a float", but you can decide the accuracy and how many you want.

Converting a float to a string is one way of making such a decision.