Assign value if not exist in MATLAB

In MATLAB, you can assign a value to a variable if it does not already exist using the exist function to check if the variable exists, and an if statement to assign a value if it does not exist. Here's an example:

% Check if the variable x exists
if ~exist('x', 'var')
    % Assign a value to x if it does not exist
    x = 10;
end

In this example, we first use the exist function to check if the variable x exists. The first argument to exist is the name of the variable, and the second argument is the string 'var', which tells exist to check if it is a variable.

The exist function returns 1 if the variable exists, and 0 if it does not exist. We use the logical NOT operator (~) to negate the result, so that the if statement will be true if the variable does not exist.

If the if statement is true, we assign the value 10 to the variable x. If the variable already exists, the if statement will be false, and the code inside the if block will not be executed.

Here's another example that uses the same approach to assign a default value to a function argument if it is not provided:

function y = myfunc(x)
    % Check if the argument x exists
    if ~exist('x', 'var')
        % Assign a default value to x if it does not exist
        x = 1;
    end
    
    % Compute the function value
    y = x + 2;
end

In this example, we define a function myfunc that takes one argument x. Inside the function, we check if x exists using the same approach as before. If x does not exist, we assign a default value of 1 to x.

We then compute y by adding 2 to the value of x, and return the result. If the caller does not provide an argument for x, the default value of 1 will be used.