Working with an equation and variables from user.

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Patrik Matta 2021-3-11

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评论： Patrik Matta 2021-3-11

Hello, I would like to get equation of two variables (x,y) from user's input. 'Enter the equation f(x,y): '. Then ask user for value of each variable 'Set value of x: ' and 'Set value of y: '. When all variables are in I want to evaluate the equation with variables. After this is it possible to countinue to work with the equation, to make a partial derivative evaluate that etc.? I'll be gratefull for any advice.

str1 = input('Enter and equation f(x,y); ','s');
x = input('Set the value of x: ');
y = input('Set the value of y: ');
% the rest was unsuccessful in my code

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James Tursa 2021-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/769907-working-with-an-equation-and-variables-from-user#answer_645372

编辑：James Tursa 2021-3-11

在 MATLAB Online 中打开

You can construct a function handle as follows:

f = str2func(['@(x,y)' str1]);

You can then use this function handle to evaluate at specific values of x and y.

If you want to take derivatives of this downstream in your code, you could create symbolic expressions to work with. E.g.,

syms x y
fxy = f(x,y)

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James Tursa 2021-3-11

编辑：James Tursa 2021-3-11

在 MATLAB Online 中打开

If you want to work with both numeric and symbolic at the same time, then pick different variable names for them. E.g., let small case x and y be the symbolic and upper case X and Y be numeric. Then you can work with both at the same time. You can use the subs( ) function to get numeric values into a symbolic expression. E.g.,

str1 = input('Enter f(x,y); ','s');
f = str2func(['@(x,y) ' str1]);
% Use f with numeric variables X and Y (uppercase)
X = input('Set x: ');
Y = input('Set y: ');
disp('Numeric Results:');
fprintf('%s evaluated at x = %d and y = %d = %d\n',str1,X,Y,f(X,Y));
% Use f with symbolic variables x and y (lowercase)
disp('Symbolic Results:');
syms x y 
F_xy = f(x,y);
disp('Expression:')
disp(F_xy);
disp('Derivative with respect to x:')
disp(diff(F_xy,x));
fprintf('Evaluated at x = %d and y = %d\n',X,Y);
subs(diff(F_xy,x),[x y],[X Y]);
disp('Derivative with respect to y:')
disp(diff(F_xy,y));
fprintf('Evaluated at x = %d and y = %d\n',X,Y);
subs(diff(F_xy,y),[x y],[X Y]);