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Problem with simple if-comparison

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Old   September 7, 2009, 15:09
Default Problem with simple if-comparison
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Sebastian Gatzka
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Hello World.

I'm completely confused ...
This is really embarrassing as it's about a simple if condition ...

Have a look at the code-(sniped):
Don't mind what it's actually doing, concentrate on the bold red if-condition.

Code:
int main(int argc, char *argv[])
{
    faceSet facesSet(mesh, setName);
    labelList faces(facesSet.toc());
    vectorField faceCenterCoordinates(facesSet.size());

     for (label i=startTime; i<endTime; i++)
    {

        for (label k=0; k<facesSet.size(); k++)
        {
            int faceNumber = faces[k];
            if (i == startTime)
            {
                faceCenterCoordinates[k] = mesh.Cf()[faceNumber];
            }

            if (i == startTime)
           {
               Info<< "faceCenterCoordinates[1].component(2) = " << faceCenterCoordinates[1].component(2) <<endl;
               Info<< "faceCenterCoordinates[2].component(2) = " << faceCenterCoordinates[2].component(2) <<endl;

               if ( faceCenterCoordinates[1].component(2) == faceCenterCoordinates[2].component(2) )
               {
                   Info << " Components identical !!! " << endl;
               }
            }
        }

}
Although this is looking rather cryptic its about the bold red if-condition.
It is not satisfied (meaning its Info-line "Components identical !!!" is not displayed while running the program) although the bold black italic Info-line displays the following:

Code:
faceCenterCoordinates[1].component(2) = 1.5625
faceCenterCoordinates[2].component(2) = 1.5625
So from the Info-output the if-condition should be satisfied as the two numbers (1.5625) appear to be equal, but it's not because the Info-line (Components identical !!!) in this particular if-bracket is not displayed!

So what's wrong with this?!
Is there any difference between the displayed values and the contents of the vectorField?!


In the further progress of this code I'm writing this faceCenterCoordinates vectorField to a file an it displayed 5-digit-numbers (4 digits after the dot). So there can't be any problem with this ...

Hope for your ideas ...
Sebastian
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Old   September 7, 2009, 15:17
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Matvej Kraposhin
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Values displayed with some tolerance (it means, that difference could be after 6 or N digits after dot)

Try to calculate and output difference between two values, or refomulate condition like inequality:

if
(
mag(faceCenterCoordinates[1].component(2) - faceCenterCoordinates[2].component(2)) < VSMALL
)
{
/// do something
}
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Old   September 8, 2009, 07:06
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Sebastian Gatzka
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Yes this worked!
The output from
Code:
mag(faceCenterCoordinates[1].component(2) - faceCenterCoordinates[2].component(2))
was 2.22045e-16 in my case and I had to use SMALL instead of VSMALL to make the comparison work.

By the way: What is the difference between SMALL and VSMALL and where is it defined?
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Old   September 8, 2009, 07:21
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VSMALL - means Very SMALL.

For single precision:
SMALL = 1e-6, VSMALL = 1e-37

For double precision:
SMALL = 1e-15, VSMALL = 1e-300

this definitions are related with machine word length, and, i think, it's done for independence of platform type

for more information, see file scalar.H
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