*** 11. Difference between i++ and ++i?

Both operators are used to increment a variable by 1, but they work differently.

Example of i++

int i = 5;
int a = i++;

Execution

• a = 5
• i = 6

Example of ++i

int i = 5;
int a = ++i;

Execution

• i = 6
• a = 6

Key Difference

Post Increment  -> Use then Increment
Pre Increment   -> Increment then Use

Interview Insight

Common interview question:

“Which is faster: i++ or ++i?”

For primitive types in C, both are almost same after compiler optimization.

** 12. What is typecasting?

Typecasting is the process of converting one data type into another.

Types of Typecasting in C

1. Implicit Typecasting

Done automatically by compiler.

int x = 10;
float y = x;

x automatically converts to float.

2. Explicit Typecasting

Done manually by programmer.

float x = 5.7;
int y = (int)x;

Output:

5

Fractional part is removed.

Why Typecasting is Used

• Data conversion
• Memory optimization
• Prevent data loss
• Mathematical operations

Interview Insight

Common follow-up:

“What is data loss in typecasting?”

Example:

float x = 9.8;
int y = (int)x;

Decimal part is lost.

** 13. What is an l-value and r-value?

l-value

An l-value refers to a memory location.

It can appear on the left side of assignment.

Example

int x = 10;

x is an l-value.

r-value

An r-value refers to a data value.

It usually appears on the right side of assignment.

Example

int x = 20;

20 is an r-value.

Important Rule

10 = x; // Invalid

Because constant values are not l-values.

Interview Insight

Simple definition interviewers like:

l-value -> Has memory address
r-value -> Temporary value/data

*** 14. What are entry-controlled and exit-controlled loops?

Loops are categorized based on when the condition is checked.

1. Entry-Controlled Loop

Condition is checked before loop execution.

Examples

• for
• while

while(i < 5) {
    printf("%d", i);
}

If condition is false initially, loop will not run.

2. Exit-Controlled Loop

Condition is checked after loop execution.

Example

• do-while

do {
    printf("Hello");
} while(i < 5);

Runs at least once.

Key Difference

Interview Insight

Very common question:

“Which loop executes at least once?”

Answer:
do-while loop.

*** 15. Difference between break, continue, and goto?

break Example

for(int i=0; i<5; i++) {
    if(i==3)
        break;
}

Loop stops when i == 3.

continue Example

for(int i=0; i<5; i++) {
    if(i==2)
        continue;
}

Skips iteration when i == 2.

goto Example

goto label;

label:
printf("Hello");

Key Difference

Interview Insight

Interviewers often ask:

“Why is goto discouraged?”

Because it makes code difficult to read and maintain.

*** 16. Difference between if-else and switch?

if-else Example

if(marks > 90)
    printf("A");
else
    printf("B");

switch Example

switch(day) {
    case 1:
        printf("Monday");
        break;
}

When to Use

• Use if-else for ranges and complex conditions.
• Use switch for fixed values/menu programs.

Interview Insight

Important point:
switch only works with:

• int
• char
• enum values

(Not with float/string in C)

*** 17. What happens if break is omitted in switch?

If break is omitted, execution continues into the next case.

This is called fall-through behavior.

Example

switch(x) {
    case 1:
        printf("One");

    case 2:
        printf("Two");
}

If x = 1

Output:

OneTwo

Because control falls into next case.

Why Sometimes Useful?

Used intentionally in:

• Grouped cases
• Menu systems

Interview Insight

Very common interview concept:

Missing break causes fall-through.

** 18. What is while(1)?

while(1)

creates an infinite loop because condition 1 is always true.

Example

while(1) {
    printf("Running...");
}

Loop never stops unless:

• break is used
• program terminates

Common Uses

• Servers
• Operating systems
• Embedded systems
• Event listeners

Interview Insight

Follow-up question:

“Difference between while(1) and while(true)?”

