# How to evaluate and plot a set of equations with multiple points for a specific variable

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aquiano on 6 Sep 2016
Commented: Joost on 8 Sep 2016
I have RE = V*D/NU and another equation C = 3.*RHOBAR/(4.*D) both variables RE and C appear in other equations, which will eventually end up in a plot. I want set multiple values for D. How do I solve this? As of right now I only have D=.01 but I would like to include 5 more values. Is there a way to set this up with making D a matrix, for example D=[.01 .05]
Attached is pdf that has the code of the 2 files that are used to plot.

Joost on 7 Sep 2016
Edited: Joost on 7 Sep 2016
In the equation C = 3.*RHOBAR/(4.*D) you use element-wise multiplication (.*) but the division is not element-wise. If you turn the / in a ./ then you can use a vector for D and get the same number of elements in C.
Possibly you need to use element-wise multiplication and division in the equation for RE as well, that is dependent on the application and domain.
I noticed in your pdf file a potentially dangerous thing: equal comparison with doubles ( if R==0 ). (note that 0.5-0.4-0.1 == 0 returns false in Matlab due to numerical significance issues.) It is better to use tolerances in the comparison.
aquiano on 7 Sep 2016
Do you have any suggestions on how to fix that part of the code (R==0)? The parts below it a are showing an error for(&&). Thanks any help would be appreciated
Joost on 8 Sep 2016
You can have a look at this page: Compare numbers for equality within a tolerance.

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