Solutions

1. Largest and smallest using pointers

Steps:

Read n and input n integers into an array.
Sort the array using bubble sort with pointer-based comparisons/swaps.
After sorting, let p point to the first element; smallest = _p, largest = _(p + n - 1).
Print the largest and smallest values.

#include <stdio.h>

int main()
{
    int n, temp;

    printf("Enter array length ->");
    scanf("%d", &n);

    int arr[n];

    printf("Enter %d numbers->", n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &arr[i]);
    }

    int largest, smallest;

    for (int i = 0; i < n - 1; i++)
    {
        for (int j = 0; j < n - 1 - i; j++)
        {
            int *p = &arr[j];

            if (*p > *(p + 1))
            {
                temp = *p;
                *p = *(p + 1);
                *(p + 1) = temp;
            }
        }
    }

    int *p=arr;
    largest = *(p+n-1);
    smallest = *p;

    printf("Largest is %d and smallest is %d", largest, smallest);

    return 0;
}

2. Armstrong numbers 1–500

Steps:

Loop j from 1 to 500.
For each number, count digits, extract digits, and sum digit^count.
If the sum equals the original number, print it.

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

int main()
{
    for (int j = 1; j <= 500; j++)
    {
        int actual=j;

        int test = actual, temp = actual;

        int count = 0;

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

        int arr[count], i = 0;

        while (temp != 0)
        {
            arr[i] = temp % 10;
            temp = temp / 10;
            i++;
        }

        int total = 0;

        for (int i = 0; i < count; i++)
        {
            total = total + (int)pow(arr[i], count);
        }

        if (total == actual)
        {
            printf("%d ",actual);
        }

    }

    return 0;
}

3. Sum of positives and negatives from file

Steps:

Open "numbers.txt" for reading.
Read integers until EOF.
If x > 0 add to positive sum, else add to negative sum.
Close the file and print both sums.

#include <stdio.h>

int main(){

    FILE *file;

    int x,sumN=0,sumP=0;

    file=fopen("numbers.txt","r");

    while (fscanf(file,"%d",&x)==1) // used 1 bcz EOF value in c is -1, which will be a problem
    {
        if(x>0)
        {
            sumP+=x;
        }
        else
        {
            sumN+=x;
        }
    }

    fclose(file);


    printf("Result is %d for positive and %d for negative",sumP,sumN);

return 0;
}

4. Factorial of 1 to n (for loop)

Steps:

Read x (how many factorials to print).
For each i from 1 to x, set factorial = 1 and multiply 1 to i in an inner loop.
Print each factorial value as it is computed.

#include <stdio.h>

int main(){

    int x;

    printf("Enter how many factorial number you want->");
    scanf("%d",&x);

    for (int i = 1; i <= x; i++)
    {
        int factorial=1;
        for (int j = 1; j <= i; j++)
        {
            factorial=factorial*j;
        }

        printf("%d ",factorial);

    }

return 0;
}

5. Prime check for N test cases (simple trial division)

Steps:

Read t (number of test cases).
For each num, count divisors by testing j from 1 to num.
If num <= 1, print specific message; else print Prime if count == 2, otherwise Not a prime.

#include <stdio.h>

int main()
{

    int x, num;

    printf("How many times you want to run the test->");
    scanf("%d", &x);

    for (int i = 0; i < x; i++)
    {
        int count = 0;
        printf("Enter Number->");
        scanf("%d", &num);

        for (int j = 1; j <= num; j++)
        {
            if (num % j == 0)
            {
                count++;
            }
        }

        if (num == 1 || num <= 0)
        {
            printf("Enter any number other than 1 ,negative integer or 0\n");
        }

        else if (count == 2)
        {
            printf("Prime\n");
        }
        else
        {
            printf("Not a prime\n");
        }
    }

    return 0;
}

6. Swap without third variable

Steps:

Read a and b.
Swap without a third variable: a = a + b; b = a - b; a = a - b.
Print the swapped values.

#include <stdio.h>

int main()
{

    int a, b;

    printf("Enter 2 numbers ->");
    scanf("%d %d", &a, &b);

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

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

7. Sum of primes 1–50

Steps:

Iterate i from 1 to 50.
For each i, count divisors using a simple loop; prime iff count == 2.
Add primes to a running sum and print the total.

#include <stdio.h>

int main(){

    int result=0,count;

    for (int i = 1; i <= 50; i++)
    {
        count=0;
        for (int j = 1; j <= i; j++)
        {
            if(i%j==0)
            {
                count++;
            }
        }

        if(count==2)
        {
            result+=i;
        }

    }

    printf("Sum of all prime(1-50) is %d",result);

return 0;
}

8. Write 1–10 to file and sum

Steps:

Open "output.txt" in write mode and write 1 to 10.
Close, reopen in read mode, and read integers until EOF accumulating the sum.
Print the sum.

