C Programming

Every C lab program in one place. Each one follows the same rhythm — read the input, apply the logic, print the result, show a sample run — so once you've got the pattern, they're all easy. Use the On this page list to jump straight to any program.

Basics quick reference

Input / output:

printf("...", values) prints; scanf("...", &var) reads (note the &).
Format specifiers: %d int, %f float, %.2f float to 2 decimals, %c char, %s string, %ld long.
Escape sequences: \n new line, \t tab.

Operators:

Arithmetic + - * / % — / on two ints drops the fraction; % is the remainder.
Relational == != > < >= <= give 1 (true) or 0 (false).
Logical && (and), || (or), ! (not).
Bitwise & | ^ ~, shift << >>; ternary condition ? a : b.

The program template

#include <stdio.h>      /* printf, scanf            */
#include <conio.h>      /* clrscr(), getch()        */
 
void main()
{
    /* declare variables */
    clrscr();           /* clear screen             */
    /* printf the prompt, scanf the input */
    /* do the calculation */
    /* printf the result  */
    getch();            /* pause so output stays    */
}

Classic Turbo C style is used throughout (void main, clrscr, getch from <conio.h>). Modern standard C uses int main(void) with return 0; and drops <conio.h> — everything else is identical.

1. Swap two variables

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int a, b, temp;
    clrscr();
 
    printf("Enter two values: ");
    scanf("%d %d", &a, &b);
 
    temp = a;
    a = b;
    b = temp;
 
    printf("\nAfter swapping: a = %d, b = %d", a, b);
    getch();
}

How it works: temp holds a's value safely while b is copied into a, then the saved value goes into b. (You can also swap with arithmetic: a=a+b; b=a-b; a=a-b;.)

Enter two values: 6 7
After swapping: a = 7, b = 6

2. Odd or even

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int n;
    clrscr();
 
    printf("Enter a number: ");
    scanf("%d", &n);
 
    if (n % 2 == 0)
        printf("\n%d is Even", n);
    else
        printf("\n%d is Odd", n);
    getch();
}

How it works: if the remainder n % 2 is 0, the number is even.

Enter a number: 7
7 is Odd

3. Largest of three numbers

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int a, b, c;
    clrscr();
 
    printf("Enter three numbers: ");
    scanf("%d %d %d", &a, &b, &c);
 
    if (a >= b && a >= c)
        printf("\n%d is the largest", a);
    else if (b >= c)
        printf("\n%d is the largest", b);
    else
        printf("\n%d is the largest", c);
    getch();
}

How it works: a wins only if it beats both others; otherwise the larger of b and c.

Enter three numbers: 4 15 9
15 is the largest

4. Area of a circle

#include <stdio.h>
#include <conio.h>
 
void main()
{
    float r, area;
    float pi = 3.14;
    clrscr();
 
    printf("Enter the radius: ");
    scanf("%f", &r);
 
    area = pi * r * r;
 
    printf("\nArea of circle = %.2f", area);
    getch();
}

How it works: area is pi x r x r with pi = 3.14.

Enter the radius: 5
Area of circle = 78.50

5. Simple calculator (switch case)

#include <stdio.h>
#include <conio.h>
 
void main()
{
    float a, b, result;
    char op;
    clrscr();
 
    printf("Enter an expression (e.g. 5 + 3): ");
    scanf("%f %c %f", &a, &op, &b);
 
    switch (op)
    {
        case '+': result = a + b; break;
        case '-': result = a - b; break;
        case '*': result = a * b; break;
        case '/': result = a / b; break;
        default:
            printf("\nInvalid operator");
            getch();
            return;
    }
 
    printf("\nResult = %.2f", result);
    getch();
}

How it works: switch picks the branch matching the operator character; break stops it falling through to the next case.

Enter an expression (e.g. 5 + 3): 8 * 4
Result = 32.00

6. Factorial of a number

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int n, i;
    long fact = 1;
    clrscr();
 
    printf("Enter a number: ");
    scanf("%d", &n);
 
    for (i = 1; i <= n; i++)
        fact = fact * i;
 
    printf("\nFactorial of %d = %ld", n, fact);
    getch();
}

How it works: multiply fact by every number from 1 to n. long because factorials grow fast.

Enter a number: 5
Factorial of 5 = 120

7. Sum of N natural numbers

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int n, i, sum = 0;
    clrscr();
 
    printf("Enter N: ");
    scanf("%d", &n);
 
    for (i = 1; i <= n; i++)
        sum = sum + i;
 
    printf("\nSum of first %d natural numbers = %d", n, sum);
    getch();
}

How it works: add 1, 2, 3 ... up to n. (Shortcut formula: n * (n + 1) / 2.)

Enter N: 10
Sum of first 10 natural numbers = 55

8. First N Fibonacci numbers

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int n, i, a = 0, b = 1, c;
    clrscr();
 
    printf("How many terms? ");
    scanf("%d", &n);
 
    printf("\n%d %d", a, b);
 
    for (i = 3; i <= n; i++)
    {
        c = a + b;
        printf(" %d", c);
        a = b;
        b = c;
    }
    getch();
}

How it works: each term is the sum of the previous two; slide the window forward each step.

