Variable arguments (varargin, varargout) in MATLAB - Step-by-Step Execution

Concept Flow - Variable arguments (varargin, varargout)

Function called with arguments

↓

Inside function: varargin collects inputs

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Process inputs using varargin

↓

Prepare outputs

↓

varargout assigns outputs

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Function returns outputs

When a function is called, varargin collects all input arguments into a cell array. The function processes them and assigns outputs to varargout, which returns variable number of outputs.

Execution Sample

MATLAB

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

This function doubles each input argument and returns them as outputs.

Execution Table

Step	Action	varargin	n	Loop k	varargout	Output Produced
1	Function called with inputs (2, 5, 10)	{2, 5, 10}	3	-	{}	-
2	Calculate n = length(varargin)	{2, 5, 10}	3	-	{}	-
3	Start loop k=1	{2, 5, 10}	3	1	{}	-
4	Assign varargout{1} = varargin{1}*2	{2, 5, 10}	3	1	{4}	-
5	Loop k=2	{2, 5, 10}	3	2	{4}	-
6	Assign varargout{2} = varargin{2}*2	{2, 5, 10}	3	2	{4, 10}	-
7	Loop k=3	{2, 5, 10}	3	3	{4, 10}	-
8	Assign varargout{3} = varargin{3}*2	{2, 5, 10}	3	3	{4, 10, 20}	-
9	End loop, return varargout	{2, 5, 10}	3	-	{4, 10, 20}	[4, 10, 20]

💡 Loop ends after k=3 because n=3, all inputs processed and outputs assigned.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
varargin	{}	{2,5,10}	{2,5,10}	{2,5,10}	{2,5,10}
n	0	3	3	3	3
k	-	1	2	3	-
varargout	{}	{}	{4}	{4,10}	{4,10,20}

Key Moments - 3 Insights

Why is varargin a cell array and not a normal array?

varargin collects inputs of any type and number, so it stores them in a cell array to hold different data types and sizes, as shown in execution_table row 1.

How does varargout know how many outputs to return?

varargout is assigned inside the loop up to n, the number of inputs. The function returns as many outputs as assigned, shown in execution_table rows 7-9.

What happens if the function is called with no inputs?

varargin is empty, n=0, so the loop does not run and varargout remains empty, returning no outputs. This is implied by the initial state in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 5. What is the value of varargout?

A {4}

B {4, 10}

C {}

D {4, 10, 20}

💡 Hint

Check the varargout column at step 5 in the execution_table.

At which loop iteration does the function assign varargout{3}?

A k=1

B k=3

C k=2

D k=4

💡 Hint

Look at the Loop k column and varargout assignments in execution_table rows 7 and 8.

If the function is called with only one input, how many outputs will varargout have?

A 0

B 2

C 1

D 3

💡 Hint

Refer to variable_tracker for n and varargout size relation.

Concept Snapshot

function varargout = funcName(varargin)
  % varargin collects inputs as cell array
  % varargout returns outputs as cell array
  n = length(varargin);
  for k = 1:n
    varargout{k} = process(varargin{k});
  end
end
Use varargin to accept any number of inputs and varargout to return any number of outputs.

Full Transcript

This visual trace shows how MATLAB functions use varargin and varargout to handle variable numbers of inputs and outputs. When the function is called, all inputs are collected into varargin, a cell array. The function calculates the number of inputs n, then loops from 1 to n. In each loop, it processes the input at position k and assigns the result to varargout at the same position. After the loop, varargout contains all outputs, which the function returns. The variable tracker shows how varargin, n, k, and varargout change step-by-step. Key moments clarify why varargin is a cell array, how varargout determines output count, and what happens with no inputs. The quiz tests understanding of varargout values at specific steps, loop iteration assignments, and output count for different input sizes.

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