• while(1) is common in C
• true requires <stdbool.h>

*** 19. How to create an infinite loop?

An infinite loop runs forever until externally stopped.

Methods to Create Infinite Loop

1. Using while

while(1) {
}

2. Using for

for(;;) {
}

3. Using do-while

do {
} while(1);

Most Common Method

for(;;)

because it avoids condition checking overhead.

Interview Insight

Common question:

“Where are infinite loops used?”

Answer:

• Web servers
• Game loops
• Operating systems
• Real-time systems

* 20. Why can't reserved keywords be used as variable names?

Reserved keywords have predefined meanings in C.

The compiler uses them for syntax and program structure.

Examples of Keywords

int
float
return
if
while

Invalid Example

int return = 10;

This causes compilation error.

Why Restriction Exists

To avoid:

• Syntax confusion
• Parsing ambiguity
• Compiler errors

Valid Alternative

int returnValue = 10;

Interview Insight

Common follow-up:

“Can keywords be used in other programming languages?”

Some modern languages allow escaped identifiers, but C does not.

** 21. What are header files?

Header files are files that contain:

• Function declarations
• Macros
• Constants
• Data type definitions

They help reuse code across multiple C programs.

Common Header Files

Example

#include <stdio.h>

int main() {
    printf("Hello");
    return 0;
}

Here:

stdio.h

contains declaration of printf().

Advantages

• Code reusability
• Easier maintenance
• Modular programming

Interview Insight

Common question:

“What happens if we do not include stdio.h?”

Compiler may show warning/error because printf() declaration is missing.

*** 22. What are preprocessor directives?

Preprocessor directives are commands processed before compilation.

They begin with:

#

Common Preprocessor Directives

Example

#define PI 3.14

Important Point

Preprocessor directives do not end with semicolon (;).

Interview Insight

Very common question:

“When does preprocessing happen?”

Answer:
Before actual compilation starts.

*** 23. Difference between #include <> and #include ""?

Example

System Header

#include <stdio.h>

User-Defined Header

#include "myheader.h"

Usage

• < > → Standard library headers
• " " → Custom/user-defined headers

Interview Insight

Common follow-up:

“Can #include "" also search system directories?”

Yes. If file is not found locally, compiler searches system paths.

*** 24. What is a macro?

A macro is a name or symbol that represents a value or code fragment.

Macros are created using:

#define

Example

#define PI 3.14

Now PI is replaced by 3.14 during preprocessing.

Function-like Macro

#define SQUARE(x) ((x) * (x))

Usage:

SQUARE(5)

Advantages

• Faster execution
• Code reusability
• Easy constant management

Interview Insight

Important concept:
Macros are expanded before compilation.

*** 25. Difference between macros and functions?

Macro Example

#define SQUARE(x) ((x)*(x))

Function Example

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

Key Difference

Macros perform text replacement, while functions execute compiled code.

Interview Insight

Very common question:

“Why are macros faster?”

Because no function call happens.

** 26. What is typedef?

typedef is used to create a new name (alias) for an existing data type.

Syntax

typedef existing_type new_name;

Example

typedef int INTEGER;

INTEGER x = 10;

Structure Example

typedef struct {
    int id;
    char name[20];
} Student;

Usage:

Student s1;

Advantages

• Improves readability
• Simplifies complex declarations
• Better code maintenance

Interview Insight

Common use:
typedef is heavily used with structures and pointers.

** 27. What are enumerations (enum)?

enum is a user-defined data type used to assign names to integer constants.

Syntax

enum Day {
    MON,
    TUE,
    WED
};

Default Values

Example

enum Day today = MON;

Advantages

• Improves readability
• Makes code meaningful
• Easy constant management

Interview Insight

Common question:

“Can enum values be customized?”

Yes.

enum Day {
    MON = 1,
    TUE = 5
};

*** 28. What is the sizeof() operator?

sizeof() returns the size of a variable or data type in bytes.