How many terms? 7
0 1 1 2 3 5 8

9. Prime numbers in a range

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int low, high, i, j, flag;
    clrscr();
 
    printf("Enter the range (low high): ");
    scanf("%d %d", &low, &high);
 
    printf("\nPrimes: ");
    for (i = low; i <= high; i++)
    {
        if (i < 2) continue;
        flag = 1;
        for (j = 2; j <= i / 2; j++)
        {
            if (i % j == 0) { flag = 0; break; }
        }
        if (flag == 1)
            printf("%d ", i);
    }
    getch();
}

How it works: for each number in the range, test if anything from 2 to i/2 divides it; if nothing does, it's prime.

Enter the range (low high): 10 30
Primes: 11 13 17 19 23 29

10. GCD of two numbers (recursion)

#include <stdio.h>
#include <conio.h>
 
int gcd(int a, int b)
{
    if (b == 0)
        return a;
    return gcd(b, a % b);   /* recursive call */
}
 
void main()
{
    int a, b;
    clrscr();
 
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
 
    printf("\nGCD = %d", gcd(a, b));
    getch();
}

How it works: Euclid's rule — gcd(a, b) = gcd(b, a % b), stopping when b becomes 0. The function calls itself with smaller values each time (recursion).

Enter two numbers: 36 24
GCD = 12

11. Roots of a quadratic equation

#include <stdio.h>
#include <conio.h>
#include <math.h>
 
void main()
{
    float a, b, c, d, r1, r2;
    clrscr();
 
    printf("Enter a, b, c: ");
    scanf("%f %f %f", &a, &b, &c);
 
    d = b * b - 4 * a * c;          /* discriminant */
 
    if (d > 0)
    {
        r1 = (-b + sqrt(d)) / (2 * a);
        r2 = (-b - sqrt(d)) / (2 * a);
        printf("\nReal and distinct roots: %.2f and %.2f", r1, r2);
    }
    else if (d == 0)
    {
        r1 = -b / (2 * a);
        printf("\nReal and equal roots: %.2f", r1);
    }
    else
    {
        printf("\nComplex roots: %.2f + %.2fi and %.2f - %.2fi",
               -b / (2 * a), sqrt(-d) / (2 * a),
               -b / (2 * a), sqrt(-d) / (2 * a));
    }
    getch();
}

How it works: the discriminant d = b^2 - 4ac decides the case — positive (two real roots), zero (one repeated root), negative (complex roots).

Enter a, b, c: 1 -5 6
Real and distinct roots: 3.00 and 2.00

12. Reverse an integer

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int n, rev = 0, digit;
    clrscr();
 
    printf("Enter a number: ");
    scanf("%d", &n);
 
    while (n > 0)
    {
        digit = n % 10;
        rev = rev * 10 + digit;
        n = n / 10;
    }
 
    printf("\nReversed = %d", rev);
    getch();
}

How it works: peel the last digit with % 10, append it to rev (shift rev up with * 10), drop the digit with / 10, repeat.

Enter a number: 1234
Reversed = 4321

13. Sort N numbers (bubble sort)

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int a[50], n, i, j, temp;
    clrscr();
 
    printf("How many numbers? ");
    scanf("%d", &n);
    printf("Enter %d numbers: ", n);
    for (i = 0; i < n; i++)
        scanf("%d", &a[i]);
 
    for (i = 0; i < n - 1; i++)
        for (j = 0; j < n - 1 - i; j++)
            if (a[j] > a[j + 1])
            {
                temp = a[j];
                a[j] = a[j + 1];
                a[j + 1] = temp;
            }
 
    printf("\nSorted: ");
    for (i = 0; i < n; i++)
        printf("%d ", a[i]);
    getch();
}

How it works: bubble sort repeatedly compares neighbours and swaps them if out of order, so the largest "bubbles" to the end each pass.

How many numbers? 5
Enter 5 numbers: 5 1 4 2 3
Sorted: 1 2 3 4 5

14. Linear search

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int a[50], n, i, key, found = 0;
    clrscr();
 
    printf("How many numbers? ");
    scanf("%d", &n);
    printf("Enter %d numbers: ", n);
    for (i = 0; i < n; i++)
        scanf("%d", &a[i]);
 
    printf("Enter the number to search: ");
    scanf("%d", &key);
 
    for (i = 0; i < n; i++)
    {
        if (a[i] == key)
        {
            printf("\nFound at position %d", i + 1);
            found = 1;
            break;
        }
    }
 
    if (found == 0)
        printf("\nNot found");
    getch();
}

How it works: check each element one by one until the key is found.

