formula node bug?

[patrick.chaopricha]

Hello,

   I can't figure out what I'm doing wrong with the formula node.  I tried this on both LabView 2011 and Labview 2013 version and got the same results.  Basically I want to past any numbers with a decimal to the formula node.  If the number is more negative than -1 then set the value to -1 and if the number is more positive than 1 then set the number to +1.  Anything else just return the input to the output.  

This arithmetic works correctly when I use the LabView discrete "Select" function.  In the formula node I have this code:

if(Xin < -1.0)
   Xout = -1;
else
   Xout = Xin;

if(Xin > 1.0)
   Xout = 1;
else
   Xout = Xin;

Where Xin is the input node and Xout is the output node. It works well for anything over +1.  If you pass 1.5, you get 1.  If you pass 0.5 you get 0.5.  It even works for negative number greater than -1.  So passing -0.5 you get -0.5 etc.   The problem shows up when passing anything more negative than -1.  If I pass -1.5, I get -1.5 as the answer.  The Labview discrete select function method gives me the correct answer as -1 when passing -1.5.

I even tried to make sure that the input is casted to float just in case it got mixed up as an integer.  But the results are the same.  So either I don't understand the formula node syntax or there's something else going on.

float32 X =  Xin;
if(X < -1.0)
   Xout = -1;
else
   Xout = Xin;

if(X > 1.0)
   Xout = 1;
else
   Xout = Xin;

[johnsold · Accepted solution]

Consider the X in= -1.5 case. 

The first test sets Xout to -1.

Then the second test runs and sets Xout to Xin = -1.5.

Last test wins.

In LabVIEW primitives using the In Range & Coerce function might simplify things a bit and it would be very clear what you were trying to do.

Lynn

[patrick.chaopricha]

Ah, that makes a lot of sense!  I should probably use (else if) for the second test.  Using primitive case, this wasn't a problem since the result of the first test is passed to the 2nd test so the Xin value is actually the Xout from the previous test.  

if(Xin < -1.0)
   Xout = -1;
else if(Xin > 1.0)
   Xout = 1;
else
   Xout = Xin;

 Or I could also do 2 formula nodes (if the test gets complicated enough).

Thanks!

[Alexander_Sobolev]

What Johnsold mentioned, LabVIEW has very compact solutions for many common tasks

[patrick.chaopricha]

Thank you for the input.  I did have the "select" example in the code but it good to see other examples as well.

[rolfk]

What's wrong about the Sign function which does exactly what you tried to implement? 

[patrick.chaopricha]

Hmm .. sign function would work.  I don't remember seeing it in my pallette though.  I'll have to go back and look. I was using version 2011 and 2013.  That would be the simplest way to do it.

I was trying to implement something to basically force the spiral radius calculation to behave correctly.  I have found other ways to do it without using a formula node now though.

[GerdW]

Hi Rolf,

the Sign() function isn't correct in this case as the requirement was:

If the number is more negative than -1 then set the value to -1 and if the number is more positive than 1 then set the number to +1.  Anything else just return the input to the output.

InRangeAndCoerce is suitable here…

[patrick.chaopricha]

I also don't have the Sign function in 2011 version of LabView.  I wasn't able to check the 2013 version.  Is the output can only be -1, 0, and 1 with the Sign fuction?  If so then GerdW is correct that it doesn't meet my criteria since I wan the original number to come through if it's not greater than either signs.

I attach the picture from my 2011 LabView Comparison Pallette.  Maybe I can access the Sign Function in a different way?  It might be handy for other uses.

[RavensFan]

The sign function is on the numeric palette.