🖥 C Programming – Practical Lab

📚 About the Lab Exam

In the practical exam, you'll be asked to write, compile, and run C programs on a computer. You must understand each program completely — the logic, syntax, and be able to explain the output. Below are the most important programs you need to master, with line-by-line explanations.

💻 Prime Number Check

A prime number is a number greater than 1 that is divisible only by 1 and itself. Examples: 2, 3, 5, 7, 11, 13, 17...

Logic: Check if the number is divisible by any number from 2 to n/2. If yes, it's NOT prime. If none divide it evenly, it IS prime.

#include <stdio.h>

int main() {
    int n, i, isPrime = 1;    // isPrime flag: 1 = assume prime initially
    
    printf("Enter a number: ");
    scanf("%d", &n);
    
    if (n <= 1) {
        isPrime = 0;           // 0 and 1 are NOT prime
    } else {
        for (i = 2; i <= n / 2; i++) {    // Check divisibility from 2 to n/2
            if (n % i == 0) {               // If remainder is 0, it's divisible
                isPrime = 0;                // So it's NOT prime
                break;                       // No need to check further
            }
        }
    }
    
    if (isPrime)
        printf("%d is a prime number.\n", n);
    else
        printf("%d is not a prime number.\n", n);
    
    return 0;
}

💻 Generate Fibonacci Series

The Fibonacci series: each number is the sum of the two before it. Starts with 0 and 1.
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

#include <stdio.h>

int main() {
    int n, i;
    int a = 0, b = 1, next;   // First two terms
    
    printf("How many terms? ");
    scanf("%d", &n);
    
    printf("Fibonacci Series: ");
    for (i = 1; i <= n; i++) {
        printf("%d ", a);       // Print current term
        next = a + b;           // Calculate next term
        a = b;                  // Shift: a takes b's value
        b = next;               // b takes the new sum
    }
    printf("\n");
    
    return 0;
}

💻 Factorial (Recursive)

Factorial: n! = n × (n-1) × (n-2) × ... × 1. Example: 5! = 5×4×3×2×1 = 120. Also: 0! = 1 (by definition).

#include <stdio.h>

// Recursive function
int factorial(int n) {
    if (n == 0 || n == 1)     // BASE CASE: stops recursion
        return 1;
    return n * factorial(n - 1);  // RECURSIVE CASE: calls itself with n-1
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);
    printf("Factorial of %d = %d\n", num, factorial(num));
    return 0;
}

💻 String Reversal

#include <stdio.h>
#include <string.h>

int main() {
    char str[100], reversed[100];
    int len, i, j;
    
    printf("Enter a string: ");
    gets(str);               // Read string (including spaces)
    
    len = strlen(str);       // Get length of string
    
    // Copy characters in reverse order
    j = 0;
    for (i = len - 1; i >= 0; i--) {
        reversed[j] = str[i];   // Last char goes first
        j++;
    }
    reversed[j] = '\0';         // Must add null terminator!
    
    printf("Reversed: %s\n", reversed);
    
    return 0;
}

💻 Palindrome Check (Number)

A palindrome reads the same forward and backward. Examples: 121, 1331, 12321. Not palindromes: 123, 456.

#include <stdio.h>

int main() {
    int num, original, reversed = 0, remainder;
    
    printf("Enter a number: ");
    scanf("%d", &num);
    
    original = num;          // Save original because we'll modify num
    
    while (num != 0) {
        remainder = num % 10;         // Extract last digit
        reversed = reversed * 10 + remainder;  // Build reversed number
        num = num / 10;               // Remove last digit
    }
    
    if (original == reversed)
        printf("%d is a palindrome.\n", original);
    else
        printf("%d is not a palindrome.\n", original);
    
    return 0;
}

💻 Bubble Sort

Bubble Sort repeatedly compares adjacent elements and swaps them if they're in the wrong order. The largest element "bubbles" to the end after each pass.

