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

Concept Flow - Variable arguments (varargin, varargout)

Function called with arguments

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Inside function: varargin collects inputs

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Process inputs using varargin

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Prepare outputs

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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.