#include <stdio.h>

int main(){

    FILE *file;

    file=fopen("output.txt","w");

    for (int i = 1; i <= 10; i++)
    {
        fprintf(file,"%d ",i);
    }

    fclose(file);

    int sum=0,n;

    file=fopen("output.txt","r");

    while(fscanf(file,"%d",&n)!=EOF)
    {
        sum+=n;
    }


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

9. Palindrome string (no string.h)

Steps:

Read a line using fgets into a char array.
Compute length until the newline character.
Compare str[i] with str[length - 1] while moving inward; decide palindrome and print.

#include <stdio.h>

int main()
{

    char str[100];

    printf("Enter String->");
    fgets(str, sizeof(str), stdin);
    fflush(stdin);

    int length = 0, j = 0;
    while (str[length] != '\n')
    {
        length++;
    }

    printf("%d\n", length);

    int count = 1;

    for (int i = 0; i < length; i++)
    {
        if (str[i] != str[length - 1])
        {
            count = -1;
            break;
        }
        length--;
    }

    if (count == 1)
    {
        printf("Palindrome");
    }
    else
    {
        printf("Not a plaindrome");
    }

    return 0;
}

10. Merge two arrays

Steps:

Read m and n, then fill two arrays of those sizes.
Create an array of size m + n; copy first array then second into it.
Print the merged array values in order.

#include <stdio.h>

int main(){

    int x=0,m,n;

    printf("Enter size of 2 arrays->");
    scanf("%d %d",&m,&n);

    int arr1[m], arr2[n], arr[m+n];


    printf("Enter inputs for first array->");
    for (int i = 0; i < m; i++)
    {
        scanf("%d",&arr1[i]);
    }

    printf("Enter inputs for second array->");
    for (int i = 0; i < n; i++)
    {
        scanf("%d",&arr2[i]);
    }

    for (int i = 0; i < m; i++)
    {
        arr[x]=arr1[i];
        x++;
    }

    for (int i = 0; i < n; i++)
    {
        arr[x]=arr2[i];
        x++;
    }

    printf("Merged array of array 1 and 2 is->");

    for (int i = 0; i < x; i++)
    {
        printf("%d",arr[i]);
    }

    return 0;
}

11. Hollow square pattern

Steps:

Read n (side length). Validate n > 1; otherwise print "Too small box." and exit.
Print the top border: '* ' repeated n times, then newline.
For each of the n-2 middle rows: print '* ', then (n-2) double-spaces, then '* ', then newline.
Print the bottom border the same as the top.

#include <stdio.h>

int main()
{
    int n;

    printf("Enter side length of the square->");
    scanf("%d", &n);

    if(n==1 || n<=0)
    {
        printf("Too small box.");
        return 1;
    }

    for (int i = 0; i < n; i++)
    {
        printf("* ");
    }
    printf("\n");

    for (int i = 0; i < n - 2; i++)
    {
        printf("* ");
        for (int j = 0; j < n - 2; j++)
        {
            printf("  ");
        }
        printf("* ");
        printf("\n");
    }

    for (int i = 0; i < n; i++)
    {
        printf("* ");
    }
    printf("\n");

    return 0;
}

12. First 20 terms of AP (a=5, d=3)

Steps:

Use term(i) = a + (i - 1)*d for i = 1 to 20.
Print each term space-separated.

#include <stdio.h>

int main(){
    int a=5,d=3;

    for (int i = 1; i <=20; i++)
    {
        int c=a+((i-1)*d);

        printf("%d ",c);
    }

return 0;
}

13. Perfect numbers 1–500

Steps:

For each n in 1 to 500, sum all divisors 1 to n, then subtract n to get proper-divisor sum.
If that sum equals n, print n.

#include <stdio.h>

int main()
{

    for (int i = 1; i <= 500; i++)
    {
        int result = 0;

        for (int j = 1; j <= i; j++)
        {
            if (i % j == 0)
            {
                result = result + j;
            }
        }
        result = result - i;

        if (result == i)
        {
            printf("%d ", i);
        }
    }

    return 0;
}

14. Count vowels in a string

Steps:

Read a line of input.
Normalize to lowercase (or check both cases).
Count occurrences of a, e, i, o, u.

