Function prototypes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function prototype in C?

A function prototype is a declaration of a function that specifies its name, return type, and parameters without the function body. It tells the compiler what to expect when the function is called.

beginner

Why do we use function prototypes in C programs?

Function prototypes help the compiler check that functions are called with the correct number and types of arguments, preventing errors before the program runs.

beginner

How do you write a function prototype for a function named add that takes two integers and returns an integer?

The prototype is:
int add(int, int);

intermediate

What happens if you call a function without a prototype in C?

If a function is called without a prototype, the compiler assumes it returns an int and does not check the arguments, which can cause unexpected behavior or errors.

beginner

Where should function prototypes be placed in a C program?

Function prototypes are usually placed at the top of the file or in a header file so that all parts of the program know about the functions before they are used.

What does a function prototype in C specify?

A Function body and variables

B Function name, return type, and parameters

C Only the function name

D Only the return type

A function prototype tells the compiler the function's name, return type, and parameter types without the body.

Where is the best place to put function prototypes in a C program?

A At the top of the file or in a header file

B Inside the main function

C After the function definitions

D Anywhere in the middle of the code

Placing prototypes at the top or in header files ensures the compiler knows about functions before they are used.

What happens if you call a function without a prototype in C?

A Program runs perfectly without warnings

B Compiler throws an error immediately

C Compiler assumes it returns int and no argument checks

D Function is ignored

Without a prototype, the compiler assumes the function returns int and does not check arguments, which can cause bugs.

Which of these is a correct function prototype for a function that returns void and takes no parameters?

A void func(void);

B void func();

C func(void);

D void func[];

The correct prototype for a function with no parameters is void func(void); to explicitly specify no arguments.

Why is it important to match the function prototype with the function definition?

A It is not important

B To make the program run faster

C To reduce the size of the executable

D To avoid compiler errors and ensure correct function calls

Matching prototypes and definitions helps the compiler check calls and prevents errors.

Explain what a function prototype is and why it is useful in C programming.

Think about how the compiler knows what a function looks like before it is used.

Describe where and how you would declare function prototypes in a C program.

Consider program organization and compiler needs.

Practice

1. What is the main purpose of a function prototype in C?

easy

A. To allocate memory for the function variables

B. To declare the function's name, return type, and parameters before its definition

C. To execute the function immediately

D. To define the function's full body and logic

Solution

1. Step 1: Understand what a function prototype declares

   A function prototype tells the compiler the function's name, return type, and parameter types without the body.

2. Step 2: Differentiate prototype from definition and execution

   It does not define the function's body or execute it; it only declares its interface.

3. Final Answer:

   To declare the function's name, return type, and parameters before its definition -> Option B

4. Quick Check:

   Function prototype = declaration before definition [OK]

Hint: Prototypes declare function signature only, no body [OK]

Common Mistakes:

• Confusing prototype with function definition
• Thinking prototype runs the function
• Believing prototype allocates memory

2. Which of the following is a correct function prototype in C for a function named sum that takes two integers and returns an integer?

easy

A. int sum(int a, int b);

B. int sum(int a, int b) {}

C. void sum(int a, int b);

D. int sum(a, b);

Solution

1. Step 1: Check the syntax of a function prototype

   A prototype includes return type, function name, parameter types, and ends with a semicolon.

2. Step 2: Evaluate each option

   int sum(int a, int b); matches the correct syntax. int sum(int a, int b) {} has braces (function body), so it's a definition. void sum(int a, int b); has wrong return type (void). int sum(a, b); lacks parameter types.

3. Final Answer:

   int sum(int a, int b); -> Option A

4. Quick Check:

   Prototype = return type + name + parameters + semicolon [OK]

Hint: Prototype ends with semicolon and no braces [OK]

Common Mistakes:

• Adding braces {} in prototype
• Omitting parameter types
• Using wrong return type

3. What will be the output of the following C code?

int add(int x, int y);
int main() {
    int result = add(3, 4);
    printf("%d", result);
    return 0;
}
int add(int a, int b) {
    return a + b;
}

medium

A. 34

B. Compilation error

C. 7

D. 0

Solution

1. Step 1: Understand the prototype and function call

   The prototype declares add before main, so the compiler knows its signature.

2. Step 2: Trace the function execution

   add(3, 4) returns 3 + 4 = 7, which is printed by printf.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Function call returns sum 3+4=7 [OK]

Hint: Prototype allows calling function before definition [OK]

Common Mistakes:

• Expecting compilation error without prototype
• Confusing concatenation with addition
• Ignoring return value

4. Identify the error in the following code snippet:

int multiply(int a, int b);
int main() {
    int x = multiply(2, 3);
    printf("%d", x);
    return 0;
}
int multiply(int a, b) {
    return a * b;
}

medium

A. No error, code runs fine

B. Missing return type in function definition

C. Missing semicolon after prototype

D. Incorrect parameter declaration in function definition

Solution

1. Step 1: Compare prototype and definition parameters

   The prototype declares int multiply(int a, int b); but the definition has int multiply(int a, b) missing type for b.

2. Step 2: Understand C parameter declaration rules

   Each parameter must have a type. Missing type for b causes a syntax error.

3. Final Answer:

   Incorrect parameter declaration in function definition -> Option D

4. Quick Check:

   All parameters need types in definition [OK]

Hint: All parameters must have types in definition [OK]

Common Mistakes:

• Omitting type for one parameter
• Assuming prototype fixes definition errors
• Missing semicolon after prototype

5. You want to declare a function prototype for processData that takes an array of integers and its size, then returns a float average. Which prototype is correct?

hard

A. float processData(int arr[], int size);

B. float processData(int arr, int size);

C. int processData(int[] arr, int size);

D. float processData(int *arr);

Solution

1. Step 1: Identify correct parameter types for array and size

   In C, an array parameter can be declared as int arr[] or int *arr, but size must be separate.

2. Step 2: Check return type and parameters

   Function returns a float average, so return type is float. float processData(int arr[], int size); matches this with int arr[] and int size.

3. Final Answer:

   float processData(int arr[], int size); -> Option A

4. Quick Check:

   Array parameter + size + float return = correct prototype [OK]

Hint: Array parameter needs type[] and size parameter [OK]

Common Mistakes:

• Using wrong return type
• Omitting size parameter
• Incorrect array parameter syntax