Value must be a double scalar error in matlab app designer?

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Robert Pritchard 2019-4-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/453980-value-must-be-a-double-scalar-error-in-matlab-app-designer

I am working on making a simple solver in matlab app designer and keep getting the error that the value must be a double standard and have no clue why it wouldnt be. When i run it just using the equation instead it works fine but i want to use the solver so I can add some more difficult equations.

 value = app.CalculateButton.Value;
            X=app.UnknownVariableListBox.Value
            R=8.314
            P=app.PressurePaEditField.Value
            n=app.MolesEditField.Value
            T=app.TemperatureKEditField.Value
            V=app.Volumem3EditField.Value
            eqn= P*V == n*R*T
             if strcmp(X,'Volume')
                 syms V
                 V=solve(eqn,V);
                 V=double(V)
%                V=n.*R.*T./P
                 app.Volumem3EditField.Value=V