How many numbers? 5
Enter 5 numbers: 10 20 30 40 50
Enter the number to search: 30
Found at position 3

15. Add two matrices

#include <stdio.h>
#include <conio.h>
 
void main()
{
    int a[10][10], b[10][10], c[10][10];
    int r, col, i, j;
    clrscr();
 
    printf("Enter rows and columns: ");
    scanf("%d %d", &r, &col);
 
    printf("Enter first matrix:\n");
    for (i = 0; i < r; i++)
        for (j = 0; j < col; j++)
            scanf("%d", &a[i][j]);
 
    printf("Enter second matrix:\n");
    for (i = 0; i < r; i++)
        for (j = 0; j < col; j++)
            scanf("%d", &b[i][j]);
 
    for (i = 0; i < r; i++)
        for (j = 0; j < col; j++)
            c[i][j] = a[i][j] + b[i][j];
 
    printf("\nSum:\n");
    for (i = 0; i < r; i++)
    {
        for (j = 0; j < col; j++)
            printf("%d ", c[i][j]);
        printf("\n");
    }
    getch();
}

How it works: add the two matrices cell by cell — c[i][j] = a[i][j] + b[i][j] — using nested loops over rows and columns.

Enter rows and columns: 2 2
First: 1 2 3 4   Second: 5 6 7 8
Sum:
6 8
10 12

16. Pass by value vs pass by address

#include <stdio.h>
#include <conio.h>
 
void swapByValue(int x, int y)      /* gets copies        */
{
    int t = x; x = y; y = t;
}
 
void swapByAddress(int *x, int *y)  /* gets addresses     */
{
    int t = *x; *x = *y; *y = t;
}
 
void main()
{
    int a = 10, b = 20;
    clrscr();
 
    swapByValue(a, b);
    printf("After pass by value:   a = %d, b = %d", a, b);
 
    swapByAddress(&a, &b);
    printf("\nAfter pass by address: a = %d, b = %d", a, b);
    getch();
}

How it works: pass by value sends copies, so the originals don't change. Pass by address sends pointers (&a), so the function edits the real variables through *x.

After pass by value:   a = 10, b = 20
After pass by address: a = 20, b = 10

17. Add two numbers using a function

#include <stdio.h>
#include <conio.h>
 
int add(int x, int y)
{
    return x + y;
}
 
void main()
{
    int a, b;
    clrscr();
 
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
 
    printf("\nSum = %d", add(a, b));
    getch();
}

How it works: the work is moved into a reusable function add that takes two values and returns their sum.

Enter two numbers: 7 8
Sum = 15

18. String operations

#include <stdio.h>
#include <conio.h>
#include <string.h>
 
void main()
{
    char s1[50], s2[50], s3[50];
    clrscr();
 
    printf("Enter first string: ");
    scanf("%s", s1);              /* no & for arrays */
    printf("Enter second string: ");
    scanf("%s", s2);
 
    printf("\nLength of s1 = %d", strlen(s1));   /* i) length      */
 
    strcpy(s3, s1);                              /* ii) copy        */
    printf("\nCopy of s1 = %s", s3);
 
    strcat(s1, s2);                              /* iii) concatenate*/
    printf("\ns1 + s2 = %s", s1);
 
    if (strcmp(s2, s3) == 0)                     /* iv) compare     */
        printf("\ns2 and copy are equal");
    else
        printf("\ns2 and copy are different");
    getch();
}

How it works: the <string.h> functions do the work — strlen (length), strcpy (copy), strcat (join), strcmp (compare; returns 0 when equal).

Enter first string: hello
Enter second string: world
Length of s1 = 5
Copy of s1 = hello
s1 + s2 = helloworld
s2 and copy are different

19. Structure — STUDENT

#include <stdio.h>
#include <conio.h>
 
struct Student
{
    int roll;
    char name[30];
    float marks;
};
 
void main()
{
    struct Student s[50];
    int n, i;
    clrscr();
 
    printf("How many students? ");
    scanf("%d", &n);
 
    for (i = 0; i < n; i++)
    {
        printf("\nEnter roll, name, marks: ");
        scanf("%d %s %f", &s[i].roll, s[i].name, &s[i].marks);
    }
 
    printf("\nStudent details:\n");
    for (i = 0; i < n; i++)
        printf("%d  %s  %.2f\n", s[i].roll, s[i].name, s[i].marks);
    getch();
}

How it works: a struct groups related fields (roll, name, marks) into one type. An array of structs holds many students; reach a field with s[i].roll.

How many students? 2
1 Asha 88.5
2 Ravi 91.0
Student details:
1  Asha  88.50
2  Ravi  91.00

20. Read & write a character using a file

#include <stdio.h>
#include <conio.h>
 
void main()
{
    FILE *fp;
    char ch;
    clrscr();
 
    fp = fopen("data.txt", "w");      /* open for writing */
    printf("Enter text (end with Enter): ");
    while ((ch = getchar()) != '\n')
        fputc(ch, fp);                /* write one char   */
    fclose(fp);
 
    fp = fopen("data.txt", "r");      /* open for reading */
    printf("\nFile contents: ");
    while ((ch = fgetc(fp)) != EOF)
        putchar(ch);                  /* read one char    */
    fclose(fp);
 
    getch();
}

How it works: fopen opens a file ("w" write, "r" read), fputc/fgetc write/read one character, EOF marks the end of the file, and fclose closes it.

Enter text (end with Enter): hello
File contents: hello