Programming in C

How to Execute a C Program in Linux

You can write, compile, and run your C programs directly from the Linux terminal. Follow these steps:

Open a text editor (e.g., gedit) and create a file:
gedit test1.c

now it opens a text area where you can type your code. type the code here.

observe the small * in the file name (its on the top left)
that star says the file is not saved

now you have to save the code.
press ctrl+s

on saving, the star goes off, it meas the file is saved

now you have to close this text editor. to do that

press ctrl+q to quit
Compile the program using the GCC compiler:
cc test1.c
Run the compiled output:
./a.out

Example:
gedit test1.c cc test1.c ./a.out

⚠️ Common Mistakes

no space between test1 .c , use test1.c instead.
Forgetting the ./ before a.out when running the program.
Saving the file with the wrong extension (e.g., test1.txt instead of test1.c).
Using cc test1 without .c — the compiler won't find the file.

Keyboard Shortcuts

Try using the keyboard as much as possible to improve speed and efficiency.

Use the left side of the keyboard with your left hand and the right side with your right hand for better flow.

Select all text or code
Ctrl + A
Copy selected text
Ctrl + C
Paste copied text
Ctrl + V
Save file in editor
Ctrl + S
Quit/Exit editor
Ctrl + Q
Shift key usage:
Hold Shift to type uppercase letters or access the top symbols on keys.

⚠️ Common Mistakes

Forgetting to hold Shift while typing uppercase letters or symbols.
Using Caps Lock instead of Shift — may lead to unexpected ALL CAPS text.
Closing the editor without saving changes (Ctrl + S is your friend!).

C Program to Print "Hello World"

To print the “Hello World”, we can use the printf function from the stdio.h library that prints the given string on the screen.

          
            // Header file for input output functions 

            #include <stdio.h>

            // Main function: entry point for execution

            int main() {

               // Writing print statement to print hello world

               printf("Hello World");

               return 0;

            }

Hello World

Explanation:

#include <stdio.h> - This line includes the standard input-output library in the program.
main() - The main function where the execution of the program begins.
printf(“Hello, World!\n”); - This function call prints “Hello, World!” followed by a new line.
return 0; -This statement indicates that the program ended successfully.

C Program To Print Your Own Name

Printing your own name means displaying your name on the computer screen. In this article, we will learn how to print your own name using a C program.

Examples

Input: name = "Pruthviraj"
Output: Pruthviraj
Input: name = "Guddu"
Output: Guddu

Print Your Own Name Using printf()

The simplest way to print something is to use the printf() function. You can provide your name in the form of string to printf() and it will print it on the output screen.

Program

          
            #include <stdio.h>

            int main() {

               printf("Pruthviraj Guddu");

               return 0;

            }

Pruthviraj Guddu

Other Different Ways to Print Your Own Name in C

Take Your Name as Input and Then Print It
Using puts()
Using fputs()
Using fprintf()
Using write() System Call

Program using scanf()

          
            #include <stdio.h>

            int main() {

               char name[100];

               printf("Enter Your Name: ");

               scanf("%s", name);

               printf("Your Name: %s \n", name);

               return 0;

            }

Enter Your Name: Pruthviraj
Your Name: Pruthviraj

Program using puts()

          
            #include <stdio.h>

            int main() {

               puts("Guddu");

               return 0;

            }

Guddu

Print an Integer Value in C

Printing an Integer in C is one of the most basic tasks. Input and output of the elements are essential for taking input from the user and then giving the output to the user. Here we are going to take an integer as input from the user with the help of the scanf() function and print that integer with the help of the printf() function in C language.

1. Printing Integer Values in C

Store the integer value in the variable of type int and print this value using the printf() method.

Syntax:

printf("%d", variableOfIntType);

Program

          
            // C Program to Print Integer value

            #include <stdio.h>

            // Driver code

            int main() {

               // Declaring integer

               int x = 5;

               // Printing values

               printf("Printing Integer value %d", x);

               return 0;

            }

Printing Integer value 5

2. Reading Integer Values in C

The user enters an integer value when asked. This value is taken from the user with the help of the scanf() method.

Syntax:

scanf("%d", &variableOfIntType);

Program

          
            #include <stdio.h>

            int main() {

               int num;

               printf("Enter the integer: ");

               scanf("%d", &num);

               printf("Entered integer is: %d", num);

               return 0;

            }

Enter the integer: 10
Entered integer is: 10

C Program to Add Two Integers

Given two integers, the task is to add these integer numbers and return their sum.

