Function prototypes - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function call with prototype

What is the output of this C program?

C

#include <stdio.h>

int add(int a, int b); // function prototype

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

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

A 0

B Compilation error

C 34

D 7

💡 Hint

Remember what the add function does and how prototypes help the compiler.

The function prototype declares add before main, so the call add(3,4) is valid. The function returns the sum 3 + 4 = 7, which is printed.

❓ Predict Output

intermediate

Effect of missing function prototype

What error or output occurs when calling a function without a prototype in C99 or later?

C

#include <stdio.h>

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

int multiply(int a, int b) {
    return a * b;
}

A Compilation error: implicit declaration of function 'multiply'

B 6

C 0

D Runtime error

💡 Hint

Check if the compiler knows about multiply before main.

In C99 and later, calling a function without a prototype causes a compilation error about implicit declaration.

🔧 Debug

advanced

Identify the prototype mismatch error

Which option shows a prototype that will cause a mismatch error with the function definition below?

C

int compute(int x, int y);

int compute(int a, float b) {
    return a + (int)b;
}

A int compute(float x, float y);

B int compute(int x, float y);

C int compute(int x, int y);

D int compute(int, int);

💡 Hint

Compare parameter types in prototype and definition carefully.

The prototype declares two ints, but the definition has int and float. This mismatch causes a compilation error.

🧠 Conceptual

advanced

Purpose of function prototypes

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

A To declare the function's return type and parameters before its use

B To define the function's body

C To allocate memory for the function

D To call the function

💡 Hint

Think about what the compiler needs before a function is called.

Function prototypes tell the compiler the function's return type and parameter types before the function is used, enabling type checking.

❓ Predict Output

expert

Output with mismatched prototype and definition

What is the output of this program?

C

#include <stdio.h>

int foo(int a, int b); // prototype

int main() {
    int result = foo(5, 2);
    printf("%d\n", result);
    return 0;
}

int foo(int a, int b) {
    return a - b;
}

A 7

B 3

C Compilation error

D Undefined behavior

💡 Hint

Check if prototype and definition match and what foo returns.

The prototype and definition match perfectly. foo returns 5 - 2 = 3, which is printed.