Calculating a new variable from a symbolic equation

I'm new to Matlab so I hope you can help me :-). I'm using MATLAB r2019a.

I have a array (Q) that I would like to use as a input in a equation and solve for a variable (y). I need the equation to calculate a y for each Q. How do I do that?

So far I've tried this (simplyfied):

Q_full = 0.5

Q = [1,2,3]

syms Q Q_full y

eqn = vpasolve(Q/Q_full == 2+y,y)

The errors I get are:

Error using mupadengine/feval (line 195)

Symbolic parameters not supported in nonpolynomial equations.

Error in sym/vpasolve (line 172)

sol = eng.feval('symobj::vpasolve',eqns,vars,X0);

Error in Model (line 30)

eqn = vpasolve(Q/Q_full == 0.46-(0.5*cos(pi*(y/D)))+(0.04*cos(2*pi*(y/D))),y); <- this is my real equation

Answer 1

[Birdman]

You already defined values for Q_full and Q. You do not need to define them as symbolic variables later. This will not work. Try the following approach:

Q_full = 0.5
Q = [1,2,3]
syms y
for i=1:numel(Q)
    eqn(i) = vpasolve(Q(i)/Q_full == 2+y,y)
end

[Helena Hjørringgaard]

Thank you. It worked.

Answer 2

[Walter Roberson]

syms Q Q_full y

That line overwrites the numeric values that you assigned to Q and Q_full. That gives you an equation with more than one unresolved variable. vpasolve can only handle multiple unresolved variables for polynomials.

Do not do the syms on Q Q_full.

In any one call to vpasolve, vpasolve will attempt to find parameter values that satisfy all of the equations simultaneously. It never treats the inputs as a sequence of separate equations, only ever as simultaneous equations.

Therefore, either loop or else arrayfun

arrayfun(@(Qr) vpasolve(Qr == etc, y), Q./Q_full)

[Helena Hjørringgaard]

Thank you. It worked.