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DJF16-Pearl
16-Pearl
December 17, 2019
Solved

Switch between input types

  • December 17, 2019
  • 3 replies
  • 2492 views

I'm often provided with a flow rate, but it can be specified in either volume flow or mass flow.  I want a simple way for my sheets to work regardless of what is specified (i.e. calculate the one that isn't defined from the other).  My goal is to have the two inputs, but the one I don't need is disabled.  I thought this would be easy, but I'm not finding a solution other than to enable/disable equations (which could get messy and dangerous) or assign flags telling mathcad what to do (also dangerous).  In excel I could use ISNUMBER or similar, but I don't seem to find that in Mathcad.  I thought TRY would work, but even that quits when it reaches an undefined variable.  I tried a few things with NaN but still no luck.  Any suggestions?  Thanks.

2019-12-17_8-24-01.jpg

 

4.0 attached.

Best answer by Fred_Kohlhepp

What might work would be to predefine both flow rates negative.  (You could put this at the top in a closed area so it doesn't show.)  Down where it matters you define the one you have, then a simple if statement will do what you want.

 

Prime 4.0 attached

3 replies

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Werner_E25-Diamond I
25-Diamond I
December 17, 2019

AFAIK there is no way to check if a variable already has been assigned a value.

You may set a variable to NaN and check using IsNaN(variablename).

Or you may set the variable to Zero or to string saying "not assigned" and check for that.

The point is, that the variable must have been assigned any value if you intend to use it in your routine.

 

T
tietjee15-Moonstone
15-Moonstone
December 17, 2019

Prime does not a function to determine is a variable is defined.  Attached is snippet that is a work around, but not a complete solution.

 

Cheers 

F
Fred_Kohlhepp23-Emerald IAnswer
23-Emerald I
December 17, 2019

What might work would be to predefine both flow rates negative.  (You could put this at the top in a closed area so it doesn't show.)  Down where it matters you define the one you have, then a simple if statement will do what you want.

 

Prime 4.0 attached

D
DJF16-PearlAuthor
16-Pearl
December 17, 2019

Thanks Fred.  A bit unusual but that should work.  A slight improvement would be to define them as imaginary numbers instead of negatives.  Either way works but if you started with multiple streams and adding flows, a negative could get lost.  However I'm unlikely to have a imaginary flow rate.

W
Werner_E25-Diamond I
25-Diamond I
December 17, 2019

In my opinion NaN would be a more "natural" initial assignment compared to negative or non-real numbers (with units!?), because NaN tells the whole story - its "not a number". Using that variables in calculations with units should not throw any error and any calculation done with them should result in NaN, too, so its easy to spot.

 

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