Local functions in MATLAB - Step-by-Step Execution

Concept Flow - Local functions

Start main function

↓

Execute main code

↓

Call local function

↓

Run local function code

↓

Return result to main

↓

Continue main or end

↓

End

The main function runs its code, calls a local function defined inside it, which executes and returns a result back to the main function.

Execution Sample

MATLAB

function result = main()
  x = 5;
  y = squareLocal(x);
  result = y;

  function y = squareLocal(n)
    y = n^2;
  end
end

This code defines a main function that calls a local function to square a number and returns the result.

Execution Table

Step	Action	Variable Values	Output/Result
1	Start main function	x: undefined, y: undefined, result: undefined
2	Assign x = 5	x: 5, y: undefined, result: undefined
3	Call local function squareLocal(5)	x: 5, y: undefined, result: undefined
4	Inside squareLocal: compute y = 5^2	n: 5, y: 25
5	Return y=25 to main	x: 5, y: 25, result: undefined
6	Assign y = 25 in main	x: 5, y: 25, result: undefined
7	Assign result = y (25)	x: 5, y: 25, result: 25
8	End main function	x: 5, y: 25, result: 25	Returns 25

💡 Main function ends after returning the result 25.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 7	Final
x	undefined	5	5	5	5	5
y	undefined	undefined	25 (local)	25	25	25
result	undefined	undefined	undefined	undefined	25	25
n (local)	undefined	undefined	5	5	5	5

Key Moments - 2 Insights

Why can't the local function squareLocal be called from outside the main function?

Because local functions in MATLAB are only visible inside the main function file after their definition, as shown in the execution_table step 3 where main calls squareLocal. Outside calls would cause an error.

Does the variable 'n' inside squareLocal affect the variable 'x' in main?

No, 'n' is local to squareLocal and separate from 'x' in main. The execution_table step 4 shows 'n' is 5 inside squareLocal, but 'x' remains 5 in main, unchanged.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the value of 'y' inside the local function?

A 25

B undefined

C 5

D 10

💡 Hint

Check the 'Variable Values' column at step 4 in the execution_table.

At which step does the main function assign the final result to return?

A Step 3

B Step 5

C Step 7

D Step 8

💡 Hint

Look for when 'result' gets assigned in the variable_tracker and execution_table.

If the local function was removed, what would happen at step 3?

A The program would run normally

B An error would occur because squareLocal is undefined

C The variable y would be assigned zero

D The main function would return 0

💡 Hint

Refer to the key_moments about local function visibility and step 3 in execution_table.

Concept Snapshot

Local functions in MATLAB are defined inside a main function file after the main function code.
They can only be called by the main function or other local functions in the same file.
Local functions have their own workspace and variables.
They help organize code into smaller parts.
Syntax:
function main()
  ...
  function localFunc()
    ...
  end
end

Full Transcript

This visual execution trace shows how MATLAB local functions work. The main function starts and assigns a value to x. It then calls the local function squareLocal with x as input. Inside squareLocal, the input n is squared and returned as y. The main function receives y and assigns it to result. Variables inside the local function are separate from main. The local function cannot be called from outside the main function file. This helps keep code organized and variables scoped properly.

Practice

1. What is a key characteristic of a local function in MATLAB?

easy

A. It automatically runs when MATLAB starts.

B. It must be saved in a separate file with the same name.

C. It can only be called from the command window.

D. It is defined inside the same file as the main function or script.

Solution

1. Step 1: Understand local function placement

   Local functions are defined inside the same file as the main function or script, not separately.

2. Step 2: Compare with other options

   Options A, B, and C describe behaviors that do not match local functions in MATLAB.

3. Final Answer:

   It is defined inside the same file as the main function or script. -> Option D

4. Quick Check:

   Local function location = same file [OK]

Hint: Local functions live inside the main file, not separate files. [OK]

Common Mistakes:

• Thinking local functions need separate files
• Believing local functions run automatically
• Assuming local functions can only be called from command window

2. Which of the following is the correct way to define a local function inside a MATLAB script file?

easy

A. def localFunc(x):
return x^2

B. localFunc(x) = x^2;

C. function y = localFunc(x)
y = x^2;
end

D. function y = localFunc(x)
y = x^2;

Solution

1. Step 1: Identify MATLAB function syntax

   MATLAB local functions start with the keyword function, followed by output, name, and input, and end with end.

