How to solve an operation written on a EditField?

I am new to Matlab, I have been using it to do some math. I created a very easy program with App Designer, the idea is that the user can enter almost any expression and then display the result, for example "7 + 4" or "4x + 2 = 0". The problem is, I don't know how to "convert" the text from the EditField to a mathematical expression. I did a little test with "2 + 4" using the "str2sym ()" function and the "num2str ()" function, but when setting the result in the other EditField, it shows me this error: Error using num2str (line 47) Input to num2str must be numeric.

methods (Access = private)
    % Button pushed function: CalculateButton
    function CalculateButtonPushed(app, event)
        expr = str2sym(app.ExpressionEditField.Value);
        
        app.ResultEditField.Value = num2str(expr);
    end
end

I don't know what to do, any recommendation is appreciated.

Answer 1

[Walter Roberson]

The result of str2sym() is symbolic. num2str() cannot convert symbolic to string.

Instead of using num2str(expr) you can use string(expr)

Note: the expression to be converted must be valid MATLAB syntax. "4x + 2 = 0" is not valid MATLAB syntax. The user would have had to have entered "4*x + 2 == 0", or else you would have to have processed what the user did enter in order to make it into valid MATLAB syntax.

MATLAB does not have any implied multiplication -- no "4x" as meaning to multiply x by 4. Not anywhere that I can find.

[Daniel Martínez]

Ok, I get it and yeah, it works with string(). This is what I done with the help you gave me:

properties (Access = private)

x = sym('x'); %creates the symbol

end

methods (Access = private)

% Button pushed function: CalculateButton

function CalculateButtonPushed(app, event)

expr = str2sym(app.ExpressionEditField.Value);

app.ResultEditField.Value = string(solve(expr, app.x));

end

end

Now I want to know if it's possible to give a value to a syms variable, for example "x = 2" and if the operation is "4*x + 2" then the result would be "10". You could help me with that or I have to ask another question?

[Walter Roberson]

Use subs(), like

syms x
y = 4 * x + 2

y = 

subs(y, x, 2)

ans = 

10

[Daniel Martínez]

Thanks, it's exactly what I was looking for.