C Programming Coding Questions

Practice most asked C programming coding interview questions with solutions.

1. Write a program to find factorial of a number

Easy 
Input: 5
Output: 120

#include <stdio.h>
int main() {
  int n, fact = 1;
  scanf("%d", &n);

  for(int i = 1; i <= n; i++)
    fact *= i;

  printf("%d", fact);
  return 0;
}

2. Check whether a number is prime

Easy 
Input: 3
Output: Prime

#include <stdio.h>
int main() {
  int n, flag = 1;
  scanf("%d", &n);

  if(n <= 1) flag = 0;
  for(int i = 2; i <= n/2; i++) {
    if(n % i == 0) {
      flag = 0;
      break;
    }
  }

  printf(flag ? "Prime" : "Not Prime");
  return 0;
}

3. Find Fibonacci series

Easy 
Input: 7
Output: 0 1 1 2 3 5 8

#include <stdio.h>
int main() {
  int n, a = 0, b = 1, c;
  scanf("%d", &n);

  for(int i = 1; i <= n; i++) {
    printf("%d ", a);
    c = a + b;
    a = b;
    b = c;
  }
  return 0;
}

4. Find largest of three numbers

Easy 
Input: 10 20 30
Output: 30

#include <stdio.h>
int main() {
  int a, b, c;
  scanf("%d%d%d", &a, &b, &c);

  if(a > b && a > c)
    printf("%d", a);
  else if(b > c)
    printf("%d", b);
  else
    printf("%d", c);

  return 0;
}

5. Reverse a string

Easy 
Input: HelloWorld
Output: dlroWolleH

#include <stdio.h>
#include <string.h>
int main() {
  char str[100];
  gets(str);

  for(int i = strlen(str) - 1; i >= 0; i--)
    printf("%c", str[i]);

  return 0;
}

6. Count digits of a number

Easy 
Input: 1451
Output: 4

#include <stdio.h>
int main() {
  int n, count = 0;
  scanf("%d", &n);

  while(n != 0) {
    count++;
    n /= 10;
  }

  printf("%d", count);
  return 0;
}

7. Check Armstrong number

Medium 
Input: 153
Output: Armstrong

#include <stdio.h>
#include <math.h>
int main() {
  int n, temp, sum = 0, digits = 0;
  scanf("%d", &n);
  temp = n;

  while(temp) {
    digits++;
    temp /= 10;
  }

  temp = n;
  while(temp) {
    sum += pow(temp % 10, digits);
    temp /= 10;
  }

  printf(sum == n ? "Armstrong" : "Not Armstrong");
  return 0;
}

8. Find sum of array elements

Easy 
Input: 5
       10 20 30 40 50
Output: 150

#include <stdio.h>
int main() {
  int n, sum = 0;
  scanf("%d", &n);
  int arr[n];

  for(int i = 0; i < n; i++) {
    scanf("%d", &arr[i]);
    sum += arr[i];
  }

  printf("%d", sum);
  return 0;
}

9. Linear search

Easy 
Input: 5
       10 20 30 40 50
       30
Output: Found

#include <stdio.h>
int main() {
  int n, key, found = 0;
  scanf("%d", &n);
  int arr[n];

  for(int i = 0; i < n; i++)
    scanf("%d", &arr[i]);

  scanf("%d", &key);

  for(int i = 0; i < n; i++) {
    if(arr[i] == key) {
      found = 1;
      break;
    }
  }

  printf(found ? "Found" : "Not Found");
  return 0;
}

10. Swap two numbers without third variable

Medium 
Input: 10 20
Output: 20 10

#include <stdio.h>
int main() {
  int a, b;
  scanf("%d%d", &a, &b);

  a = a + b;
  b = a - b;
  a = a - b;

  printf("%d %d", a, b);
  return 0;
}

11. Find second largest element in an array

Medium 
Input: 6
       10 45 20 8 45 30
Output: 30

#include <stdio.h>
int main() {
  int n;
  scanf("%d", &n);
  int a[n], max, second;
  for(int i=0;i max){
      second = max;
      max = a[i];
    } else if(a[i] > second && a[i] != max){
      second = a[i];
    }
  }
  printf("%d", second);
  return 0;
}

