Local functions in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a script with local function call

What is the output of this MATLAB script when run?

MATLAB

disp(mainFunction())

function out = mainFunction()
    out = helperFunction(5);
end

function y = helperFunction(x)
    y = x^2;
end

A 10

B 5

C Error: Undefined function or variable 'helperFunction'.

D 25

💡 Hint

Local functions can be called by functions in the same file.

The mainFunction calls helperFunction with input 5. helperFunction squares it, returning 25.

❓ Predict Output

intermediate

Value of variable after local function modifies it

What is the value of variable 'result' after running this code?

MATLAB

function result = testLocal()
    x = 3;
    result = increment(x);

    function y = increment(a)
        y = a + 2;
    end
end

r = testLocal();

A Error: Undefined function 'increment'.

B 3

C 5

D 2

💡 Hint

Local functions can access variables passed as arguments.

The local function increment adds 2 to input 3, returning 5.

❓ Predict Output

advanced

Output when local function shadows variable

What is the output of this script?

MATLAB

function output = shadowTest()
    x = 10;
    output = localFunc();

    function y = localFunc()
        x = 5;
        y = x + 1;
    end
end

result = shadowTest();
disp(result)

A 6

B 11

C Error: Variable 'x' is undefined.

D 10

💡 Hint

Local function has its own variable 'x' that hides outer 'x'.

Inside localFunc, x is set to 5, so y = 5 + 1 = 6.

❓ Predict Output

advanced

Output of a script with chained local function calls

What is the output of this MATLAB script when run?

MATLAB

disp(outsideCall())

function y = outsideCall()
    y = localOnly();
end

function z = localOnly()
    z = 42;
end

A 42

B 0

C Error: Function 'outsideCall' not found.

D Error: Undefined function or variable 'localOnly'.

💡 Hint

Local functions can call other local functions in the same file.

The script calls outsideCall(), which calls localOnly() returning 42.

🧠 Conceptual

expert

Number of local functions accessible inside a nested function

Given this MATLAB file with three local functions, how many local functions can the nested function 'inner' call directly?

MATLAB

function main()
    disp(inner())

    function a = inner()
        a = f1() + f2() + f3();
    end

    function x = f1()
        x = 1;
    end

    function y = f2()
        y = 2;
    end

    function z = f3()
        z = 3;
    end
end

A 0

B 3

C 1

D 2

💡 Hint

Local functions in the same file are visible to each other.

The nested function 'inner' can call all three local functions f1, f2, and f3 directly.

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