shift registers and while loop problem

[kingkong]

Hi frens,

I got a problem here:

1. I want to continously operate a loop that compares SET power with MEASURED power:

    If SET = MEASURED power, loop stops.

       SET > MEASURED power, an increment of 1 will be added into existing hexadecimal string vice versa for SET < MEASURED.

So in the vi I have passed the string of hexadecimal code from previous loop via shift registers. Please help me check if I've done this procedure properly.

At present, this vi seems to be incrementing continously w/o stopping. Where is the problem?

Thanx alot.

KK 

Message Edited by kingkong on 04-25-2006 01:33 AM

[Dynamik]

Hi Kong,

      Look carefully at the condition being used to stop the loop.  Do the values being compared change while the loop is running?

You're comparing two floats for equality, so even if "Set" and "Measured" agree to 14 decimal places, you'll keep looping.    Use the "In Range and Coerce"  Function to terminate when values are close.

There's an uninitialized string shift-register which will have a value after the first run.  Subsequently, when you press run, control values will start with some (arbitrary) value.   Give the left shift-register an initial value (empty-string.)

Cheers.

Message Edited by Dynamik on 04-25-2006 02:29 AM

[kingkong]

Yup, you're right!

But what if my SET and MEASURED are variables? Then how do I use the 'in range and coerce' function?  

From what I gather from this 'in range and coerce' function, I hafta set 2 fixed values for upper and lower limit.

How about if I use 'string subset' function to extract out the number of digits of precision I require and then compare them? 

Rgds,

KK

[Ben]

You can make the upper limit

+Inf

Ben

[kingkong]

Sorry Ben, I dun quite get it.

If so, then how do I compare 2 values(which are variables) at 2 decimals place precision?  

[Ben]

Please disregard my last.

its a typical pre-cafeine reply.

Ben

[smercurio_fc]

Just create the upper/lower limit based on one of the values that you're using in your comparison:

    upper limit = SET + 0.1
    lower limit = SET - 0.1

or whatever offset is appropriate for your application.

[kingkong]

Hi friends,

Please have a look at the compare(testing).vi I created for comparing.

In this vi, 2 values(variables) are being compared and if both values are same, program stops. Else result is printed.  

Eg. SET power = 20.9333  Display = 20.9

       MEASURED power = 20.94 Display = 20.9

Result = SET > MEASURED

The above was input into the vi and it should take care of the comparing problem as mentioned by Dynamik earlier.

Am I right?

[kingkong]

Hi Dynamik,

Yes, the MEASURED value should always change as loop iterates. However when I debug, it stays constant => Hence my loop never stop incrementing!!

I have already ensured the points of precision for the values of MEASURED and SET, so it should only compare up to a decimal's place.

What should I do now?

Rgds,

KK

Message Edited by kingkong on 04-26-2006 05:42 AM

[smercurio_fc]

---

@kingkong wrote:

What should I do now?

---

Answer: Follow the advice given on this board, which said to NOT use an equality operator when comparing floats. Period, end of story. I'm sorry to be so blunt and forceful, but you don't seem to be understanding this point. You're confusing value with display. Your control is set to display only one digit of resolution, but the underlying value doesn't change! It's irrelevant how you display it - it only matters what its value is. Bottom line: DO NOT USE THE EQUALITY OPERATOR TO COMPARE FLOATING POINT VALUES.