Code generation for C/C++ in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generated C code from MATLAB function

Consider the following MATLAB function that will be converted to C code using MATLAB Coder. What will be the output when the generated C code is run with input x = 3?

MATLAB

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

A 16

B 9

C 7

D 10

💡 Hint

Remember to square the input and then add one.

The function squares the input (3^2 = 9) and then adds 1, resulting in 10.

🧠 Conceptual

intermediate

Understanding MATLAB Coder limitations

Which of the following MATLAB features cannot be directly converted to C code using MATLAB Coder?

A For-loops with fixed iteration counts

B Basic arithmetic operations

C Dynamic memory allocation using <code>eval</code>

D Conditional <code>if</code> statements

💡 Hint

Think about features that require runtime interpretation.

MATLAB Coder does not support dynamic features like eval because C code is statically compiled.

🔧 Debug

advanced

Identify the error in generated C code from MATLAB

The following MATLAB function is converted to C code. The generated C code crashes with a segmentation fault. What is the most likely cause?

function y = getElement(arr, idx)
    y = arr(idx);
end

A Indexing in C starts at 0, but MATLAB indexing starts at 1

B The function uses unsupported data types

C The function uses recursion which is not supported

D The function does not have an output argument

💡 Hint

Think about how MATLAB and C handle array indices differently.

MATLAB arrays start at index 1, but C arrays start at 0. The generated C code must adjust indices accordingly to avoid out-of-bounds access.

📝 Syntax

advanced

Syntax error in MATLAB code for C code generation

Which of the following MATLAB code snippets will cause a syntax error during C code generation?

A

function y = add(a,b)
    y = a + b;
end

B

function y = f(x)
    y = x++;
end

C

function y = g(x)
    if x > 0
        y = x;
    else
        y = -x;
    end
end

D

function y = test()
    y = 1:5;
end

💡 Hint

Check for invalid MATLAB syntax that looks like C syntax.

MATLAB does not support the ++ operator; using x++ causes a syntax error.

🚀 Application

expert

Optimizing MATLAB code for efficient C code generation

You want to generate efficient C code from MATLAB for a function that computes the sum of squares of elements in a vector. Which MATLAB code version will produce the most efficient C code?

A

function s = sumSquares(v)
    s = 0;
    for i = 1:numel(v)
        s = s + v(i)^2;
    end
end

B

function s = sumSquares(v)
    s = 0;
    for i = 1:length(v)
        s = s + v(i)^2;
    end
end

C

function s = sumSquares(v)
    s = sum(v.^2);
end

D

function s = sumSquares(v)
    s = 0;
    i = 1;
    while i <= length(v)
        s = s + v(i)^2;
        i = i + 1;
    end
end

💡 Hint

Consider which function is preferred for code generation and efficiency.

Using numel is preferred over length for code generation because it works correctly for all array shapes and helps generate efficient loops.

Practice

1. What is the primary purpose of using codegen in MATLAB for C/C++?

easy

A. To visualize MATLAB data in 3D plots

B. To debug MATLAB scripts line by line

C. To create graphical user interfaces in MATLAB

D. To convert MATLAB code into C/C++ code for faster execution or integration

Solution

1. Step 1: Understand the role of codegen

   codegen is used to generate C/C++ code from MATLAB functions.

2. Step 2: Identify the main benefit

   This conversion helps in faster execution and easier integration with other C/C++ projects.

3. Final Answer:

   To convert MATLAB code into C/C++ code for faster execution or integration -> Option D

4. Quick Check:

   Code generation = convert MATLAB to C/C++ [OK]

Hint: Remember: codegen means generate code in C/C++ [OK]

Common Mistakes:

• Confusing codegen with debugging tools
• Thinking codegen creates plots
• Assuming codegen builds GUIs

2. Which of the following is the correct syntax to generate C code for a MATLAB function myFunc with input type double scalar?

easy

A. codegen myFunc -args {double(0)}

B. codegen myFunc -args {single(1)}

C. codegen myFunc -args {int32(1)}

D. codegen myFunc -args {1+0i}

Solution

1. Step 1: Understand input argument specification

   For codegen, input types are specified using MATLAB data types like double(0) for a scalar double.