2. Step 2: Check each option

   function y = localFunc(x) y = x^2; end uses correct MATLAB syntax with function and end. def localFunc(x): return x^2 is Python syntax. localFunc(x) = x^2; is invalid MATLAB syntax. function y = localFunc(x) y = x^2; misses the end keyword.

3. Final Answer:

   function y = localFunc(x) y = x^2; end -> Option C

4. Quick Check:

   MATLAB function syntax = function y = localFunc(x) y = x^2; end [OK]

Hint: MATLAB functions need 'function' and 'end' keywords. [OK]

Common Mistakes:

• Omitting the 'end' keyword
• Using Python or other language syntax
• Trying to assign function like a variable

3. Consider this MATLAB code in a file named mainFile.m:

function result = mainFile(x)
  result = squarePlusOne(x);

  function y = squarePlusOne(n)
    y = n^2 + 1;
  end
end

What is the return value of mainFile(3)?

medium

A. 10

B. 9

C. 7

D. Error: undefined function

Solution

1. Step 1: Understand the local function call

   The main function calls squarePlusOne(3). The local function computes 3^2 + 1 = 9 + 1 = 10.

2. Step 2: Confirm output assignment

   The result from the local function is assigned to result, which is returned by mainFile. So mainFile(3) returns 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   3 squared plus 1 = 10 [OK]

Hint: Calculate local function output then return it. [OK]

Common Mistakes:

• Forgetting to add 1 after squaring
• Thinking local functions are inaccessible
• Confusing variable names

4. This MATLAB script contains a local function:

function output = calc(x)
  output = doubleValue(x);

  function y = doubleValue(n)
    y = n * 2
  end

result = calc(5);

What is the error in this code?

medium

A. Missing semicolon after y = n * 2 in local function

B. Missing end keyword for the main function calc

C. Local function doubleValue is not accessible inside calc

D. Local functions cannot return values

Solution

1. Step 1: Check function endings

   The local function doubleValue has an end, but the main function calc does not have its own end keyword.

2. Step 2: Verify MATLAB syntax rules

   Each function in MATLAB must end with end. Missing end for calc causes a syntax error.

3. Final Answer:

   Missing end keyword for the main function calc -> Option B

4. Quick Check:

   Every function needs 'end' [OK]

Hint: Count 'end' keywords: one per function needed. [OK]

Common Mistakes:

• Assuming semicolon omission causes error
• Thinking local functions are inaccessible
• Believing local functions can't return values

5. You want to write a MATLAB function that computes the factorial of a number using a local helper function to multiply numbers recursively. Which of the following code snippets correctly uses a local function for this purpose?

hard

A. function f = factorialMain(n)
f = helper(n);
function val = helper(k)
if k <= 1
val = 1;
else
val = k * helper(k-1);
end
end
end

B. function f = factorialMain(n)
f = helper(n);
end
function val = helper(k)
if k <= 1
val = 1;
else
val = k * helper(k-1);
end

C. function f = factorialMain(n)
f = factorial(n);
function val = helper(k)
val = k * helper(k-1);
end
end

D. function f = factorialMain(n)
f = n * factorialMain(n-1);
end

Solution

1. Step 1: Identify correct local function usage

   function f = factorialMain(n) f = helper(n); function val = helper(k) if k <= 1 val = 1; else val = k * helper(k-1); end end end defines the local function helper inside factorialMain and uses recursion properly with base case.

2. Step 2: Check other options for errors

   function f = factorialMain(n) f = helper(n); end function val = helper(k) if k <= 1 val = 1; else val = k * helper(k-1); end defines helper outside main function (not local). function f = factorialMain(n) f = factorial(n); function val = helper(k) val = k * helper(k-1); end end calls factorial (built-in) and has incomplete helper. function f = factorialMain(n) f = n * factorialMain(n-1); end is a recursive main function but lacks base case and local function.

3. Final Answer:

   function f = factorialMain(n) f = helper(n); function val = helper(k) if k <= 1 val = 1; else val = k * helper(k-1); end end end -> Option A

4. Quick Check:

   Local recursive helper inside main function = function f = factorialMain(n) f = helper(n); function val = helper(k) if k <= 1 val = 1; else val = k * helper(k-1); end end end [OK]

Hint: Local functions go inside main function with 'function' and 'end'. [OK]

Common Mistakes:

• Defining helper function outside main function
• Missing base case in recursion
• Calling built-in factorial instead of local helper