varargin

what is varargin used for ?and how to use it? and can I change it to an ordinary input variable?

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/techdoc/ref/varargin.html

There are situations in which it is not possible to use any fixed number of arguments, so conversion depends upon what your code does.

Answer 2

[Matt Tearle]

It is used for situations where you want to allow any number of input arguments (so you can't list them). A common reason for that is optional settings -- in MATLAB this is often done with property name/property value pairs. For example, think of how you can pass extra options to plot:

plot(x,y) % plain
plot(x,y,'LineWidth',4)
plot(x,y,'Marker','o','LineWidth',2)
plot(x,y,'Marker','o','LineWidth',2,'MarkerFaceColor','r')

etc. Here you don't want to force the user to input all the possible optional parameters in a specific order, so instead you allow them to specify 'MagicPropertyName' takes the value PropertyValue (eg 'LineWidth' takes the value 2).

To write a function that can do this, you'd make a function declaration like

function plot(x,y,varargin)

Then inside this function, x takes the value of the first input, y takes the value of the second, and all the rest go into cells of the cell array varargin. So, in the last example, varargin would be a 1-by-6 cell array; varargin{1} would be the string 'Marker', varargin{4} would be the number 2, etc.

So obviously there's a bunch of programming you have to do as the developer to parse the elements of varargin, and make your code behave accordingly.

I don't really understand the last question. Could you elaborate?

(Note: the actual plot function is actually even more flexible than this -- the above function declaration line is for illustration only.)

[Ahmed Tawfeeq]

I have a simulink project and I encounter a problem in calling the function adaptfilt.lms to matlab function block because of two reasons first : I can't find a way to define the data type of this function and second: adaptfilt.lms obviously have a dot..one of mathworks contributors suggests to use the lms.m (contains varargin) inside the [C:\Program Files\MATLAB\R2011a\toolbox\dsp\filterdesign\@adaptfilt\@lms\lms.m % adaptfilt.lms constructor

] to get rid of the problem of the dot but there is not benefit because an error message appears says: varargin cannot be used in top level functions...(that once a run the simulink of the block contains the lms.m)...so I decided that i use an ordinary variable to get rid of this problem....

[Walter Roberson]

You cannot pass adaptfilt.lms objects around in Simulink . No efforts along the line of calling lms.m directly are going to work.

[Walter Roberson]

Alternatives discussed in one of your other Questions.

Answer 3

[Benjamin Schwabe]

varargin is used when the number of input parameter might change. Basically, it puts all input arguments into a cell array. The number of input parameters is given by

nargin.

You can access any of the input values inside the function by

varargin{ind}

where ind is the index number of your argument.