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

int main()
{

    char str[100];
    int vcount = 0;

    printf("Enter string->");
    fgets(str, sizeof(str), stdin);
    str[strcspn(str, "\n")] = '\0';

    int count = strlen(str);

    for (int i = 0; i < count; i++)
    {
        char c = str[i];
        str[i] = tolower(c);
    }

    for (int i = 0; i < count; i++)
    {
        char c = str[i];

        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u')
        {
            vcount++;
        }
    }

    printf("Vowels present at %s is %d", str, vcount);

    return 0;
}

15. Stats from mixed-number file

Steps:

Open file (numbers.txt).
Read doubles until EOF; track count, sum, min, max.
Compute average = sum/count and print with two decimals.

#include <stdio.h>

int main()
{
    FILE *file;

    file = fopen("numbers.txt", "r");

    double sum = 0, min = 100000, max = -min, average, n;
    int count = 0;

    while (fscanf(file, "%lf", &n) == 1)
    {
        count++;

        sum = sum + n;

        if (min > n)
        {
            min = n;
        }

        else if (max < n)
        {
            max = n;
        }
    }

    printf("Max-> %lf\n", max);
    printf("Min-> %lf\n", min);
    printf("Sum-> %lf\n", sum);
    printf("Average-> %.2lf\n", sum / count);
    printf("Count-> %d\n", count);

    return 0;
}

16. Even/odd sum using ?:

Steps:

Read two integers a and b.
Compute s = a + b.
Use conditional operator to print Even or Odd.

#include <stdio.h>

int main()
{

    int a, b;
    printf("Enter 2 integer number->");
    scanf("%d %d", &a, &b);

    int c = a + b;

    printf((c % 2 == 0) ? "Even\n" : "odd\n");
    return 0;
}

17. Count positive/negative/zero in array

Steps:

Read n and the array.
Traverse once and count positives, negatives, zeros.
Print the three counts.

#include <stdio.h>

int main()
{

    int n;

    printf("Enter array size->");
    scanf("%d", &n);

    int arr[n];

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

    int positive = 0, negative = 0, zero = 0;

    for (int i = 0; i < n; i++)
    {
        if (arr[i] > 0)
        {
            positive++;
        }

        else if (arr[i] < 0)
        {
            negative++;
        }

        else
        {
            zero++;
        }
    }

    printf("Positive-> %d\nNegative-%d\nZero->%d\n", positive, negative, zero);

    return 0;
}

18. Sum of digits (iterative)

Steps:

Use recursion: sum(n) returns n % 10 + sum(n / 10) with base case n == 0.
Read n and print sum(n).

#include <stdio.h>

int sum(int n)
{
    if (n == 0)
    {
        return 0;
    }
    else
    {
        return n % 10 + sum(n / 10);
    }
}

int main()
{

    int n;

    printf("Enter number->");
    scanf("%d", &n);

    printf("Sum of all digits are->%d", sum(n));
    return 0;
}

19. Highest mark and average from file

Steps:

Open marks file; each line has Name and Mark.
Track best (max) mark with name and running sum/count.
After reading, print top student and average.

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

int main()
{

    FILE *file;

    file = fopen("numbers.txt", "r");

    int max = -1, mark, sum = 0, count = 0;
    char name[100], topname[100];

    while (fscanf(file, "%s %d", name, &mark) == 2)
    {
        if (mark > max)
        {
            max = mark;
            strcpy(topname, name);
        }

        sum += mark;
        count++;
    }

    float average = (float)sum / (float)count;

    printf("Highest mark obtained by %s is %d\n And average of mark is %.3f", topname, max, average);
    fclose(file);

    return 0;
}

20. Reverse array in place

Steps:

Read n and array a.
Use two-pointer swap from ends moving inward.
Print reversed array.

#include <stdio.h>

int main(){

    int n;
    printf("Enter array size->");
    scanf("%d",&n);
    int arr[n];

    printf("Enter values->");

    for (int i = 0; i < n; i++)
    {
        scanf("%d",&arr[i]);
    }
    int j=n-1;

    for (int i = 0; i <= n/2; i++)
    {
        int temp;
        temp=arr[i];
        arr[i]=arr[j];
        arr[j]=temp;
        j--;
    }

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

return 0;
}

21. Rotate array right by one

Steps:

Read n and array a.
Store last element; shift right; place last at index 0.
Print rotated array.

#include <stdio.h>

int main()
{

    int n;
    printf("Enter array size->");
    scanf("%d", &n);

    int arr[n];

    int temp;

    printf("Enter elements->");
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &arr[i]);
    }

    temp = arr[n - 1];

    for (int i = n - 1; i > 0; i--)
    {
        arr[i] = arr[i - 1];
    }
    arr[0] = temp;

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

    return 0;
}

22. Multiplication table

Steps:

Read n.
For i=1 to 10, print n x i = n*i.

