Why Variable arguments (varargin, varargout) in MATLAB? - Purpose & Use Cases

The Big Idea

Discover how to make your MATLAB functions smart enough to handle any number of inputs and outputs effortlessly!

The Scenario

Imagine you are writing a function in MATLAB that needs to handle different numbers of inputs and outputs depending on the situation. Without variable arguments, you would have to write many versions of the same function or check each input one by one manually.

The Problem

This manual approach is slow and messy. You might forget to handle some cases, leading to errors. Also, your code becomes long and hard to read, making it difficult to maintain or update later.

The Solution

Using varargin and varargout lets your function accept any number of inputs and outputs flexibly. This keeps your code clean, easy to read, and adaptable to many situations without rewriting the function.

Before vs After

✗ Before

function result = addNumbers(a, b, c)
  if nargin == 2
    result = a + b;
  elseif nargin == 3
    result = a + b + c;
  else
    error('Wrong number of inputs');
  end
end

✓ After

function result = addNumbers(varargin)
  result = 0;
  for k = 1:nargin
    result = result + varargin{k};
  end
end

What It Enables

You can write one flexible function that adapts to many input and output needs, saving time and avoiding errors.

Real Life Example

Think of a calculator app that can add any number of numbers you enter, without needing a new function for each count of numbers.

Key Takeaways

Manual input handling is slow and error-prone.

varargin and varargout make functions flexible and clean.

One function can handle many input and output cases easily.

Practice

1. What does varargin do in a MATLAB function?

easy

A. Returns multiple output arguments as a cell array

B. Collects all input arguments into a cell array

C. Limits the number of input arguments to one

D. Specifies the data type of input arguments

Solution

1. Step 1: Understand the role of varargin

   varargin is used to collect any number of input arguments passed to a function into a single cell array.

2. Step 2: Differentiate from varargout

   varargout is used for output arguments, not input. So it does not collect inputs.

3. Final Answer:

   Collects all input arguments into a cell array -> Option B

4. Quick Check:

   varargin = input collector [OK]

Hint: Remember: 'varargin' collects inputs, 'varargout' returns outputs [OK]

Common Mistakes:

• Confusing varargin with varargout
• Thinking varargin limits inputs
• Assuming varargin specifies data types

2. Which of the following is the correct way to define a MATLAB function that accepts a fixed input followed by any number of additional inputs using varargin?

easy

A. function output = myFunc(input1, varargin)

B. function output = myFunc(varargout)

C. function output = myFunc(varargin)

D. function output = myFunc(varargin, input1)

Solution

1. Step 1: Check correct varargin placement

   varargin must be the last input argument to collect all remaining inputs.

2. Step 2: Evaluate options

   function output = myFunc(input1, varargin): function output = myFunc(input1, varargin) correctly places varargin after the fixed input input1. function output = myFunc(varargout) incorrectly uses varargout as an input. function output = myFunc(varargin) has no fixed input. function output = myFunc(varargin, input1) places varargin before a fixed input, which is invalid.

3. Final Answer:

   function output = myFunc(input1, varargin) -> Option A

4. Quick Check:

   varargin must be last input [OK]

Hint: Put varargin last in input list to accept extra arguments [OK]

Common Mistakes:

• Placing varargin before fixed inputs
• Using varargout as input
• Omitting parentheses in function definition

3. What will be the output of the following MATLAB code?

function varargout = exampleFunc(varargin)
  n = length(varargin);
  for k = 1:n
    varargout{k} = varargin{k} * 2;
  end
end

[a, b] = exampleFunc(3, 5); disp([a, b]);

medium

A. Error: varargout not assigned properly

B. [3 5]

C. [6 10]

D. [9 25]

Solution

1. Step 1: Understand varargin and varargout usage

   varargin collects inputs {3, 5}. The loop doubles each input and assigns to varargout.

2. Step 2: Calculate output values

   3 * 2 = 6, 5 * 2 = 10. So outputs are 6 and 10.

3. Final Answer:

   [6 10] -> Option C

4. Quick Check:

   Each input doubled = output [OK]

Hint: varargin inputs, varargout outputs; loop doubles inputs here [OK]

Common Mistakes:

• Confusing input and output values
• Forgetting to assign varargout elements
• Assuming varargout is a numeric array, not cell

4. Identify the error in this MATLAB function using varargin:

function result = sumAll(varargin)
  total = 0;
  for i = 1:varargin
    total = total + varargin{i};
  end
  result = total;
end

medium

A. Incorrect indexing of varargin with parentheses

B. Missing output argument varargout

C. No error, function works correctly

D. Using varargin as a number in the for loop

Solution

1. Step 1: Check for loop range

   The for loop uses 1:varargin, but varargin is a cell array, not a number.

2. Step 2: Correct way to get number of inputs

   Use length(varargin) or nargin to get number of inputs for the loop.

3. Final Answer:

   Using varargin as a number in the for loop -> Option D

4. Quick Check:

   Loop needs numeric length, not cell array [OK]

Hint: Use length(varargin) to loop over inputs, not varargin itself [OK]

Common Mistakes:

• Using varargin directly as a number
• Forgetting to use curly braces for cell access
• Assuming varargout is required for outputs

5. You want to write a MATLAB function that accepts any number of numeric inputs and returns their squares as multiple outputs. Which function definition and body correctly uses varargin and varargout to achieve this?

hard

A. function varargout = squareAll(varargin)
for k = 1:length(varargin)
varargout{k} = varargin{k}^2;
end
end

B. function varargout = squareAll(varargin)
for k = 1:nargin
varargout{k} = varargin(k)^2;
end
end

C. function varargout = squareAll(varargin)
for k = 1:length(varargin)
varargout(k) = varargin{k}^2;
end
end

D. function varargout = squareAll(varargin)
for k = 1:nargout
varargout{k} = varargin{k}^2;
end
end

Solution

1. Step 1: Check input count method

   Using length(varargin) correctly gets the number of inputs. nargin inside the function counts all inputs including fixed ones, but here only varargin inputs matter.

2. Step 2: Check cell indexing and assignment

   Accessing inputs with varargin{k} and assigning outputs with varargout{k} using curly braces is correct. Parentheses would cause errors.

3. Final Answer:

   function varargout = squareAll(varargin) for k = 1:length(varargin) varargout{k} = varargin{k}^2; end end -> Option A

4. Quick Check:

   Use length(varargin) and curly braces for cell access [OK]

Hint: Use length(varargin) and curly braces {} for cell arrays [OK]

Common Mistakes:

• Using parentheses instead of curly braces for cell access
• Using nargin instead of length(varargin)
• Assigning outputs with parentheses instead of curly braces