Programming using C
Interview Questions with Answers

21. What is the difference between while and do-while loop?

Answer:

 Example

int i = 0;
// while loop
while (i < 5) {
    printf("%d ", i);
    i++;
}
// do-while loop
do {
    printf("%d ", i);
    i++;
} while (i < 5);

22. How does the for loop work internally?

Answer:

A for loop has the structure:

for (initialization; condition; increment) {

// loop body

}

 Execution steps:

1. Initialization (runs once at the start)
2. Condition check
   • If true, execute loop body.
   • If false, exit the loop.
3. Increment or Update
4. Go back to step 2.

 Example

for (int i = 0; i < 3; i++) {
    printf("%d ", i);
}

Internally, it works like:

int i = 0;
while (i < 3) {
    printf("%d ", i);
    i++;
}

23. What is the use of break and continue statements?

Answer:

• break:
  • Immediately terminates the loop or switch.
  • Control moves to the statement after the loop or switch.
• continue:
  • Skips the current iteration and jumps to the next loop cycle.

 Example – break

for (int i = 1; i <= 5; i++) {
    if (i == 3)
        break;
    printf("%d ", i);           // Output: 1 2
}

Example – continue

for (int i = 1; i <= 5; i++) {
    if (i == 3)
        continue;
    printf("%d ", i);           // Output: 1 2 4 5
}

24. How does switch-case work and what is its fall-through behavior?

Answer:

A switch statement executes code based on matching a case label with the provided expression.

 Syntax

switch (expression) {
    case value1:
        // code
        break;
    case value2:
        // code
        break;
    default:
        // default code
} 

Fall-through behavior:

• If break is not used, execution continues to the next case, even if it does not match.
• This is known as fall-through.

 Example

int grade = 2;
switch (grade) {
    case 1:
    case 2:
        printf("Good\n");  // Fall-through: case 1 and 2 share output
        break;
    case 3:
        printf("Average\n");
        break;
    default:
        printf("Invalid\n");
}

25. What is a nested loop? Give an example.

Answer:

A nested loop is a loop inside another loop. It is used when we need to perform repetitive operations within another repetitive operation, such as matrix traversal or pattern printing.

 Example

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        printf("i=%d, j=%d\n", i, j);
    }
}

26. How is goto used and why is it discouraged?

Answer:

• goto is a control transfer statement that jumps to a labeled part of code.

Syntax

goto label;
...
label:
// code to execute

 Example

int x = 1;
if (x == 1)
    goto skip;
printf("This will not print\n");
skip:
printf("Goto executed\n");

 Why it’s discouraged:

• Makes code hard to read, debug, and maintain.
• Violates structured programming principles.
• Can lead to spaghetti code if misused.

Use loops and functions instead of goto whenever possible.

27. Can return be used to exit a loop?

Answer:

Yes. The return statement can exit not only the loop but also the entire function.

Example

void findNumber(int arr[], int size, int target) {

    for (int i = 0; i < size; i++) {
        if (arr[i] == target) {
            printf("Found\n");
            return;  // Exits loop and function
        }
    }
    printf("Not Found\n");
}

 Note: Use break if you only want to exit the loop without terminating the function.

28. What is the difference between if-else and switch?

Answer:

 Example using switch

switch (choice) {
    case 1: // do task A
    case 2: // do task B
}

 Example using if-else

if (score >= 90) { ... }
else if (score >= 80) { ... }

29. What are infinite loops? Give an example.

Answer:

An infinite loop is a loop that never terminates, either due to a condition that is always true or a missing termination condition.

Use Cases: Game engines, OS kernels, server processes.

 Example using while

while (1) {
// infinite loop
}

 Example using for

for (;;) {
// infinite loop
}

 Example using do-while

do {
// infinite loop
} while (1);

30. How do you prevent logical errors in control structures?

Answer:

To prevent logical errors (which compile but produce wrong results), follow these best practices:

• Clearly define logic and flow before coding (e.g., use flowcharts or pseudocode).
• Use meaningful variable and function names to avoid confusion.
• Test with different input cases – normal, boundary, and edge cases.
• Use debugging tools and printf() for tracing flow.
• Avoid deeply nested loops or conditions – refactor into functions.
• Comment your code and follow consistent indentation.
• Use break, continue, return judiciously – avoid excessive jumping.
• Write unit tests for each block of logic.

