Calculating intersection of circles giving NaN first 'round'.

Question

See attachment.
When I give the correct parameters as input, the output is always NaN.
However, when I run the VI again, the correct x,y coordinates are returned.
Obviously this is not correct behaviour I want in my VI. What did I do wrong here?
Thanks,
Marvin
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Attachments:
Calculate Intersection Circles.vi ‏32 KB

Answer 1

marvin117 wrote:
What's the point of local variables if they don't work that way?
Although local variables are often demonized, there are proper uses for them.  Coming from a text-programming background, you're used to using variables and, presumably, not accustomed to things happening in parallel.  The first thing I noticed, which is certainly not obvious to anyone, is that your local variables are read in every frame of the Sequence structure immediately, but the indicators aren't written until the calculations happen in the first frame.  In this way, the Sequence structure is highly non-intuitive.  National Instruments owes you an apology for that.
The second thing I noticed was, in your second frame, that you were using the results of calculations with a local variable in the same frame.  You'll never be able to know that the calculations will happen before the locals are read.  There's your classic race condition.
I (almost) only use local variables to update front panel control values.  If I find myself using them to read control values, I hang my head and wish I had designed my code better.  Try to think in terms of "The wire is the variable."  It's almost always the right way to think about data, and data flow.
Jim
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