Extracting decimals from a number in Python

Question

I am writing a function to extract decimals from a number. Ignore the exception and its syntax, I am working on 2.5.2 (default Leopard version). My function does not yet handle 0's. My issue is, the function produces random errors with certain numbers, and I don't understand the reason. I will post an error readout after the code.

Function:

def extractDecimals(num):
    try:
        if(num > int(num)):
            decimals = num - int(num)
            while(decimals > int(decimals)):
                print 'decimal: ' + str(decimals)
                print 'int: ' + str(int(decimals))
                decimals *= 10
            decimals = int(decimals)
            return decimals
        else:
            raise DecimalError(num)
    except DecimalError, e:
        e.printErrorMessage()

Exception Class:

class DecimalError(Exception):
    def __init__(self, value):
        self.value = value

    def printErrorMessage(self):
        print 'The number, ' + str(self.value) + ', is not a decimal.'

Here is error output when I input the number 1.988:
decimal: 0.988 int: 0 decimal: 9.88 int: 9 decimal: 98.8 int: 98 decimal: 988.0 int: 987 decimal: 9880.0 int: 9879 decimal: 98800.0 int: 98799 decimal: 988000.0 int: 987999 decimal: 9880000.0 int: 9879999 decimal: 98800000.0 int: 98799999 decimal: 988000000.0 int: 987999999 decimal: 9880000000.0 int: 9879999999 decimal: 98800000000.0 int: 98799999999 decimal: 988000000000.0 int: 987999999999 decimal: 9.88e+12 int: 9879999999999 decimal: 9.88e+13 int: 98799999999999 decimal: 9.88e+14 int: 987999999999999 9879999999999998

I do not know why this error is popping up. Hopefully you guys can help me out.

score 5 · Accepted Answer · edited May 23 '17 at 12:13

5

The problem is that (binary) floating point numbers aren't precisely representable as decimals. See Why can't decimal numbers be represented exactly in binary? for more information.

edited May 23 '17 at 12:13

Community

1
1

answered Oct 25 '09 at 02:58

Ned Batchelder

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Thanks for all of the help, everyone. There are definitely better ways that I could have gone about doing this program, as you guys pointed out, but I am less interested in the result and more interested in what I learn along the way. That's part of the reason why I don't use all of the built in capabilities of Python. Thanks for the answers. – dbmikus Oct 25 '09 at 16:25

unutbu · Answer 2 · 2009-10-25T03:23:42.130

As Ned Batchelder said, not all decimals are exactly representable as floats. A float is represented by a certain number of binary digits which are used to approximate the decimal as closely as possible. You can never assume a float is exactly equal to a decimal.

In [49]: num
Out[49]: 1.988

In [50]: decimals=num - int(num)

In [51]: decimals
Out[51]: 0.98799999999999999

In [52]: print decimals   # Notice that print rounds the result, masking the inaccuracy.
0.988

See http://en.wikipedia.org/wiki/Floating_point for more info on the binary representation of floats.

There are other ways to achieve you goal. Here is one way, using string operations:

def extractDecimals(num):
    try:
        numstr=str(num)
        return int(numstr[numstr.find('.')+1:])
    except ValueError, e:
        print 'The number, %s is not a decimal.'%num

score 1 · Answer 3 · answered Oct 25 '09 at 03:24

1

As others have already pointed out, the issue you are seeing is due to the inexact representation of floating point numbers

Try your program with Python's Decimal

from decimal import Decimal
extractDecimals(Decimal("0.988"))

answered Oct 25 '09 at 03:24

John La Rooy

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does `Decimal` have `extractDecimals`? – Gathide Aug 11 '21 at 09:11

score 1 · Answer 4 · answered Oct 25 '09 at 03:31

As has already been said, floating point numbers are not exactly equal to decimals. You can see this by using the modulus operator like so:

>>> 0.988 % 1
0.98799999999999999
>>> 9.88 % 1
0.88000000000000078
>>> 98.8 % 1
0.79999999999999716

This gives the remainder of division by 1, or the decimal.

score 0 · Answer 5 · answered Oct 25 '09 at 03:23

As others have said in their answers, arithmetic with floats doesn't always result in what you expect due to rounding errors. In this case, perhaps converting the float into a string and back is better?

In [1]: num = 1.988

In [2]: num_str = str(num)

In [3]: decimal = num_str.split('.')[1]

In [4]: decimal = int(decimal)

In [5]: decimal
Out[5]: 988

Extracting decimals from a number in Python

5 Answers5