Examples

Input: a = 5, b = 3
Output: 8
Explanation: The sum of 5 and 3 is 8.
Input: a = -2, b = 7
Output: 5
Explanation: The sum of -2 and 7 is 5.

In C, we have multiple methods to add two numbers, such as using the addition operator (+), the increment operator (++ ), or bitwise operators.

1. Adding Two Numbers using + Operator

Adding two numbers is a simple task in C language that can be accomplished using the '+' operator that takes two operands and returns their sum as the result.

Program

          
            #include <stdio.h>

            int main() {

               int a, b, sum = 0;

               printf("Enter two integers: ");

               scanf("%d %d", &a, &b);

               sum = a + b;

               printf("Sum: %d", sum);

               return 0;

            }

Enter two integers: 5 3
Sum: 8

Complexity

Time Complexity: O(1)
Auxiliary Space: O(1)

Explanation

In the above program, the user is first asked to enter two numbers. The input is taken using the scanf() function and stored in the variables a and b. Then, the variables a and b are added using the arithmetic addition operator +, and the result is stored in the variable sum.

C Program to Check Voting Eligibility

This program takes the user's age as input and checks if they are eligible to vote. If the entered age is 18 or above, it displays a message saying the person can vote.

          
            #include <stdio.h>

            int main() {

              int age;

              printf("Enter your age: ");

              scanf("%d", &age);

              if (age > = 18) {

                printf("You can vote.\n");

              }

              return 0;

            }

Enter your age: 20
You can vote.

Explanation:

The program reads an integer input (age) from the user.
It then checks if the age is greater than or equal to 18.
If true, the program prints You can vote.
Otherwise, nothing is printed and the program ends.

⚠️ Common Mistakes

Forgetting to use &age in scanf("%d", &age); will cause errors.
Missing braces { } after if when writing multiple statements.

C Program to Check if a Number is Even or Odd

This program reads a number from the user and checks whether it is even or odd using the modulo (%) operator.

          
            #include <stdio.h>

            int main() {

              int number;

              printf("Enter a number: ");

              scanf("%d", &number);

              if (number % 2 == 0) {

                printf("The number is even.\n");

              } else {

                printf("The number is odd.\n");

              }

              return 0;

            }

Enter a number: 7
The number is odd.

Explanation:

The program takes an integer input from the user.
The condition number % 2 == 0 checks if the number is divisible by 2.
If true, the program prints The number is even.
Otherwise, it prints The number is odd.

⚠️ Common Mistakes

Forgetting &number in scanf("%d", &number); will not store the input.
Writing if (number % 2 = 0) instead of == — single = is assignment, not comparison.

C Program to Find the Largest of Two Numbers

This program takes two numbers as input from the user and compares them to determine which one is larger.

          
            #include <stdio.h>

            int main() {

              int num1, num2;

              printf("Enter two numbers: ");

              scanf("%d %d", &num1, &num2);

              if (num1 > num2) {

                printf("%d is the largest.\n", num1);

              } else if (num2 > num1) {

                printf("%d is the largest.\n", num2);

              } else {

                printf("Both numbers are equal.\n");

              }

              return 0;

            }

Enter two numbers: 12 25
25 is the largest.

Explanation:

The program takes two integer inputs from the user using scanf().
If num1 > num2, it prints that num1 is larger.
If num2 > num1, it prints that num2 is larger.
If neither condition is true, both numbers are equal.

⚠️ Common Mistakes

Forgetting &num1 and &num2 in scanf("%d %d", &num1, &num2);.
Using = instead of == when checking equality.
Not handling the case when both numbers are equal.

Different Ways to Find the Largest of Three Numbers Using If Conditions

These examples show various ways to find the largest among three numbers using if-based logic in C.

Method 1: Using Nested if

          
            #include <stdio.h>

            int main() {

              int a, b, c;

              printf("Enter three numbers: ");

              scanf("%d %d %d", &a, &b, &c);

              if (a > b) {

                if (a > c)

                  printf("%d is the largest.\n",
            a);

                else

                  printf("%d is the largest.\n",
            c);

              } else {

                if (b > c)

                  printf("%d is the largest.\n",
            b);

                else

                  printf("%d is the largest.\n",
            c);

              }

              return 0;

            }

Method 2: Using if...else if...else

          
            #include <stdio.h>

            int main() {

              int a, b, c;

              printf("Enter three numbers: ");

              scanf("%d %d %d", &a, &b, &c);

              if (a > = b && a > = c)

                printf("%d is the largest.\n", a);

              else if (b > = a && b > = c)

                printf("%d is the largest.\n", b);

              else

                printf("%d is the largest.\n", c);

              return 0;