#include <stdio.h>

int main() {
    int arr[100], n, i, j, temp;
    
    printf("Enter number of elements: ");
    scanf("%d", &n);
    
    printf("Enter %d elements:\n", n);
    for (i = 0; i < n; i++)
        scanf("%d", &arr[i]);
    
    // Bubble Sort - n-1 passes
    for (i = 0; i < n - 1; i++) {           // Outer: number of passes
        for (j = 0; j < n - 1 - i; j++) {   // Inner: comparisons per pass
            if (arr[j] > arr[j + 1]) {       // Compare adjacent elements
                temp = arr[j];                // SWAP them
                arr[j] = arr[j + 1];
                arr[j + 1] = temp;
            }
        }
    }
    
    printf("Sorted array: ");
    for (i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");
    
    return 0;
}

💻 Adding Two Matrices

Add corresponding elements from two matrices of the same size: C[i][j] = A[i][j] + B[i][j]

#include <stdio.h>

int main() {
    int a[10][10], b[10][10], sum[10][10];
    int rows, cols, i, j;
    
    printf("Enter rows and columns: ");
    scanf("%d %d", &rows, &cols);
    
    printf("Enter Matrix A:\n");
    for (i = 0; i < rows; i++)
        for (j = 0; j < cols; j++)
            scanf("%d", &a[i][j]);
    
    printf("Enter Matrix B:\n");
    for (i = 0; i < rows; i++)
        for (j = 0; j < cols; j++)
            scanf("%d", &b[i][j]);
    
    // Add corresponding elements
    for (i = 0; i < rows; i++)
        for (j = 0; j < cols; j++)
            sum[i][j] = a[i][j] + b[i][j];
    
    printf("Sum Matrix:\n");
    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++)
            printf("%d\t", sum[i][j]);
        printf("\n");
    }
    
    return 0;
}

💻 Linear Search

Linear Search checks each element one by one from start to end until the target is found or the array is exhausted.

#include <stdio.h>

int main() {
    int arr[100], n, key, i, found = 0;
    
    printf("Enter number of elements: ");
    scanf("%d", &n);
    
    printf("Enter %d elements:\n", n);
    for (i = 0; i < n; i++)
        scanf("%d", &arr[i]);
    
    printf("Enter element to search: ");
    scanf("%d", &key);
    
    for (i = 0; i < n; i++) {
        if (arr[i] == key) {
            printf("Element %d found at position %d (index %d).\n", key, i+1, i);
            found = 1;
            break;
        }
    }
    
    if (!found)
        printf("Element %d not found in the array.\n", key);
    
    return 0;
}

💻 Pointer-Based Swap

Demonstrates call by reference using pointers, where the function actually changes the original variables.

#include <stdio.h>

void swap(int *a, int *b) {   // Receives ADDRESSES
    int temp = *a;             // *a = value at address a
    *a = *b;                   // Value at a = value at b
    *b = temp;                 // Value at b = old value of a
}

int main() {
    int x = 10, y = 20;
    
    printf("Before swap: x = %d, y = %d\n", x, y);
    swap(&x, &y);              // Pass ADDRESSES using &
    printf("After swap:  x = %d, y = %d\n", x, y);
    
    return 0;
}

💻 Check Armstrong Number

An Armstrong number (narcissistic number) is a number where the sum of its digits each raised to the power of the number of digits equals the number itself.

Examples: 153 = 1³ + 5³ + 3³ = 1 + 125 + 27 = 153 ✔
370 = 3³ + 7³ + 0³ = 27 + 343 + 0 = 370 ✔

#include <stdio.h>
#include <math.h>

int main() {
    int num, original, remainder, sum = 0, digits = 0;
    
    printf("Enter a number: ");
    scanf("%d", &num);
    
    original = num;
    
    // Count number of digits
    int temp = num;
    while (temp != 0) {
        digits++;
        temp /= 10;
    }
    
