Check for optional inputs in a function using exist

Hi everyone. Is it wrong, prone to mistakes, or slower to check the existance of optional arguments in a function using exist()? For the purposes of the illustration, assume I have 2 required and 1 optional inputs named r1,r2 and op1, respectively.

function y = mafun(r1,r2,op1)

    if ~(exist('op1', 'var')==1)
        op1 = 'smthn';
    end

    %do smthn with r1,r2 and op1 (when needed)
    
end

[Walter Roberson]

if ~exist('op1', 'var')
    op1 = 'smthn';
end

would be cleaner.

Answer 1

[Walter Roberson]

I suspect that it is faster to check nargin.

Note: starting R2019b, it is possible to use an "arguments" block to do automatic default values.

https://www.mathworks.com/help/matlab/ref/arguments.html