            }

Method 3: Using Simple if Conditions

          
            #include <stdio.h>

            int main() {

              int a, b, c;

              printf("Enter three numbers: ");

              scanf("%d %d %d", &a, &b, &c);

              if (a > b && a > c)

                printf("%d is the largest.\n", a);

              if (b > a && b > c)

                printf("%d is the largest.\n", b);

              if (c > a && c > b)

                printf("%d is the largest.\n", c);

              if (a == b && b == c)

                printf("All numbers are equal.\n");

              return 0;

            }

🔍 Tips & Notes

Always validate input when using scanf.
Be cautious of using only if without else — multiple blocks may execute.
Handle equal values when necessary.

C Program to Classify Score using if-else

This program takes a score as input and classifies it into distinction, first class, second class, pass class, or fail using if-else statements.

          
            #include <stdio.h>

            int main() {

              int score;

              printf("Enter your score (0-100): ");

              scanf("%d", &score);

              if (score > = 90) {

                printf("Distinction\n");

              } else if (score > = 80) {

                printf("First Class\n");

              } else if (score > = 70) {

                printf("Second Class\n");

              } else if (score > = 50) {

                printf("Pass Class\n");

              } else {

                printf("Fail\n");

              }

              return 0;

            }

Enter your score (0-100): 85
First Class

Explanation:

User inputs a score using scanf().
if-else conditions check ranges and print the corresponding class.
Final else handles the "fail" case.

C Program to Classify Score using switch

This program uses switch to classify a score. Since switch doesn't support ranges directly, we divide the score by 10 and use the result.

          
            #include <stdio.h>

            int main() {

              int score;

              printf("Enter your score (0-100): ");

              scanf("%d", &score);

              switch (score / 10) {

                case 10:

                case 9:

                  printf("Distinction\n");

                  break;

                case 8:

                  printf("First Class\n");

                  break;

                case 7:

                  printf("Second Class\n");

                  break;

                case 5:

                case 6:

                  printf("Pass Class\n");

                  break;

                default:

                  if (score < 0 || score >
            100) {

                    printf("Invalid
            Score\n");

                  } else {

                    printf("Fail\n");

                  }

              }

              return 0;

            }

Enter your score (0-100): 73
Second Class

Explanation:

Score is divided by 10 to get a category (e.g., 85 becomes 8).
switch checks the case values to classify.
Special handling for out-of-range scores in the default case.

C Program to Print Weekday Name using switch

This program takes a number (1 to 7) and uses switch to print the corresponding weekday.

          
            #include <stdio.h>

            int main() {

              int day;

              printf("Enter day number (1-7): ");

              scanf("%d", &day);

              switch (day) {

                case 1:

                  printf("Monday\n");

                  break;

                case 2:

                  printf("Tuesday\n");

                  break;

                case 3:

                  printf("Wednesday\n");

                  break;

                case 4:

                  printf("Thursday\n");

                  break;

                case 5:

                  printf("Friday\n");

                  break;

                case 6:

                  printf("Saturday\n");

                  break;

                case 7:

                  printf("Sunday\n");

                  break;

                default:

                  printf("Invalid day
            number\n");

              }

              return 0;

            }

Enter day number (1-7): 3
Wednesday

Explanation:

User enters a number from 1 to 7.
The switch matches the number to a case and prints the corresponding weekday.
If the number is out of range, the default case prints an error message.

C Program to Print Squares of First 10 Numbers (for loop)

This program uses a for loop to print the squares of numbers from 1 to 10.

          
            #include <stdio.h>

            int main() {

              int i;

              for (i = 1; i < = 10; i++) {

                printf("%d^2 = %d\n", i, i * i);

              }

              return 0;

            }

Output:
1^2 = 1
2^2 = 4
3^2 = 9
...
10^2 = 100

C Program to Display Even Numbers up to 20 (for loop)

This program prints all even numbers between 1 and 20 using a for loop.

          
            #include <stdio.h>

            int main() {

              printf("Even numbers from 1 to 20:\n");

              for (int i = 2; i < = 20; i += 2) {

                printf("%d ", i);

              }

              return 0;

            }

Output: 2 4 6 8 10 12 14 16 18 20

C Program to Check if a Number is Prime (for loop)

This program checks whether a given number is prime using a for loop.

