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Postings from February 2001
Ilya Zakharevich <ilya@math.ohio-state.edu> writes:
>On Fri, Feb 16, 2001 at 09:28:32PM +0000, nick@ing-simmons.net wrote:
>> >Etc etc etc. Checking that a float is an integer is
>> >a non-trivial and a very expensive operation.
>>
>> No it isn't (at least for IEEE). It is a simple matter of comparing
>> the (binary) exponent with the (known, constant) number of bits in the mantissa.
>> and then seeing if there are any non-zero bits to the right of the radix point.
>> (am I missing something here?).
>
>AFAIK, there is no API to extract the exponent and the mantissa from
>a double. Did I miss it?
My copy of IEEE-754 is in a dusty cuboard at work, so I will not try
and remember the SHALLs and MUSTs vs SHOULDs. I probably have the
code that implemented them round here-at-home somewhere.
However most UNIXs have
SYNOPSIS
#include <math.h>
double frexp(double x, int *exp);
DESCRIPTION
The frexp() function is used to split the number x into a
normalized fraction and an exponent which is stored in
exp.
double modf(double x, double *iptr);
DESCRIPTION
The modf() function breaks the argument x into an integral
part and a fractional part, each of which has the same
sign as x. The integral part is stored in iptr.
RETURN VALUE
The modf() function returns the fractional part of x.
Both are "easy" to do by bit twiddling for IEEE format.
(As is ldexp())
modf() is the one you want for "is integer" testing.
if (modf(value,&ipart) == 0.0 && MIN_INT <= ipart && ipart <= MAX_INT)
That modf actually does rather more than is necessary in this case
as it re-formats the fraction bits of mantissa as a valid double,
which one does not need to do for testing against zero.
The main pain with doing bit-twiddling in software version of IEEE
is the "hidden" MS bit of mantissa.
--
Nick Ing-Simmons
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