LabWindows/CVI
CVI Nugget - Dealing with Doubles
I'm not sure if there were past Nuggets in the CVI forum, but here is my first nugget: Dealing with Doubles
I read an interesting article today: http://www.exploringbinary.com/the-answer-is-one-unless-you-use-floating-point/
As I am mostly active in the LabVIEW Forum, I will do a cross post because I want to do a comparison between CVI and LabVIEW when dealing with doubles. I have not yet implemented the code in LabVIEW, but I do expect the same results.
In the article by Rick Regan, he explains the results that was obtained when running his function. I have duplicated is function and add my own main function below.
#include <utility.h>
#include <ansi_c.h>
#include <userint.h>
// Prototypes
double f(double a);
void main (void)
{
int i;
double result;
double myArray[21] = { 5.1, 91.3, 451.9, 7341.4, 51367.7, 897451.7,
1923556.4, 59567771.9, 176498634.7,
2399851001.7, 60098761442.7,
772555211114.1, 1209983553611.9,
59871426773404.9, 190776306245331.2,
2987154209766221.6, 19843566622234755.9,
719525522284533115.3, 8399871243556765103.9,
39847765103525225199.1, 553774558711019983333.9 };
for( i = 0; i < 21; i++)
{
result = f ( myArray[i] );
printf("f(%22.1f) \t = %18.17f \n", myArray[i], result );
}
while ( !KeyHit() )
GetKey(); // to see the io screen before the program closes
}
double f(double a)
{
double b, c;
b = 10*a - 10;
c = a - 0.1*b;
return (c);
}
As expected, I get the same output as Rick.
Rick's output:
My output (and yes, I was lazy and did not create a pretty table.. I just wanted to show a comparison of the values).
You can read the article (link provided above) for more details on the values obtained above. However, the message I am trying to convey is that you have to be careful when "dealing with doubles". Often I see code where there is an if statement that compares the value to zero.
For instance something like:
if ( double zero == 0 )
{
// then do something when true
}
and people get upset and frustrated because they do not understand why the true case never gets executed. As a matter of fact, I do recall several discussions on this topic in the LabVIEW forum. I have actually seen the discussion turn into an argument. 😄
What is important to understand is how the processor interprets double values.
I hope this information will save someone some headaches..
RayR
Thanks Ray for this nugget.
I had fallen across such cases in the past enough times to have learned not to perform any exact match on doble values 
Now your example gives me a reason for this.
The accuracy of binary representation of doubles has been discussed several times in this forums: in addition to posts that explicitly mention IEEE754 standard I remember a few ones that also cover this subject (for example this thread and this one), but indeed having it explained in a good way is a valuable addition on this subject!
And yes: you should be awarded for posting the first nugget on the CVI board!
Of course one can use it... but I don't think it is a good idea...
For example, if one is calculating the second, third or forth moment of a distribution (number of measurement results ...) these numbers are small, 10-10 to 10-30..., but their roots (second third, forth) yield interesting information. Setting 10-20 equal to zero would ruin this approach...
I agree that EPSILON may be useful to check e.g. for convergence, but then I would compare the result to epsilon, not zero...