2. Step 2: Check each option

   codegen myFunc -args {double(0)} uses double(0) which correctly specifies a double scalar input. Others either use incorrect syntax or values.

3. Final Answer:

   codegen myFunc -args {double(0)} -> Option A

4. Quick Check:

   Input type double scalar = double(0) [OK]

Hint: Use double(0) to specify a double scalar input [OK]

Common Mistakes:

• Using numeric literals directly without type
• Using curly braces incorrectly
• Confusing input value with input type

3. Given the MATLAB function

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

What will be the output C code function signature generated by codegen squarePlusOne -args {zeros(3,1)}?

medium

A. void squarePlusOne(const double x[3], double y[3])

B. double squarePlusOne(double x)

C. void squarePlusOne(double x, double *y)

D. double squarePlusOne(const double x[3])

Solution

1. Step 1: Analyze input argument

   The input is a 3x1 double vector, so the generated C function will take an array of 3 doubles as input.

2. Step 2: Determine output type

   The output is also a 3x1 vector, so the output is an array passed by reference.

3. Step 3: Match function signature

   void squarePlusOne(const double x[3], double y[3]) matches the expected signature: void function with input array and output array parameters.

4. Final Answer:

   void squarePlusOne(const double x[3], double y[3]) -> Option A

5. Quick Check:

   Array input/output = void func(const double x[3], double y[3]) [OK]

Hint: Array inputs generate functions with array parameters [OK]

Common Mistakes:

• Assuming scalar input/output signatures
• Missing const qualifier for input arrays
• Returning output instead of using output parameter

4. You try to generate C code for this MATLAB function:

function y = addVectors(a, b)
y = a + b;
end

using codegen addVectors -args {zeros(3,1), zeros(4,1)}. What is the main issue?

medium

A. Function syntax is incorrect for code generation

B. Input vectors have different sizes causing a code generation error

C. You must specify input types as integers, not doubles

D. codegen cannot handle vector inputs

Solution

1. Step 1: Check input argument sizes

   The inputs are vectors of sizes 3x1 and 4x1, which cannot be added element-wise due to size mismatch.

2. Step 2: Understand codegen requirements

   Code generation requires consistent input sizes for operations like addition.

3. Final Answer:

   Input vectors have different sizes causing a code generation error -> Option B

4. Quick Check:

   Size mismatch in inputs = error [OK]

Hint: Ensure input sizes match for element-wise operations [OK]

Common Mistakes:

• Ignoring input size mismatch
• Assuming codegen auto-resizes inputs
• Confusing data type with size issues

5. You want to generate C++ code from this MATLAB function that computes the sum of positive elements only:

function s = sumPositive(v)
s = sum(v(v > 0));
end

Which input argument specification is best for codegen to handle variable-length vectors?

hard

A. codegen sumPositive -args {double(0)}

B. codegen sumPositive -args {zeros(10,1)}

C. codegen sumPositive -args {coder.typeof(0,[Inf,1],[1,0])}

D. codegen sumPositive -args {coder.typeof(0,[10,1],[0,0])}

Solution

1. Step 1: Understand variable-size input specification

   To handle variable-length vectors, coder.typeof with [Inf,1] size and [1,0] variable-size flags is used.

2. Step 2: Evaluate options

   codegen sumPositive -args {coder.typeof(0,[Inf,1],[1,0])} correctly specifies a variable-length column vector of doubles. Others specify fixed sizes or scalars.

3. Final Answer:

   codegen sumPositive -args {coder.typeof(0,[Inf,1],[1,0])} -> Option C

4. Quick Check:

   Variable-size vector = coder.typeof with Inf size [OK]

Hint: Use coder.typeof with Inf size for variable-length inputs [OK]

Common Mistakes:

• Using fixed-size arrays for variable-length inputs
• Not setting variable-size flags correctly
• Confusing scalar input with vector input