Experimental Physics and
Industrial Control System

"J. Lewis Muir" <[email protected]> · Wed, 30 May 2012 12:06:05 -0500

On 5/30/12 11:11 AM, Andrew Johnson wrote:
> Hi Lewis,
> 
> On 2012-05-30 Allison, Stephanie wrote:
>> Could this be a round-off error (long to double)?  Does this CALC work:
>>
>> field(CALC, "A>0.5?1:0")
> 
> If that works, also try the expression "!!A" which is more efficient than 
> using ?: anyway.  In general I never assume that a true value is 1, it's 
> better to check for non-zero, so use "A!=0?1:0" or just "A?1:0".

Hi, Andrew.

Yes, "!!A" works too.

I'm not actually assuming a true value is 1.  I'm simply
comparing the value of A with 1 to see if the bi record value is
1.  That should work.  The bi record does not indicate true or
false; it indicates two values: 0 or 1.  I could just as easily
be using an mbbi record and doing a comparison of A with the 2
to see if the mbbi record value is 2.

My CALC expression shows the problem, but it is not what I would
normally write by hand.  The real situation is that I have a
database where the bi value corresponding to the "inserted"
state of a pneumatically controlled backlight is specified as a
macro (in this case named "INSERTED").  So the CALC expression
in the database actually looks like this:

  field(CALC, "A=$(INSERTED)?1:0")

This enables the database to work with a pneumatically
controlled backlight where the actual state (i.e. inserted or
retracted) is reported as a high *or* low digital I/O signal.

Maybe I could do something tricky like setting the INSERTED
macro to "!" or the empty string "" depending on how the actual
state is reported to achieve your CALC expression, but that
would be way less readable and therefore a poorer choice to me.

> However I admit I would be very surprised if the integer to double conversion 
> is not exact for the value 1.

Right.  And I'm sure I'm not the only one writing CALC
expressions like this.  There have got to be hundreds of
expressions (probably thousands) out there that would break
under these circumstances.

I also tried "nint(A)=1?1:0".  This does not work.

Thanks,

Lewis

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System