Trouble using solve function in App Designer

Question

Thomas Engel 2018-4-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/392204-trouble-using-solve-function-in-app-designer

评论： Mukesh Kumar 2020-10-10

I can't solve the error "Struct contents reference from a non-struct array object." with the code below. The code works well in Matlab. This code is a function inside App Designer and that's where error occurs. Error occurs on last line when I try to change editbox value. Thanks.

        function SolveButtonPushed(app, event)
            clear x1 x2 x3 x4 x5 x6 b1 b2 b3 b4 b5 b6        %Clear symbolic variables
            syms x1 x2 x3 x4 x5 x6 b1 b2 b3 b4 b5 b6         %Declare symbolic variables
                                                             %Read matrix entries and create arrays based on SysOrder
            if app.SysOrder >= 1
                b1 = str2num(app.Source1.Value);
                eqns = [x1 * str2num(app.Entry11.Value) == b1];
            end
            S = solve(eqns);
            app.Solution1.Value = [num2str(S.x1)];
        end

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Answer 1

Alberto Rota 2019-1-2

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https://ww2.mathworks.cn/matlabcentral/answers/392204-trouble-using-solve-function-in-app-designer#answer_354672

在 MATLAB Online 中打开

In this case, eqns is not a symbolic expression, but a logical value that will be true if

x1 * str2num(app.Entry11.Value) == b1

and false otherwise.

When you call the solve function, the parameter passed to the function is either a 0 or a 1, not a symbolic expression. To make it work you should counsider writing this code:

if app.SysOrder >= 1
    b1 = str2num(app.Source1.Value);
    eqns = x1 * str2num(app.Entry11.Value) - b1;
end
S = solve(eqns, x1);
app.Solution1.Value = [num2str(S.x1)];

Instead of considering the equation as f(x) = b, you consider it as f(x) - b = 0, because the only form that the solve function can work on is f(x) = 0, and then solve for x.

Please respond on this post to let me know if this solves your problem!