C Programs Practice Solutions

Program Practice Method

These programs turn individual C concepts into complete console exercises. Trace the input, identify the loop or function that performs the work, predict the output, and then test an edge case before comparing your result with the solution.

C Language Programs

1. Hello World

#include <stdio.h>
int main() {
    printf("Hello, World!\n");
    return 0;
}

2. Sum of Two Numbers

#include <stdio.h>
int main() {
    int a, b, sum;
    printf("Enter two integers: ");
    scanf("%d %d", &a, &b);
    sum = a + b;
    printf("Sum of %d and %d = %d\n", a, b, sum);
    return 0;
}

3. Factorial (Iterative)

#include <stdio.h>
int main() {
    int n;
    long long fact = 1;
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    if (n < 0) {
        printf("Factorial not defined for negative numbers.\n");
    } else {
        for (int i = 1; i <= n; i++) fact *= i;
        printf("%d! = %lld\n", n, fact);
    }
    return 0;
}
/* Output: 5! = 120 */

4. Factorial (Recursive)

#include <stdio.h>
long long factorial(int n) {
    if (n == 0 || n == 1) return 1;
    return n * factorial(n - 1);
}
int main() {
    int n;
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    printf("%d! = %lld\n", n, factorial(n));
    return 0;
}

5. Fibonacci Series

#include <stdio.h>
int main() {
    int n;
    printf("Enter number of terms: ");
    scanf("%d", &n);
    long long a = 0, b = 1, c;
    printf("Fibonacci series: ");
    for (int i = 0; i < n; i++) {
        printf("%lld ", a);
        c = a + b;
        a = b;
        b = c;
    }
    printf("\n");
    return 0;
}
/* Output (n=8): 0 1 1 2 3 5 8 13 */

6. Check Prime Number

#include <stdio.h>
#include <math.h>
int isPrime(int n) {
    if (n < 2) return 0;
    if (n == 2) return 1;
    if (n % 2 == 0) return 0;
    for (int i = 3; i <= (int)sqrt(n); i += 2) {
        if (n % i == 0) return 0;
    }
    return 1;
}
int main() {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    printf("%d is %s\n", n, isPrime(n) ? "prime" : "not prime");

    // Print all primes up to 50
    printf("Primes up to 50: ");
    for (int i = 2; i <= 50; i++) {
        if (isPrime(i)) printf("%d ", i);
    }
    printf("\n");
    return 0;
}
/* Compile: gcc prime.c -o prime -lm */

7. Check Palindrome (Number)

#include <stdio.h>
int main() {
    int n, original, reversed = 0, digit;
    printf("Enter a number: ");
    scanf("%d", &n);
    original = n;
    while (n != 0) {
        digit    = n % 10;
        reversed = reversed * 10 + digit;
        n       /= 10;
    }
    if (original == reversed)
        printf("%d is a palindrome\n", original);
    else
        printf("%d is not a palindrome\n", original);
    return 0;
}
/* 121 -> palindrome, 123 -> not palindrome */

8. Reverse a String

#include <stdio.h>
#include <string.h>
void reverseString(char str[]) {
    int len = strlen(str);
    for (int i = 0; i < len / 2; i++) {
        char temp       = str[i];
        str[i]          = str[len - 1 - i];
        str[len - 1 - i] = temp;
    }
}
int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);
    str[strcspn(str, "\n")] = '\0';  // remove newline
    printf("Original: %s\n", str);
    reverseString(str);
    printf("Reversed: %s\n", str);
    return 0;
}
/* "Hello" -> "olleH" */

9. Check Palindrome (String)

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int isPalindrome(char str[]) {
    int len = strlen(str);
    for (int i = 0; i < len / 2; i++) {
        if (tolower(str[i]) != tolower(str[len - 1 - i])) return 0;
    }
    return 1;
}
int main() {
    char words[][20] = {"racecar", "hello", "madam", "level", "world"};
    for (int i = 0; i < 5; i++) {
        printf("%-10s -> %s\n", words[i],
               isPalindrome(words[i]) ? "palindrome" : "not palindrome");
    }
    return 0;
}
/* racecar -> palindrome, hello -> not palindrome */

10. Bubble Sort

#include <stdio.h>
void bubbleSort(int arr[], int n) {
    for (int i = 0; i < n - 1; i++) {
        int swapped = 0;
        for (int j = 0; j < n - 1 - i; j++) {
            if (arr[j] > arr[j + 1]) {
                int temp   = arr[j];
                arr[j]     = arr[j + 1];
                arr[j + 1] = temp;
                swapped    = 1;
            }
        }
        if (!swapped) break;  // already sorted
    }
}
int main() {
    int arr[] = {64, 34, 25, 12, 22, 11, 90};
    int n = sizeof(arr) / sizeof(arr[0]);
    printf("Before: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    bubbleSort(arr, n);
    printf("\nAfter:  ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");
    return 0;
}
/* After: 11 12 22 25 34 64 90 */

11. Binary Search

#include <stdio.h>
int binarySearch(int arr[], int n, int target) {
    int low = 0, high = n - 1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == target) return mid;
        else if (arr[mid] < target) low  = mid + 1;
        else                        high = mid - 1;
    }
    return -1;  // not found
}
int main() {
    int arr[] = {2, 5, 8, 12, 16, 23, 38, 56, 72, 91};
    int n = sizeof(arr) / sizeof(arr[0]);
    int target = 23;
    int idx = binarySearch(arr, n, target);
    if (idx != -1)
        printf("%d found at index %d\n", target, idx);
    else
        printf("%d not found\n", target);
    return 0;
}
/* 23 found at index 5 */

