fmod error

[matth3w6247]

Hello

I am programming in CVI 8.5.1.  I simply want to see if a number divides evenly into another.  To do this I'm using fmod and wrote a small program to see if it works as I think.

int CVICALLBACK Divide (int panel, int control, int event, void *callbackData, int eventData1, int eventData2)
{
 double divisor, dividend, remainder;
 switch (event)
 {
  case EVENT_COMMIT:
   GetCtrlVal(panel, PANEL_DIVIDEND, &dividend);
   GetCtrlVal(panel, PANEL_DIVISOR, &divisor);
   remainder = fmod(dividend, divisor);
   
   SetCtrlVal(panel, PANEL_REMAINDER, remainder);
   
   break;
 }
 return 0;
}

This function does not respond as I expect.  For example 

dividend: 1.00

divisor: 0.10

The function returns 0.1 as the remainder of (dividend/divisor)

And

dividend: 0.30

divisor: 0.10

returns remainder = 0.1

And

dividend: 0.40

divisor: 0.10

returns remainder = 0.0

And

dividend: 0.50

divisor: 0.10

returns remainder = 0.1

I do not know what to think when I check it with a calculator and the answer is not the same

Can I get an answer to

(1) why I don't get 0.00 for all the remainder for the above cases

(2) how can I compute if two number evenly divide into each other.

Thank you

Matt

[Wolfgang]

hm, don't know, but did you check that your numeric control PANEL_REMAINDER permits displaying all values? So if you open the control panel, check if lower limit and upper limit are set properly and allow all possible results...

Wolfgang

[Wolfgang]

you may forget my previous comment, I have tried calling this function without the help of any control;

here are some results:

fmod ( 1.0; 1.0 ) works fine (0)

fmod ( 1.0; 0.5 ) works fine (0)

fmod ( 1.0; 0.2 ) seems buggy: ( 0.2 )

fmod ( 1.0; 0.1 ) seems buggy, too ( 0.1 ), as reported by you

So it appears that you have discovered a bug...

Wolfgang

[RobertoBozzolo]

This item has been discussed a while ago in the forum: look at this discussion and the linked ones. Basically it seems that there is a problem given by the accuracy on floating number internal representation, that influences fmod operator. In one of the linked discussions a LabVIEW vi is described that covers this issue; the description is detailed so it should not be difficult to port this approach to CVI even without opening the VI.

Another approach could be to define a significant level of approximation (tenths, hundredths,... ?), multiply bt this factor, cast to an integer and apply integer remainder operator %