Re: Decimals after the point

[Dennis Lee Bieber <wlfraed@ix.netcom.com>]

Genevieve B. fed this fish to the penguins on Wednesday 06 November 
2002 03:01 pm:

> Hi!
> 
> For a project at university, there is a bonus that is to check for
> only 2 decimals after the point in a float.
> does anybody have an idea?
> 
> I have an idea of putting my float into a string instead and then
> check for the point...and then check for only 2 numbers after it...
>
        Probably not valid -- after all, you could code the float->string 
write operation to only write two decimal places... Hence you'd always 
see just two decimals <G>


        However, the assignment is rather error-prone, due to the problems of 
decimal <> binary conversions. What was input as 1.40 may be output as 
1.3999999. The assignment should properly specify what behavior is to 
be taken in such a case.

{The following is from a Python interpreter -- float is double <G>}

>>> a = 1.40
>>> a
1.3999999999999999
>>> print "%4f" % a
1.400000
>>> print "%7f" % a
1.400000
>>> print "%7.7f" % a
1.4000000
>>> print "%10.9f" % a
1.400000000
>>> print "%10.10f" % a
1.4000000000
>>> print "%16.15f" % a
1.400000000000000
>>> print "%16.16f" % a
1.3999999999999999
>>> print "%20.16f" % a
  1.3999999999999999

        So what is the correct output for your assignment? The internal (and 
16 decimal place representation) turned 1.40 -> 1.399999999999, so 
effectively, 1.40 does NOT have only two decimal places. How close do 
two floating point numbers have to be to be accepted as "the same".

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