Function declaration and definition - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple function call

What is the output of this C program?

C

#include <stdio.h>

int add(int a, int b);

int main() {
    int result = add(3, 4);
    printf("%d\n", result);
    return 0;
}

int add(int a, int b) {
    return a + b;
}

A 7

B 34

C 0

D Compilation error

💡 Hint

Look at what the function 'add' returns and what is printed.

The function 'add' returns the sum of its two parameters. Calling add(3,4) returns 7, which is printed.

❓ Predict Output

intermediate

Function declaration without definition

What happens when you compile and run this C code?

C

#include <stdio.h>

int multiply(int x, int y);

int main() {
    printf("%d\n", multiply(2, 3));
    return 0;
}

A Linker error

B 0

C 6

D Compilation error

💡 Hint

The function is declared but not defined anywhere.

The compiler knows the function signature but the linker cannot find the function's code, causing a linker error.

🔧 Debug

advanced

Identify the error in function definition

What error does this code produce?

C

#include <stdio.h>

int square(int n) {
    return n * n;
}

int main() {
    printf("%d\n", square(5));
    return 0;
}

A Undefined function 'square' (Linker error)

B No error, outputs 25

C Type mismatch error

D Missing semicolon after return statement (SyntaxError)

💡 Hint

Check punctuation at the end of the return line.

The return statement now has a semicolon, so the code compiles and outputs 25.

❓ Predict Output

advanced

Function with pointer parameter output

What is the output of this program?

C

#include <stdio.h>

void increment(int *p) {
    (*p)++;
}

int main() {
    int a = 10;
    increment(&a);
    printf("%d\n", a);
    return 0;
}

A Compilation error

B 10

C 11

D Garbage value

💡 Hint

The function changes the value at the address passed.

The function increments the value pointed to by p, so a becomes 11.

🧠 Conceptual

expert

Function declaration and definition order

Which option correctly explains why this code compiles and runs without errors?

C

#include <stdio.h>

void greet();

int main() {
    greet();
    return 0;
}

void greet() {
    printf("Hello!\n");
}

A Because the function is defined before main, so no declaration is needed.

B Because the function is declared after main, so the compiler ignores the call.

C Because the function is defined inside main, so it is local and accessible.

D Because the function is declared before main and defined after, the compiler knows its signature when main calls it.

💡 Hint

Think about how the compiler processes declarations and definitions.

The declaration before main tells the compiler the function signature, so the call in main is valid. The definition after main provides the actual code.

Practice

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

easy

A. To allocate memory for variables inside the function

B. To execute the function's code

C. To tell the compiler about the function's name, return type, and parameters before its use

D. To print the function's output

Solution

1. Step 1: Understand function declaration role

   A function declaration informs the compiler about the function's name, return type, and parameters so it can be used before its definition.

2. Step 2: Differentiate declaration from definition

   Declaration does not contain the function's code; that is done in the definition.

3. Final Answer:

   To tell the compiler about the function's name, return type, and parameters before its use -> Option C

4. Quick Check:

   Function declaration = Inform compiler [OK]

Hint: Declaration tells compiler about function before use [OK]

Common Mistakes:

• Confusing declaration with definition
• Thinking declaration runs the function
• Assuming declaration allocates memory

2. Which of the following is a correct function declaration in C?

easy

A. int add { int a, int b; };

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

C. add(int a, int b) int;

D. int add(int a, int b);

Solution

1. Step 1: Identify correct syntax for declaration

   A function declaration includes the return type, function name, parameter list, and ends with a semicolon.

2. Step 2: Check each option

   int add(int a, int b); matches correct syntax: return type 'int', name 'add', parameters '(int a, int b)', and semicolon. Others have syntax errors or missing semicolon.

3. Final Answer:

   int add(int a, int b); -> Option D

4. Quick Check:

   Declaration ends with semicolon [OK]

Hint: Declaration ends with semicolon and no body [OK]

Common Mistakes:

• Missing semicolon at end
• Including function body in declaration
• Wrong parameter syntax

3. What will be the output of this C program?

#include <stdio.h>

int multiply(int x, int y);

int main() {
    printf("%d", multiply(3, 4));
    return 0;
}

int multiply(int x, int y) {
    return x * y;
}

medium

A. 12

B. 7

C. 34

D. Compilation error

Solution

1. Step 1: Understand function declaration and call

   The function 'multiply' is declared before main and defined after. main calls multiply(3, 4).

2. Step 2: Calculate function return value

   multiply returns 3 * 4 = 12, which is printed by printf.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Declaration allows call before definition [OK]

Common Mistakes:

• Thinking declaration runs code
• Confusing output with parameters
• Expecting compilation error due to declaration

4. Identify the error in this function declaration and definition:

int sum(int a, int b)
{
    return a + b
}

medium

A. Function name is invalid

B. Missing semicolon after return statement

C. Parameters must be declared outside parentheses

D. Return type should be void

Solution

1. Step 1: Check syntax inside function body

   The return statement must end with a semicolon in C.

2. Step 2: Identify missing semicolon

   Here, 'return a + b' lacks a semicolon, causing a syntax error.

3. Final Answer:

   Missing semicolon after return statement -> Option B

4. Quick Check:

   Statements end with semicolon [OK]

Hint: Return statements need semicolon [OK]

Common Mistakes:

• Omitting semicolon after return
• Misunderstanding parameter syntax
• Changing return type unnecessarily

5. You want to declare and define a function that calculates the average of three integers and returns a float. Which of the following is the correct declaration and definition?

hard

A. float average(int a, int b, int c);

float average(int a, int b, int c) {
return (a + b + c) / 3.0;
}

B. int average(int a, int b, int c);

int average(int a, int b, int c) {
return (a + b + c) / 3.0;
}

C. float average(int a, int b, int c);

float average(int a, int b, int c) {
return (a + b + c) / 3;
}

D. float average(int a, int b, int c) {
return (a + b + c) / 3;
}

Solution

1. Step 1: Check function declaration matches definition

   Declaration and definition must have same return type (float) and parameters (three ints).

2. Step 2: Ensure correct calculation for float average

   To get float result, divide by 3.0 (float), not 3 (int).

3. Step 3: Verify other options

   Integer division (/ 3) causes truncation. Declaring int return but dividing by 3.0 causes type mismatch. Missing declaration prevents prior use.

4. Final Answer:

   float average(int a, int b, int c); float average(int a, int b, int c) { return (a + b + c) / 3.0; } -> Option A

5. Quick Check:

   Float division needs 3.0 [OK]

Hint: Use float division (3.0) and matching declaration [OK]

Common Mistakes:

• Using integer division instead of float
• Mismatching declaration and definition types
• Omitting function declaration