31. What is a function pointer? Give an example.

Answer:

A function pointer is a pointer that stores the address of a function and can be used to call the function indirectly.

Syntax

return_type (*pointer_name)(parameter_list);

Example

#include <stdio.h>
void greet() {
    printf("Hello from function pointer!\n");
}
int main() {
    void (*fp)();                  // Declare function pointer
    fp = greet;                    // Assign address of greet
    fp();                          // Call function using pointer
    return 0;
}

Function pointers are useful for callbacks, dynamic function selection, and implementing polymorphism in C.

32. What is the difference between call by value and call by reference in C?

Answer:

 Example

void modifyValue(int x) {
    x = 100;
}
void modifyReference(int *x) {
    *x = 100;
}
int main() {
    int a = 10;
    modifyValue(a);              // a remains 10
    modifyReference(&a);         // a becomes 100
}

33. What are inline functions? Are they available in C?

Answer:

An inline function is a function where the compiler replaces the function call with the actual function code to eliminate overhead.

• Introduced in C99 standard using the inline keyword.
• It’s only a request to the compiler; not mandatory.
• Mostly used for small, frequently called functions.

 Syntax (C99)

inline int square(int x) {
    return x * x;
}

Note: Inline functions in C are less powerful than in C++ (C does not support default arguments, function overloading, etc.).

34. What is recursion? Give an example.

Answer:

Recursion is a process in which a function calls itself directly or indirectly to solve a problem by breaking it into smaller sub-problems.

 Example – Factorial

int factorial(int n) {
    if (n == 0) return 1;
    return n * factorial(n - 1);
}

Usage: Problems like factorial, Fibonacci, Tower of Hanoi, tree traversals, etc.

35. What are the advantages and disadvantages of recursion?

Answer:

Advantages:

• Simplifies complex problems.
• Cleaner code for tree and graph algorithms, backtracking, etc.
• Natural fit for problems with recursive structure.

Disadvantages:

• High memory usage (stack frames).
• Risk of stack overflow if not written properly.
• Usually slower than iteration due to overhead of function calls.

36. What are storage classes and their impact on functions?

Answer:

Storage classes in C determine scope, visibility, lifetime, and linkage of variables and functions.

 Example of static function

static void helper() {
// This function is accessible only in this file
}

37. Can you return multiple values from a function in C?

Answer:

C functions cannot return multiple values directly, but you can achieve this by:

1. Using pointers as arguments:

void calc(int a, int b, int *sum, int *prod) {
    *sum = a + b;
    *prod = a * b;
}

1. Using a structure:

struct Result {
    int sum;
    int prod;
};
struct Result calc(int a, int b) {
    struct Result r = {a + b, a * b};
    return r;
}

38. What is a static function?

Answer:

A static function in C is a function declared with the static keyword, which restricts its visibility to the current source file.

• Cannot be accessed from other files (no external linkage).
• Helps in encapsulation and modular design.

 Example

static void logError() {
    printf("Error occurred\n");
}

Useful in library or utility files where helper functions should not be exposed externally.

39. What is the default return type of functions in C?

Answer:

Before C99, if no return type is specified, C defaults the return type to int.

Example (Old Style)

func() {  // Treated as int func()
    return 5;
}

However, in modern C standards (C99 and later), explicit declaration is mandatory. Implicit int is considered deprecated or invalid.

Best Practice: Always declare the return type explicitly.

40. What is the purpose of void in function declaration?

Answer:

void has two main purposes in function declarations:

1. As return type – Indicates the function does not return any value.

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

1. As parameter list – Indicates the function does not accept any parameters.

void show(void) {
printf("Nothing to input\n");
}

Using void improves code readability and helps the compiler catch errors.