Syntax

sizeof(data_type)

or

sizeof(variable)

Example

int x;
printf("%lu", sizeof(x));

Output

Depends on system architecture.

Example:

4

for int on many systems.

Important Uses

• Dynamic memory allocation
• Array size calculation
• System-level programming

Interview Insight

Common question:

“Is sizeof() a function?”

Answer:
No. It is an operator.

*** 29. What is recursion?

Recursion is a technique where a function calls itself.

Basic Structure

function() {
    function();
}

Important Components

1. Base Condition

Stops recursion.

2. Recursive Call

Function calls itself.

Example

void test(int n) {
    if(n == 0)
        return;

    printf("%d ", n);
    test(n - 1);
}

Advantages

• Simplifies complex problems
• Useful for tree and graph problems

Disadvantages

• More memory usage
• Stack overflow risk

Interview Insight

Very common question:

“What happens without base condition?”

Answer:
Infinite recursion causing stack overflow.

*** 30. Recursive factorial program

Factorial of n:

n! = n × (n-1)!

Recursive Factorial Program

#include <stdio.h>

int factorial(int n) {

    // Base condition
    if(n == 0 || n == 1)
        return 1;

    // Recursive call
    return n * factorial(n - 1);
}

int main() {

    int num = 5;

    printf("Factorial = %d", factorial(num));

    return 0;
}

Output

Factorial = 120

How It Works

factorial(5)
= 5 * factorial(4)
= 5 * 4 * factorial(3)
...
= 120

Interview Insight

Common follow-up:

“What is time complexity of recursive factorial?”

Answer:

O(n)

because function is called n times.

*** 31. Fibonacci using recursion

In recursion, a function calls itself to generate Fibonacci numbers.

Fibonacci Series

0 1 1 2 3 5 8 13 ...

Each number is the sum of previous two numbers.

Recursive Fibonacci Program

#include <stdio.h>

int fibonacci(int n) {

    // Base conditions
    if(n == 0)
        return 0;

    if(n == 1)
        return 1;

    // Recursive call
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {

    int n = 7;

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

    return 0;
}

Output

0 1 1 2 3 5 8

Drawback

Recursive Fibonacci is slower because same values are calculated multiple times.

Interview Insight

Common question:

“What is time complexity of recursive Fibonacci?”

Answer:

O(2^n)

*** 32. Fibonacci without recursion

This approach uses loops instead of recursive function calls.

Iterative Fibonacci Program

#include <stdio.h>

int main() {

    int n = 7;
    int a = 0, b = 1, c;

    printf("%d %d ", a, b);

    for(int i = 2; i < n; i++) {

        c = a + b;
        printf("%d ", c);

        a = b;
        b = c;
    }

    return 0;
}

Output

0 1 1 2 3 5 8

Advantages over Recursion

• Faster
• Less memory usage
• No stack overhead

Interview Insight

Very common comparison question:

“Which Fibonacci approach is better?”

Iterative approach is more efficient.

*** 33. Prime number program

A prime number is divisible only by:

• 1
• itself

Examples:

2, 3, 5, 7, 11

Prime Number Program

#include <stdio.h>

int main() {

    int num = 13;
    int isPrime = 1;

    if(num <= 1) {
        isPrime = 0;
    }

    for(int i = 2; i <= num / 2; i++) {

        if(num % i == 0) {
            isPrime = 0;
            break;
        }
    }

    if(isPrime)
        printf("Prime Number");
    else
        printf("Not Prime Number");

    return 0;
}

Output

Prime Number

Interview Insight

Optimization question:

“Why not loop till num-1?”

Because checking till num/2 or sqrt(num) is enough.

** 34. Armstrong number program

An Armstrong number is a number equal to the sum of cubes of its digits.

Example:

153 = 1³ + 5³ + 3³

Armstrong Number Program

#include <stdio.h>

int main() {

    int num = 153;
    int original = num;
    int rem, sum = 0;

    while(num > 0) {

        rem = num % 10;
        sum += rem * rem * rem;

        num /= 10;
    }

    if(sum == original)
        printf("Armstrong Number");
    else
        printf("Not Armstrong Number");

    return 0;
}

Output

Armstrong Number

Interview Insight

Common follow-up:

“Is 370 an Armstrong number?”

Yes.

*** 35. Reverse a number program

This program reverses digits of a number.

Reverse Number Program

#include <stdio.h>

int main() {

    int num = 1234;
    int reverse = 0;
    int rem;

    while(num > 0) {

        rem = num % 10;

        reverse = reverse * 10 + rem;

        num /= 10;
    }

    printf("Reverse = %d", reverse);

    return 0;
}

Output

Reverse = 4321

Logic

1234
-> 4
-> 43
-> 432
-> 4321

Interview Insight

Common follow-up:

“How to check palindrome number?”

Compare original number with reversed number.

*** 36. Pass by value vs pass by reference

Pass by Value Example

void test(int x) {
    x = 20;
}

Original variable remains unchanged.

Pass by Reference Example

void test(int *x) {
    *x = 20;
}

Original variable changes.

Important Point

C supports:

Pass by value only

Pass by reference is achieved using pointers.

Interview Insight

Very frequently asked question:

“Does C support true pass by reference?”

Answer:
No. It uses pointers to simulate it.

*** 37. What is an array?

An array is a collection of similar data types stored in contiguous memory locations.

Syntax

data_type array_name[size];

Example

int arr[5] = {1, 2, 3, 4, 5};

Features

• Stores multiple values
• Fixed size
• Indexed access

Array Index

arr[0]
arr[1]
arr[2]

Index starts from 0.

Interview Insight

Common question:

“Why are arrays fast?”

Because elements are stored in contiguous memory.

** 38. Difference between arrays and strings?

Array Example

int arr[3] = {1, 2, 3};

String Example

char str[] = "Hello";

Internally:

H e l l o \0

Interview Insight

Important concept:
Every string is an array, but every array is not a string.

*** 39. What is the null character ('\0')?

'\0' is a special character that marks the end of a string in C.

Its ASCII value is:

0

Example

char str[] = "Hi";

Stored as:

H i \0

Why Important?

String functions stop reading when '\0' is found.

Example

printf("%s", str);

Prints characters until null character appears.

Interview Insight

Common question:

“What happens if string has no null character?”

String functions may read garbage values causing undefined behavior.

*** 40. Difference between strlen() and sizeof()?

Example

char str[] = "Hello";

strlen()

strlen(str)

Output:

5

sizeof()

sizeof(str)

Output:

6

Because:

H e l l o \0

Key Difference

strlen() -> Counts characters
sizeof() -> Counts memory bytes

Interview Insight

Very common interview trap:

“Why does sizeof() return larger value for strings?”

Because it includes the null character '\0'.

*** 51. Difference between & and *?

Example

int x = 10;

int *p = &x;

Explanation

& Operator

Returns memory address of variable.

&p

or

&x

* Operator

Accesses value stored at address.

*p

Output:

10

Memory Representation

x = 10
p = address of x
*p = value at address = 10

Interview Insight

Very common question:

“Can * be used for declaration and dereferencing?”

Yes.

int *p;

Declaration.

*p

Dereferencing.

*** 52. What is dereferencing?

Dereferencing means accessing the value stored at the memory address held by a pointer.

It uses:

*

operator.

Example

int x = 20;

int *p = &x;

printf("%d", *p);

Output

20

Explanation

p  -> stores address of x
*p -> accesses value of x

Interview Insight

Common follow-up:

“What happens if invalid pointer is dereferenced?”

It may cause:

• Segmentation fault
• Undefined behavior

*** 53. What is NULL pointer?

A NULL pointer is a pointer that does not point to any valid memory location.