12. Check if a string is palindrome

Medium 
Input: madam
Output: Palindrome

#include <stdio.h>
#include <string.h>
int main() {
  char s[100];
  gets(s);
  int l=0, r=strlen(s)-1;

  while(l

13. Remove duplicate elements from array

Medium 
Input: 8
       1 2 3 2 4 1 5 3
Output: 1 2 3 4 5

#include <stdio.h>
int main(){
  int n;
  scanf("%d",&n);
  int a[n];
  for(int i=0;i

14. Count frequency of characters in string

Medium 
Input: hello
Output: h = 1
        e = 1
        l = 2
        o = 1


#include <stdio.h>
int main(){
  char s[100];
  gets(s);
  int freq[256]={0};

  for(int i=0;s[i];i++)
    freq[s[i]]++;

  for(int i=0;i<256;i++)
    if(freq[i])
      printf("%c = %d\n", i, freq[i]);
  return 0;
}

15. Binary search

Medium 
Input: 7
       2 5 8 12 16 23 38
       16
Output: Found

#include <stdio.h>
int main(){
  int n,key;
  scanf("%d",&n);
  int a[n];
  for(int i=0;i

16. Implement stack using array

Hard 
Output: 10

#include <stdio.h>
#define SIZE 5
int stack[SIZE], top=-1;

void push(int x){
  if(top==SIZE-1) return;
  stack[++top]=x;
}

void pop(){
  if(top==-1) return;
  top--;
}

int main(){
  push(10); push(20);
  pop();
  for(int i=0;i<=top;i++)
    printf("%d ",stack[i]);
}

17. Implement queue using array

Hard 
Output: 2

#include <stdio.h>
#define SIZE 5
int q[SIZE], front=0, rear=-1;

void enqueue(int x){
  if(rear==SIZE-1) return;
  q[++rear]=x;
}

void dequeue(){
  if(front>rear) return;
  front++;
}

int main(){
  enqueue(1); enqueue(2);
  dequeue();
  for(int i=front;i<=rear;i++)
    printf("%d ",q[i]);
}

18. Reverse linked list

Hard 
Input: 10 20 30
Output: 30 20 10

struct Node{
  int data;
  struct Node* next;
};

struct Node* reverse(struct Node* head){
  struct Node *prev=NULL,*curr=head,*next=NULL;
  while(curr){
    next=curr->next;
    curr->next=prev;
    prev=curr;
    curr=next;
  }
  return prev;
}

19. Check balanced parentheses

Hard 
Input: ((a+b)*(c+d))
Output: Balanced

#include <stdio.h>
#include <string.h>
int main(){
  char s[100];
  int count=0;
  gets(s);

  for(int i=0;s[i];i++){
    if(s[i]=='(') count++;
    if(s[i]==')') count--;
    if(count<0){
      printf("Unbalanced");
      return 0;
    }
  }
  printf(count==0 ? "Balanced" : "Unbalanced");
}

20. Find missing number in array

Hard 
Input: 5
       1 2 3 5
Output: 4

#include <stdio.h>
int main(){
  int n,sum=0;
  scanf("%d",&n);
  int a[n];
  for(int i=0;i

21. Write a program to reverse a number

Easy

Problem: Reverse a given integer number.

Example:

Input: 1234
Output: 4321

#include <stdio.h>
int main() {
  int n, rev = 0;
  scanf("%d", &n);

  while(n != 0) {
    rev = rev * 10 + n % 10;
    n /= 10;
  }

  printf("%d", rev);
  return 0;
}

22. Check whether a number is palindrome

Easy 
Input: 121
Output: Palindrome

#include <stdio.h>
int main() {
  int n, temp, rev = 0;
  scanf("%d", &n);
  temp = n;

  while(n != 0) {
    rev = rev * 10 + n % 10;
    n /= 10;
  }

  if(temp == rev)
    printf("Palindrome");
  else
    printf("Not Palindrome");

  return 0;
}