#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    for (int i = 1; i <= 10; i++) {
        printf("%d x %d = %d\n", n, i, n * i);
    }
    return 0;
}

23. GCD (iterative Euclid)

Steps:

Implement Euclid's algorithm recursively.
Read a and b; print gcd(a, b).

#include <stdio.h>

int gcd(int a, int b)
{
    if (b == 0)
    {

        return a;
    }
    else
    {
        return gcd(b, a % b);
    }
}

int main()
{
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);

    printf("GCD of %d and %d is %d\n", a, b, gcd(a, b));

    return 0;
}

24. Sign of product without multiplying (avoid overflow)

Steps:

Read three integers a, b, c.
If any number is zero, print "Zero".
Otherwise, compute product = a _ b _ c and print "Positive" if product > 0, else "Negative".

#include <stdio.h>

int main()
{

    int a, b, c, product;

    printf("Enter 3 integers->");
    scanf("%d%d%d", &a, &b, &c);

    if (a == 0 || b == 0 || c == 0)
    {
        printf("Zero");
    }
    else
    {
        product = a * b * c;

        printf(product>0 ? "Positive":"Negative");
    }

    return 0;
}

25. Divisibility using ?:

Steps:

Read a and b.
If b != 0 and a % b == 0, print Divisible; else Not Divisible.
Use conditional operator.

#include <stdio.h>

int main() {
    int a, b;
    scanf("%d %d", &a, &b);
    printf((b != 0 && a % b == 0) ? "Divisible\n" : "Not Divisible\n");
    return 0;
}

26. Binary of integer (iterative, no arrays/strings)

Steps:

Read unsigned integer n.
Find the highest power of 2 not exceeding n, then print bits from high to low.
If n is 0, print 0.

#include <stdio.h>

int main() {
    unsigned int n;
    scanf("%u", &n);
    if (n == 0) {
        putchar('0');
        putchar('\n');
        return 0;
    }
    unsigned int p = 1;
    while (p <= n / 2) {
        p *= 2;
    }
    while (p > 0) {
        if (n >= p) {
            putchar('1');
            n -= p;
        } else {
            putchar('0');
        }
        p /= 2;
    }
    putchar('\n');
    return 0;
}

27. Count uppercase and lowercase

Steps:

Read a line character-by-character.
If 'A'..'Z' increment upper; if 'a'..'z' increment lower.
Print counts.

#include <stdio.h>

int main() {
    int c, up = 0, low = 0;
    while ((c = getchar()) != EOF && c != '\n') {
        if (c >= 'A' && c <= 'Z') {
            up++;
        } else if (c >= 'a' && c <= 'z') {
            low++;
        }
    }
    printf("Upper=%d Lower=%d\n", up, low);
    return 0;
}

28. Reverse integer (iterative)

Steps:

Read an integer n.
Print digits in reverse using a recursive helper that prints last digit then recurses on n/10.
Print a newline at the end.

#include <stdio.h>

void reverse(int num)
{
    if (num == 0)
        return;

    printf("%d", num % 10);
    reverse(num / 10);
}

int main()
{
    int number;

    printf("Enter an integer: ");
    scanf("%d", &number);

    printf("Reversed number: ");
    reverse(number);
    printf("\n");

    return 0;
}

29. Diamond pattern

Steps:

Read n for half-height.
Print increasing width odd stars with leading spaces, then decreasing.
Use loops; no arrays needed.

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);

    for (int i = 1; i <= n; i++) {
        int sp = n - i;
        int st = 2 * i - 1;
        while (sp--) putchar(' ');
        while (st--) putchar('*');
        putchar('\n');
    }

    for (int i = n - 1; i >= 1; i--) {
        int sp = n - i;
        int st = 2 * i - 1;
        while (sp--) putchar(' ');
        while (st--) putchar('*');
        putchar('\n');
    }
    return 0;
}

30. Remove duplicates from array

Steps:

Read n and array a.
For each i, scan j>i and remove duplicates by shifting left and reducing size.
Print remaining elements.

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);
    int a[n];
    for (int i = 0; i < n; i++) {
        scanf("%d", &a[i]);
    }
    int m = n;
    for (int i = 0; i < m; i++) {
        for (int j = i + 1; j < m;) {
            if (a[j] == a[i]) {
                for (int k = j; k < m - 1; k++) {
                    a[k] = a[k + 1];
                }
                m--;
            } else {
                j++;
            }
        }
    }
    for (int i = 0; i < m; i++) {
        printf("%d ", a[i]);
    }
    printf("\n");
    return 0;
}

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