          
            #include <stdio.h>

            int main() {

              int n, i, isPrime = 1;

              printf("Enter a number: ");

              scanf("%d", &n);

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

                if (n % i == 0) {

                  isPrime = 0;

                  break;

                }

              }

              if (isPrime && n > 1)

                printf("%d is a prime number.\n", n);

              else

                printf("%d is not a prime number.\n", n);

              return 0;

            }

Output:
Enter a number: 20
20 is not a prime number.

C Program to Find Sum of Digits (while loop)

This program finds the sum of digits of a given number using a while loop.

          
            #include <stdio.h>

            int main() {

              int n, sum = 0, digit;

              printf("Enter a number: ");

              scanf("%d", &n);

              while (n ! = 0) { /*no spaces*/

                digit = n % 10;

                sum + = digit; /*no spaces*/

                n / = 10; /*no spaces*/

              }

              printf("Sum of digits = %d\n", sum);

              return 0;

            }

Enter a number: 1234
Sum of digits = 10

C Program to Find Sum of First N Numbers (while loop)

This program calculates the sum of the first N natural numbers using a while loop.

          
            #include <stdio.h>

            int main() {

              int n, sum = 0, i = 1;

              printf("Enter a number: ");

              scanf("%d", &n);

              while (i < = n) {

                sum += i;

                i++;

              }

              printf("Sum = %d\n", sum);

              return 0;

            }

Enter a number: 5
Sum = 15

C Program to Reverse a Number (while loop)

This program reverses a given number using a while loop.

          
            #include <stdio.h>

            int main() {

              int num, rev = 0, digit;

              printf("Enter a number: ");

              scanf("%d", &num);

              while (num ! = 0) {

                digit = num % 10;

                rev = rev * 10 + digit;

                num / = 10;

              }

              printf("Reversed number = %d\n", rev);

              return 0;

            }

Enter a number: 1234
Reversed number = 4321

C Program to Check Palindrome Number (while loop)

This program checks if a number is a palindrome using a while loop.

this is follows the same logis as above to reverse the number, additionally we will compare the original number with reversed number

          
            #include <stdio.h>

            int main() {

              int num, temp, rev = 0, digit;

              printf("Enter a number: ");

              scanf("%d", &num);

              temp = num;

              while (temp ! = 0) {

                digit = temp % 10;

                rev = rev * 10 + digit;

                temp / = 10;

              }

              if (rev == num)

                printf("Palindrome\n");

              else

                printf("Not Palindrome\n");

              return 0;

            }

Enter a number: 121
Palindrome

C Program to Check if a String is Palindrome (while loop, flag, start/end)

This program checks if a string is a palindrome using a while loop, a flag, and start/end variables.

          
            #include <stdio.h>

            #include <string.h>

            int main() {

              char str[100];

              int start = 0, end, flag = 1;

              printf("Enter a string: ");

              scanf("%s", str);

              end = strlen(str) - 1;

              while (start < end) {

                if (str[start] ! = str[end]) {

                  flag = 0;

                  break;

                }

                start++;

                end--;

              }

              if (flag = = 1)

                printf("Palindrome\n");

              else

                printf("Not Palindrome\n");

              return 0;

            }

Enter a string: radar
Palindrome

C Program to Check if a String is Palindrome (while loop)

This program checks if a string is a palindrome using a while loop.

          
            #include <stdio.h>

            #include <string.h>

            int main() {

              char str[100];

              int start = 0, end, isPalindrome = 1;

              printf("Enter a string: ");

              scanf("%s", str);

              end = strlen(str) - 1;

              while (start < end) {

                if (str[start] ! = str[end]) {

                  isPalindrome = 0;

                  break;

                }

                start++;

                end--;

              }

              if (isPalindrome)

                printf("Palindrome\n");

              else

                printf("Not Palindrome\n");

              return 0;

            }

Enter a string: gadag
Palindrome

Enter a string: madam
Palindrome

Enter a string: apple
Not Palindrome

C Program to Find Factorial of a Number (while loop)

This program calculates the factorial of a number using a while loop.

          
            #include <stdio.h>

            int main() {

              int n, fact = 1;

              printf("Enter a number: ");

              scanf("%d", &n);

              int i = 1;

              while (i < = n) {

                fact *= i;

                i++;

              }

              printf("Factorial = %d\n", fact);

              return 0;

            }

Output: Enter a number: 5
Factorial = 120

Output: Enter a number: 8
Factorial = 40320

C Program to Count Digits in a Number (while loop)

This program counts the total number of digits in a given number using a while loop.

