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functionmatlab
Akshay Madan
Akshay Madan

Reputation: 85

Use varargin for multiple arguments with default values in MATLAB

Is there a way to supply arguments using varargin in MATLAB in the following manner?

Function

func myFunc(varargin)
  if a not given as argument
    a = 2;
  if b not given as argument
    b = 2;
  if c not given as argument
    c = a+b;
  d = 2*c;
end

I want to call the above function once with b = 3 and another time while the previous one is running in the same command window with a = 3 and c = 3 and letting b take the default value in the function this time. How can it be done using varargin?

Upvotes: 1

Views: 1305

Answers (2)

Wolfie
Wolfie

Reputation: 30165

Here are two ways to do this which have been available since 2007a (i.e. a long time!). For a much newer approach, see Edric's answer.

  1. Use nargin and ensure your inputs are always in order
  2. Use name-value pairs and an input parser

nargin: slightly simpler but relies on consistent input order

function myFunc( a, b, c )
    if nargin < 1 || isempty(a)
        a = 2;
    end
    if nargin < 2 || isempty(b)
        b = 2;
    end
    if nargin < 3 || isempty(c)
        c = a + b;
    end
end

Using the isempty check you can optionally provide just later arguments, for example myFunc( [], 4 ) would just set b=4 and use the defaults otherwise.

inputParser: more flexible but can't directly handle the c=a+b default

function myFunc( varargin )
    p = inputParser;
    p.addOptional( 'a', 2 );
    p.addOptional( 'b', 2 );
    p.addOptional( 'c', NaN ); % Can't default to a+b, default to NaN
    p.parse( varargin{:} );

    a = p.Results.a;
    b = p.Results.b;
    c = p.Results.c;
    if isnan(c) % Handle the defaulted case
        c = a + b;
    end
end

This would get used like myFunc( 'b', 4 );. This approach is also agnostic to the input order because of the name-value pairs, so you can also do something like myFunc( 'c', 3, 'a', 1 );

Upvotes: 1

Edric
Edric

Reputation: 25160

Here's the latest and greatest way to write the function (using arguments blocks from R2019b)

function out = someFcn(options)
arguments
    options.A = 3;
    options.B = 7;
    options.C = [];
end
if isempty(options.C)
    options.C = options.A + options.B;
end
out = options.A + options.B + options.C;
end

Note that this syntax does not allow you to say options.C = options.A + options.B directly in the arguments block.

In MATLAB < R2021a, you call this like so

someFcn('A', 3)

In MATLAB >= R2021a, you can use the new name=value syntax

someFcn(B = 7)

Upvotes: 1

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