Syntax

int *p = NULL;

Important Point

NULL means:

Pointer contains no valid address.

Why Used?

• Prevents accidental memory access
• Useful for initialization
• Indicates empty pointer

Example

if(p == NULL) {
    printf("Pointer is NULL");
}

Interview Insight

Best practice:
Always initialize unused pointers with NULL.

*** 54. What happens when NULL is dereferenced?

Dereferencing a NULL pointer causes undefined behavior.

Usually, program crashes with:

Segmentation Fault

Example

int *p = NULL;

printf("%d", *p);

Why Crash Happens?

Because program tries to access invalid memory location.

Safe Practice

Always check:

if(p != NULL)

before dereferencing.

Interview Insight

Very common interview question:

“Is dereferencing NULL always guaranteed to crash?”

No. Behavior is undefined, but crashes are common.

*** 55. Difference between array and pointer?

Array Example

int arr[3] = {1, 2, 3};

Pointer Example

int *p = arr;

Important Difference

arr = arr + 1; // Invalid

But:

p = p + 1; // Valid

Interview Insight

Frequently asked concept:
Array name acts like constant pointer to first element.

*** 56. What is pointer arithmetic?

Pointer arithmetic means performing arithmetic operations on pointers.

Supported Operations

• Increment (p++)
• Decrement (p--)
• Addition
• Subtraction

Example

int arr[] = {10, 20, 30};

int *p = arr;

p++;

Now pointer moves to next integer.

Important Point

Pointer increases by:

size of data type

For int:

Usually 4 bytes

Example

Address = 1000
After p++ = 1004

Interview Insight

Very common question:

“Why pointer arithmetic depends on data type?”

Because compiler must move pointer correctly in memory.

** 57. What is a double pointer?

A double pointer is a pointer that stores the address of another pointer.

Also called:

Pointer to pointer

Syntax

int **ptr;

Example

int x = 10;

int *p = &x;

int **pp = &p;

Accessing Values

Output Example

printf("%d", **pp);

Output:

10

Interview Insight

Used heavily in:

• Dynamic 2D arrays
• Command-line arguments
• Linked data structures

*** 58. What is dynamic memory allocation?

Dynamic memory allocation means allocating memory during runtime.

Memory is allocated from:

Heap memory

Why Needed?

• Flexible memory usage
• Runtime-sized arrays
• Efficient memory management

Functions Used

malloc()
calloc()
realloc()
free()

Example

int *p;

p = (int*) malloc(5 * sizeof(int));

Advantages

• Efficient memory usage
• Dynamic data structures possible

Interview Insight

Very common question:

“What happens if allocated memory is not freed?”

It causes:

Memory Leak

*** 59. Difference between stack and heap memory?

Stack Example

int x = 10;

Heap Example

int *p = (int*) malloc(sizeof(int));

Important Difference

Stack memory is automatically released.

Heap memory must be manually released using:

free()

Interview Insight

Common question:

“Which memory causes memory leaks?”

Heap memory.

*** 60. What are malloc(), calloc(), realloc(), and free()?

These are dynamic memory management functions from:

<stdlib.h>

1. malloc()

Allocates memory block.

Syntax

ptr = (type*) malloc(size);

Example

int *p = (int*) malloc(5 * sizeof(int));

Features

• Memory contains garbage values
• Faster than calloc()

2. calloc()

Allocates multiple memory blocks.

Syntax

ptr = (type*) calloc(n, size);

Example

int *p = (int*) calloc(5, sizeof(int));

Features

• Initializes memory with 0

3. realloc()

Resizes previously allocated memory.

Syntax

ptr = realloc(ptr, new_size);

Example

p = realloc(p, 10 * sizeof(int));

4. free()

Releases dynamically allocated memory.

Syntax

free(ptr);

Example

free(p);

Why Important?

Prevents:

Memory leaks

Key Difference Table

Interview Insight

Very frequently asked:

“Difference between malloc() and calloc()?”