          
            #include <stdio.h>

            int main() {

              int n, count = 0;

              printf("Enter a number: ");

              scanf("%d", &n);

              while (n ! = 0) {

                n / = 10;

                count++;

              }

              printf("Number of digits = %d\n", count);

              return 0;

            }

Output: Enter a number: 2142
Number of digits = 4

Output: Enter a number: 32345
Number of digits = 5

C Program to Print Numbers from 1 to N (do-while loop)

This program prints numbers from 1 to N using a do-while loop.

          
            #include <stdio.h>

            int main() {

              int n, i = 1;

              printf("Enter the value of N: ");

              scanf("%d", &n);

              do {

                printf("%d ", i);

                i++;

              } while (i < = n);

              return 0;

            }

Output:
Enter the value of N: 5
1 2 3 4 5

C Program to Display Menu using do-while loop

This program demonstrates a simple menu-driven system using a do-while loop.

          
            #include <stdio.h>

            int main() {

              int choice;

              do {

                printf("\n1. Add\n2. Subtract\n3.
            Exit\n");

                printf("Enter choice: ");

                scanf("%d", &choice);

                switch (choice) {

                  case 1:

                    printf("Addition
            selected\n");

                    break;

                  case 2:

                    printf("Subtraction
            selected\n");

                    break;

                  case 3:

                    printf("Exiting...\n");

                    break;

                  default:

                    printf("Invalid
            choice!\n");

                }

              } while (choice ! = 3);

              return 0;

            }

Output:
1. Add
2. Subtract
3. Exit
Enter choice: 1
Addition selected
1. Add
2. Subtract
3. Exit
Enter choice: 2
Subtraction selected
1. Add
2. Subtract
3. Exit
Enter choice:

C Program to Find Sum of Positive Numbers (do-while loop)

This program adds positive numbers entered by the user until a negative number is entered.

          
            #include <stdio.h>

            int main() {

              int num, sum = 0;

              do {

                printf("Enter a number: ");

                scanf("%d", &num);

                if (num > 0)

                  sum += num;

              } while (num > = 0);

              printf("Sum = %d\n", sum);

              return 0;

            }

Output:
Enter a number: 20
Enter a number: 30
Enter a number: 0
Enter a number: -1
Sum = 50

Output:
Enter a number: 33
Enter a number: 5
Enter a number: 11
Enter a number: 25
Enter a number: 35
Enter a number: -1
Sum = 109

C Program to Reverse a Number (do-while loop)

This program reverses a given number using a do-while loop.

          
            #include <stdio.h>

            int main() {

              int n, rev = 0, digit;

              printf("Enter a number: ");

              scanf("%d", &n);

              do {

                digit = n % 10;

                rev = rev * 10 + digit;

                n / = 10;

              } while (n ! = 0);

              printf("Reversed Number = %d\n", rev);

              return 0;

            }

Output:
Enter a number: 245316
Reversed Number = 613542

Output:
Enter a number: 391245
Reversed Number = 542193

C Program - Simple Calculator (do-while loop)

A menu-driven calculator using a do-while loop to perform basic arithmetic operations.

          
            #include <stdio.h>

            int main() {

              int choice;

              float a, b, result;

              do {

                printf("\n1. Add\n2. Subtract\n3.
            Multiply\n4. Divide\n5. Exit\n");

                printf("Enter your choice: ");

                scanf("%d", &choice);

                if (choice >= 1 && choice <=
            4) {

                  printf("Enter two numbers:
            ");

                  scanf("%f %f", &a,
            &b);

                }

                switch (choice) {

                  case 1: result = a + b;
            break;

                  case 2: result = a - b;
            break;

                  case 3: result = a * b;
            break;

                  case 4: 

                    if (b != 0)

                      result =
            a / b;

                    else {

                      printf("Cannot
            divide by zero!\n");

                      continue;

                    }

                  break;

                  case 5: printf("Exiting...\n");
            break;

                  default: printf("Invalid
            choice!\n"); continue;

                }

                if (choice > = 1 && choice <=
            4)

                  printf("Result = %.2f\n",
            result);

              } while (choice ! = 5);

              return 0;

            }

Output:
1. Add
2. Subtract
3. Multiply
4. Divide
5. Exit
Enter your choice: 1
Enter two numbers: 5 3
Result = 8.00
...

Output:
1. Add
2. Subtract
3. Multiply
4. Divide
5. Exit
Enter your choice: 1
Enter two numbers: 15 20
Result = 35.00

1. Add
2. Subtract
3. Multiply
4. Divide
5. Exit
Enter your choice: 3
Enter two numbers: 4 2
Result = 8.00

1. Add
2. Subtract
3. Multiply
4. Divide
5. Exit
Enter your choice: