double/float precision question

Dave Angel davea at ieee.org
Wed Apr 1 20:48:34 CEST 2009


It's not at all clear what you really want.  You say you want to "use" 
the %e format, but then imply you're then going to turn it back into a 
float.  Since I don't know what the end goal is, I'll just comment 
generally.

All Python floating point is equivalent to the 'double' type of the C 
implementation.  That may vary by C compiler, or by processor, but what 
you see is typical of an 8 byte double type.  I don't really remember my 
values, but it's around 16 digits of precision.   Anything displayed 
beyond those digits is noise, and shouldn't be considered reproducible.  
In other words, those two results are the same, the rounding is just 
different.

To put it simply, in Python,  float() on a floating point number does 
not reduce the precision in the least.

As for having the user specify the number of digits he/she wants, that 
just means you have to construct the format string in a variable.  
Something like:
     digits = 14  (this you'd have gotten from a user)
     format = "%." + str(digits) + "e"
     result = format % number         #this does the formatting

TP wrote:
> Hi everybody,
>
> Try the following python statements:
>
>   
>>>> "%.40f" % 0.2222222222222222222222222222222
>>>>         
> '0.2222222222222222098864108374982606619596'
>   
>>>> float( 0.2222222222222222222222222222222)
>>>>         
> 0.22222222222222221
>
> It seems the first result is the same than the following C program:
> ################
> #include <stdio.h>
>
> int main(void)
> {
>     double a = 0.2222222222222222222222222222222;
>
>     printf( "%.40f\n", a );
>     return 0;
> }
> #################
>
> My problem is the following:
> * the precision "40" (for example) is given by the user, not by the
> programmer.
> * I want to use the string conversion facility with specifier "e", that
> yields number is scientific format; so I cannot apply float() on the result
> of "%.40e" % 0.2222222222222222222222222222222, I would lost the scientific
> format.
>
> Is there any means to obtain the full C double in Python, or should I limit
> the precision given by the user (and used in "%.*e") to the one of a Python
> float?
>
> Thanks in advance
>
> Julien
>
>   



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