Checking existence of a input argument to a Matlab function

Question

I'm trying to check the input of a Matlab function to see whether the user has forgotten about it or not (which is easy to do in this case).

If the user has not supplied number_obs then I want to pause the program and wait for the user to input this information.

Some other StackOverflow posts seem to suggest using ~exist however this doesn't seem to work. Can anybody suggest what I'm doing wrong here?

function output=test(number_obs) 
if ~exist('number_obs'),
    number_obs=input('How many observations do you have in your experiments?')
end

The Python equivalent would be something like:

def test(number_obs):
    if nummber_obs != None:
        output=raw_input('How many observations do you have in your experiments? :')
    return output

Your Python code does not work correctly, as the number_obs does not have a default value. — Jørgen
– Jørgen, Commented Jan 14, 2016 at 11:46
What is not working? Asking for an input when you already passed it or the opposite case? — Daniel
– Daniel, Commented Jan 14, 2016 at 13:43
What do you mean by "this doesn't seem to work"? The MATLAB version works flawlessly. — dasdingonesin
– dasdingonesin, Commented Jan 14, 2016 at 13:59

Steve · Accepted Answer · 2016-01-14 11:13:25Z

6

You can do this with nargin

function output=test(number_obs) 
if nargin<1
    number_obs=input('How many observations do you have in your experiments?')
end

answered Jan 14, 2016 at 11:13

Steve

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Comments

Carl Witthoft · Accepted Answer · 2016-01-14 18:26:32Z

2

(Edited to correct the truth) It may not matter here, but to be on the safe side, you should always specify the type of object you're checking. In your case, its a 'var', so

if ~exist('number_obs','var'),

Thanks to dasdingonesin for pointing this out.

edited Jan 14, 2016 at 18:26

answered Jan 14, 2016 at 13:38

Carl Witthoft

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2 Comments

dasdingonesin Over a year ago

It doesn't need to. The second argument is optional (see de.mathworks.com/help/matlab/ref/exist.html).

Carl Witthoft Over a year ago

@dasdingonesin I plead guilty to not reading up-to-date help files.

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