Wrong in qt calculation?

[bollibompa]

Hi,
I am pulling my hair.....
I am trying to send a Rate to a pump (throung RS-232). I have done this is qt3 about 6 years ago and I am know converting it all to qt4.

The problem is that I (according to me and my calculations in excel) that qt is calculating the rate almost right, but every other timevalue(second) is calculates it wrong.

Part of source code:

void pump::loop()
{
    float amount;
    float rate;
 
 amount = getAmount(kB, vf, (timeValue), bC);
  rate = getRate(kA, kB, vf, (timeValue),bC);
 
timeValue = timeValue + 1000;
 
float myQtApp::getAmount(float kB, float vf, float timeValue, float bC)
{
    float amount;
amount=expf(-kB*timeValue)*bC*vf;
float myQtApp::getRate( float kA, float kB, float vf, float timeValue, float bC)
   return(amount);
}
 
float myQtApp::getRate( float kA, float kB, float vf, float timeValue, float bC)
{
float rate;
float ga1 = getAmount(kB,vf,timeValue, bC);
float ga2 = getAmount(kB,vf, (timeValue+1), bC);
 
rate = -(expf(-kA) * ga1 - ga2);  // it is here the calculation problem ocuurs
return(rate);
}

void pump::loop()
{
    float amount;
    float rate;

 amount = getAmount(kB, vf, (timeValue), bC);
  rate = getRate(kA, kB, vf, (timeValue),bC);

timeValue = timeValue + 1000;

float myQtApp::getAmount(float kB, float vf, float timeValue, float bC)
{
    float amount;
amount=expf(-kB*timeValue)*bC*vf;
float myQtApp::getRate( float kA, float kB, float vf, float timeValue, float bC)
   return(amount);
}

float myQtApp::getRate( float kA, float kB, float vf, float timeValue, float bC)
{
float rate;
float ga1 = getAmount(kB,vf,timeValue, bC);
float ga2 = getAmount(kB,vf, (timeValue+1), bC);

rate = -(expf(-kA) * ga1 - ga2);  // it is here the calculation problem ocuurs
return(rate);
}

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it if very small number so I think an expanation could possibly be that qt rounds off the values. How can I tell qt to include as many decimals as possible in this calculations?

Thanks in advance!
Thomas

[wysota]

Qt is not calculating anything (you're not even using a single line of code related to Qt here), it's your CPU that performs calculations based on what the C/C++ runtime provides to it. Remember that float type has a limited precision.

[SixDegrees]

I agree with wysota. Try using type 'double' instead of type 'float', and use the double form of exp().

[bollibompa]

Hi,
Thank for answer
I changed float to double but I still have the same problem.

Can I add some code that tells c++ to use as many decimals as possible.
/Thomas

[wysota]

You can use a fixed point arithmetics library.

[SixDegrees]

C++ always performs computations at the full precision allowed by the data type. If doubles aren't working, you can try to convert the problem into an integer expression, or analyze the order of operations to try and minimize effects caused by limited precision, such as adding a very small number to a relatively very large number. Again, wysota's suggestion of using fixed point point math is also viable, although it may not be necessary.

If you could post the inputs, intermediate results and outputs of your functions with two calls, and the expected results and discrepancies, we might be able to lend more assistance.

------------------------

Something else to consider: what, exactly, is "wrong" with the results? It's entirely possible that Excel is doing some rounding itself when displaying results; C++ may also not be displaying the full precision of the numbers used, depending on how you're outputting them.

[bollibompa]

You are the best!
I removed the expf and replaced with exp and it now works perfectly!
Many many thanks!!
/Thomas

[wysota]

Just to be sure - you did change the expf() call to exp(), right? And you are sure the order of operations is correct, I mean that the brackets are positioned correctly?

rate = -(exp(-kA) * ga1 - ga2);

rate = -(exp(-kA) * ga1 - ga2);

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vs

rate = -(exp(-kA) * (ga1 - ga2));

rate = -(exp(-kA) * (ga1 - ga2));

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