Using the command "eval"

I want to use the command eval, but I want my program to display an error message like: "Please try again" if the eval command doesn't work.

For example, if the user inputs:

eval('2^3')

then it's all good and the answer should be 8.

But if the user inputs:

eval('dg@^4')

then MATLAB displays: "Please try again"

[Rik]

Why do you want to do this in the first place?

[Image Analyst]

What do you want it to say instead, when the user enters some illegal input?

See Adam's Answer below for some more graceful ways to authenticate user input.

Answer 1

[Adam Danz]

Don't use eval() at all.

• Matlab documentation: Alternative eval()
• Matlab Fandom: Why is it advised to avoid using the "eval" function?
• Lots of experienced Matlab users strongly recommend to avoid eval()

If the user should enter an expression and you want to test that the expression is functional,

responseOK = false;
while ~responseOK
    try
        response = input('Enter an expression (example: 2^3): '); % without quotes
        if isnumeric(response)
            responseOK = true;
        else
            fprintf('The result must be numeric.\n')
        end
    catch ME
        fprintf('Please try again\n')
        % you could use the MException (ME) to provide 
        % additional feedback on the error
    end
end
disp(response)

If a string (or character vector) is supplied that should be evaluated as an expression, see Karan Gill's suggestion to use str2sym or you can convert it to an anonymous function but this introduces some of the risks associated with eval().

responseOK = false;
while ~responseOK
    try
        response = input('Enter an expression (example: "2^3"): '); % without quotes
        if ischar(response) || isstring(response)
            anonFcn = str2func(['@()',char(response)]); 
            z = anonFcn(); 
            responseOK = true;
        else
            fprintf('Please enter a string or character array.\n')
        end
    catch ME
        fprintf('Please try again\n')
        % you could use the MException (ME) to provide 
        % additional feedback on the error
    end
end
disp(z)