12. Swap Two Numbers (3 Methods)

#include <stdio.h>
int main() {
    int a = 5, b = 10;
    printf("Original: a=%d, b=%d\n", a, b);

    // Method 1: Using temp variable
    int temp = a; a = b; b = temp;
    printf("After temp swap: a=%d, b=%d\n", a, b);

    // Method 2: Using XOR (no temp variable)
    a = 5; b = 10;
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;
    printf("After XOR swap:  a=%d, b=%d\n", a, b);

    // Method 3: Using pointers
    void swapPtr(int *x, int *y);
    a = 5; b = 10;
    swapPtr(&a, &b);
    printf("After ptr swap:  a=%d, b=%d\n", a, b);
    return 0;
}
void swapPtr(int *x, int *y) {
    int t = *x; *x = *y; *y = t;
}

13. Find Largest in Array

#include <stdio.h>
int main() {
    int arr[] = {34, 78, 12, 56, 90, 23, 45};
    int n = sizeof(arr) / sizeof(arr[0]);
    int largest = arr[0], smallest = arr[0];
    int largestIdx = 0, smallestIdx = 0;

    for (int i = 1; i < n; i++) {
        if (arr[i] > largest)  { largest  = arr[i]; largestIdx  = i; }
        if (arr[i] < smallest) { smallest = arr[i]; smallestIdx = i; }
    }

    printf("Array: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\nLargest:  %d (index %d)\n", largest,  largestIdx);
    printf("Smallest: %d (index %d)\n", smallest, smallestIdx);
    return 0;
}
/* Largest: 90 (index 4), Smallest: 12 (index 2) */

14. Matrix Multiplication

#include <stdio.h>
#define N 3
int main() {
    int a[N][N] = {{1,2,3},{4,5,6},{7,8,9}};
    int b[N][N] = {{9,8,7},{6,5,4},{3,2,1}};
    int c[N][N] = {0};

    // Matrix multiplication: c[i][j] = sum of a[i][k] * b[k][j]
    for (int i = 0; i < N; i++)
        for (int j = 0; j < N; j++)
            for (int k = 0; k < N; k++)
                c[i][j] += a[i][k] * b[k][j];

    printf("Result of A x B:\n");
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) printf("%5d", c[i][j]);
        printf("\n");
    }
    return 0;
}
/*
   30   24   18
   84   69   54
  138  114   90
*/

15. Count Vowels and Consonants

#include <stdio.h>
#include <ctype.h>
#include <string.h>
int isVowel(char c) {
    c = tolower(c);
    return c=='a' || c=='e' || c=='i' || c=='o' || c=='u';
}
int main() {
    char str[200];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);
    int vowels = 0, consonants = 0, spaces = 0, digits = 0;
    for (int i = 0; str[i] != '\0'; i++) {
        if (isalpha(str[i])) {
            if (isVowel(str[i])) vowels++;
            else                 consonants++;
        } else if (isspace(str[i])) spaces++;
        else if (isdigit(str[i]))   digits++;
    }
    printf("Vowels:     %d\n", vowels);
    printf("Consonants: %d\n", consonants);
    printf("Spaces:     %d\n", spaces);
    printf("Digits:     %d\n", digits);
    return 0;
}
/* "Hello World 123" -> Vowels:3, Consonants:7, Spaces:2, Digits:3 */

Define the Program Contract First

Before writing a small C program, state the accepted input domain, expected output format, and behavior for invalid input. This prevents an exercise from silently assuming positive numbers, non-empty arrays, or successful conversion. Keep input parsing at the boundary and pass validated values into a function that implements the calculation.

Compile with warnings enabled and treat the first warning as a defect to understand. A program that prints the expected value for one sample can still contain undefined behavior, overflow, or an unchecked read.

Use a return value from scanf and reject incomplete conversion.
Choose integer types from the required range rather than habit.
Document whether output includes labels, separators, and a trailing newline.

Test Boundary Inputs Systematically

For numeric programs, test zero, one, negative values when allowed, and the largest supported input. For arrays and strings, test empty, one-element, duplicate, already ordered, and maximum-length cases. Predict the result before running the program so a surprising output becomes evidence instead of noise.

When a program loops or recurses, identify the measure that changes on every iteration. A boundary test should confirm both termination and the final state, not merely that the process did not crash.

Separate Calculation from Input and Output

Put reusable logic in a function whose arguments and return value can be tested without terminal input. Keep main responsible for reading, validating, calling the function, printing, and choosing an exit status. This separation makes the same calculation usable from a test, another program, or a future user interface.

Verify each program with a small table of input, expected output, actual output, and exit status. Add one invalid case and one boundary case before considering the exercise complete.

Before you move on

C Programs Practice Solutions Mastery Check

1 checks

After the code runs, verify the lesson by doing this: change one input and explain the changed output.

C Language Questions Learners Ask

scanf returns the number of successful conversions. If input does not match the requested type, the target variable may remain unchanged and the invalid characters can stay in the input stream.

Test an empty logical range, one element, duplicates, negative values, already sorted data, reverse order, and a missing search target. Also test the maximum supported input count.

Undefined behavior often produces the expected output during one run. Options such as -Wall, -Wextra, and relevant sanitizers catch type mismatches, uninitialized values, invalid formats, out-of-bounds access, and lifetime bugs that printed output may not reveal. A clean sample result is weaker evidence than warnings plus edge-case tests.

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