After requesting input of str2func, how do I make sure the user wrote a valid function?

Question

Santiago Oliveira 2018-11-20

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编辑： Adam Danz 2018-11-20

I have a part of a script that requires the user to type a function, which needs to be only dependant of x. something like sin(x), or cos(2*x), or x^2+1, or etc. I have used:

func=str2func(['@(x)' input('Please, write f(x):','s')]);

Now, if the user writes: c^2, or c+2, or abcdef, it gives me a red error message. I really want it to give an error message saying something like:

"The function you wrote is invalid. Repeat."

"Please, write f(x):"

allowing the user to write a correct function of x.

Thank you!

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Answer 1

Walter Roberson 2018-11-20

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try

func(0)

catch

.... do appropriate rejection here

end

note: consider using vectorize()

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Jan 2018-11-20

在 MATLAB Online 中打开

Or with the loop:

ready = false;
while ~ready
   func = str2func(['@(x)', input('Please, write f(x):','s')]);
   try
       func(0);
       ready = true;
   catch ME
       fprintf('Function rejected. Please, write f(x):\n');
   end
end

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Answer 2

Adam Danz 2018-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/431026-after-requesting-input-of-str2func-how-do-i-make-sure-the-user-wrote-a-valid-function#answer_348047

在 MATLAB Online 中打开

The solution below uses a try/catch statement nested within a while loop that tests the function. If the function passes, the while loop ends. If the function fails, 1) a message appears indicating why it failed and 2) the user is propted to try again. I also include a maximum number of failures (5) before the code gives up on the user and quits.

One word of caution, to test if the function works, I'm using the input value of 1. In the unlikely event that this input breaks the user's function, you could either replace that as the test value or build in a 2nd test in the 'catch' section.

errorCount = 0; 
confirmed = false; 
while ~confirmed
    func=str2func(['@(x)' input('Please, write f(x):','s')]);
    try
        func(1);
        confirmed = true; 
    catch ME
        % If the user makes 5 mistakes in a row, quit and throw error
        errorCount = errorCount + 1; 
        fprintf('The function you wrote is invalid: %s\n', ME.message)
        if errorCount >= 5
            rethrow(ME)
        end
    end
end