    // Calculate sum of digits^(number of digits)
    temp = num;
    while (temp != 0) {
        remainder = temp % 10;
        sum += (int)pow(remainder, digits);  // pow() needs math.h
        temp /= 10;
    }
    
    if (sum == original)
        printf("%d is an Armstrong number.\n", original);
    else
        printf("%d is not an Armstrong number.\n", original);
    
    return 0;
}

💻 Writing to and Reading from a File

#include <stdio.h>

int main() {
    FILE *fp;
    char name[50];
    int age;
    
    // === WRITE TO FILE ===
    fp = fopen("student.txt", "w");      // Open for writing
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    
    printf("Enter name: ");
    gets(name);
    printf("Enter age: ");
    scanf("%d", &age);
    
    fprintf(fp, "Name: %s\n", name);     // Write to file
    fprintf(fp, "Age: %d\n", age);
    fclose(fp);                           // Close file
    printf("Data written to student.txt\n\n");
    
    // === READ FROM FILE ===
    fp = fopen("student.txt", "r");      // Open for reading
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    
    char line[100];
    printf("Contents of student.txt:\n");
    while (fgets(line, 100, fp) != NULL) {  // Read line by line
        printf("%s", line);                  // Print each line
    }
    fclose(fp);
    
    return 0;
}

💻 Using Structures to Store Student Data

#include <stdio.h>
#include <string.h>

struct Student {
    char name[50];
    int roll;
    float marks[3];     // Marks in 3 subjects
    float percentage;
};

int main() {
    struct Student s;
    float total = 0;
    
    printf("Enter Name: ");
    gets(s.name);
    printf("Enter Roll Number: ");
    scanf("%d", &s.roll);
    
    printf("Enter marks in 3 subjects:\n");
    for (int i = 0; i < 3; i++) {
        printf("Subject %d: ", i + 1);
        scanf("%f", &s.marks[i]);
        total += s.marks[i];
    }
    
    s.percentage = total / 3.0;
    
    printf("\n--- Student Record ---\n");
    printf("Name: %s\n", s.name);
    printf("Roll: %d\n", s.roll);
    printf("Marks: %.1f, %.1f, %.1f\n", s.marks[0], s.marks[1], s.marks[2]);
    printf("Percentage: %.2f%%\n", s.percentage);
    
    if (s.percentage >= 60)
        printf("Result: FIRST DIVISION\n");
    else if (s.percentage >= 45)
        printf("Result: SECOND DIVISION\n");
    else if (s.percentage >= 33)
        printf("Result: THIRD DIVISION\n");
    else
        printf("Result: FAIL\n");
    
    return 0;
}

💻 Common Star Patterns

Right Triangle:

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}
/*  Output:
    *
    * *
    * * *
    * * * *
    * * * * *
*/

Pyramid (center-aligned):

int n = 5;
for (int i = 1; i <= n; i++) {
    // Print spaces for alignment
    for (int j = 1; j <= n - i; j++)
        printf(" ");
    // Print stars
    for (int j = 1; j <= 2 * i - 1; j++)
        printf("*");
    printf("\n");
}
/*  Output:
        *
       ***
      *****
     *******
    *********
*/

Inverted Triangle:

for (int i = 5; i >= 1; i--) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}
/*  Output:
    * * * * *
    * * * *
    * * *
    * *
    *
*/

Number Pattern:

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

📚 Important Tips for the Practical Exam

• Always include headers: <stdio.h> for printf/scanf, <string.h> for string functions, <math.h> for math functions, <stdlib.h> for dynamic memory
• Every statement ends with semicolon (;) — forgetting this is the #1 beginner error
• Curly braces { } must match — every opening brace needs a closing brace
• Use %d for int, %f for float, %c for char, %s for string in printf and scanf
• scanf needs & for all types EXCEPT strings: scanf("%d", &n) but scanf("%s", name) (no &)
• Array indices start from 0, not 1. An array of size 5 has indices 0,1,2,3,4
• Test your code with edge cases: 0, negative numbers, very large numbers, empty strings
• Comment your code: Viva examiners appreciate well-commented programs
• Practice dry runs: Trace through the code with specific input values on paper
• Common compilation errors: missing semicolons, undeclared variables, wrong format specifiers, mismatched parentheses/braces