# plotting ode warning with nor plot returned

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jg on 18 Jan 2020
Answered: Walter Roberson on 18 Jan 2020
Trying to plot both of these ODEs against each other.
he error that is being returned is.
Warning: Error updating ParameterizedFunctionLine.
The following error was reported evaluating the function in FunctionLine update: Unable to convert expression into double array.
and the code is.
syms A(t) B(t)
N = 1000;
gamma = 0.1;
beta = 0.95;
eqns = [diff(A,t) == -beta*(B/N)*A+gamma*B, diff(B,t) == beta*(B/N)*(N-B)-gamma*B];
S = dsolve(eqns);
fplot(A,B)

Star Strider on 18 Jan 2020
It helps to look at the dsolve results:
Bsol = S.B
Asol = S.A
that are:
Bsol =
0
17000/19
-17000/(19*(exp(C1 - (17*t)/20) - 1))
Asol =
C3
exp(-(17*t)/20)*(C2 + (2000*exp((17*t)/20))/19)
2000/19 - C4/(19*(exp((17*t)/20) - exp(C1)))
Provide numerical values for the initial conditions ‘C1’ and ‘C2’ and the rest, and the plot should magically appear!

Walter Roberson on 18 Jan 2020
You dsolve() and store the answer but then you try to fplot the unprocessed A and B. At the very least you would want to plot S.A and S.B
However you would need to add boundary conditions in order to get S.A and S.B in a form that you can plot.