Re: How to determine if double contains an integer value?
Jiang wrote:
Greg Herlihy wrote:
Martin Eisenberg wrote:
[snip]
Assigning a floating point value to an int variable is not the purpose
of the routine we have been asked to write - testing whether a floating
point value is exactly divisible by 1.0 - on the other hand - is the
problem we need to solve. And as it happens there are (at least) two
standard functions that can provide the needed answer - either of which
will do so reliably.
I am not sure they are reliable for decimal floating literals or not,
You can't apply them directly to a decimal floating point
literal, only to a double. (Versions for float are also
available.)
and I tested your method.
$ cat fmod.cpp
#include <cmath>
#include <iostream>
int main()
{
double d = 1.0000000000000001;
I don't know about your machine, but on the machines I currently
have access to, this initializes d with the exact value 1.0.
Which is the closest representable value to the value you have
written.
if(std::fmod(d,1.0) == 0.0){
std::cout << "exact division\n";
}else{
std::cout << "not exact division\n";
}
}
$ ./fmod
exact division
$
Is it the problem of double value representation?
I'd say that it's more a problem with your understanding of what
a double is. It's a characteristic of the double
representation, but whether it is a problem or not depends on
what you are trying to do.
Or, the comprison is not reliable?
For all intents and purposes, I would say that if the test
didn't work, it was a bug in the library. Technically speaking,
the standard doesn't require any minimum precision for the
functions in <math.h> (and I once encountered an implementation
where sin(x) gave results with less than 5 decimal digits of
precision), but from a QoI point of view, I can't see any
reasonable implemenation failing in this case.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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