Check if a function input exists in workspace

Dear all,

Simple question about checking if a variable exists. I want my function to check if a given parameter exists in the workspace before attempting to modify it. If it doesn't exist, my function should behave differently.

Something like this:

function out1 = myFun(in1,in2)

if( exist(in1, 'var') )
    out1 = in1;
else
    out1 = in2;
end

Unfortunately, exist requires the varname in a string. I'm sorry to say that I don't know how to get around this. I've read that using eval should be avoided, otherwise I considered making the first input called as a string, e.g. myFun('in1',in2)...

function out1 = myFun(in1,in2)

if( exist(in1, 'var') )
    out1 = eval(in1);
else
    out1 = in2;
end

I tried using exist( inputname(1), 'var' ) without any luck. Possibly, this is a scope issue that eludes me. Could someone point me in the right direction?

Regards, Eric

Answer 1

[Stephen23]

The fastest and most reliable way to check which inputs have been provided is to use nargin, like this:

function out1 = myFun(in1,in2)
switch nargin
    case 0
        ...
    case 1
        ...
    otherwise
        ...
end
end

If this variable is simply popping into existence in your workspace (via eval, assignin, or one of the other awful methods that beginners insist on using to pass variable between workspace), then you are learning why these methods are awful ways to pass variables:

http://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

http://www.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html

How does it end up in the workspace without you knowing its name, or whether it exists?

Answer 2

[Star Strider]

I would use the who or whos function to check for workspace variables.

For example:

variables = who