C programs for practise

all_prime_numbers_from_1_to_n.c

/*
 * Print all the prime numbers from 1...n
 * and also show the sum of all the prime numbers within this range.
 */

#include <stdio.h>

int main()
{
    // loop controllers
    int i, outer_loop_counter;

    // count of prime numbers
    int count = 0;

    // sum of the prime numbers
    int sum = 0;

    // user input
    int user_input_number;

    // this variable will indicate whether a number is prime or not
    // 1 - TRUE; 0 - FALSE
    int is_prime;

    // take user input
    printf("Input a number: ");
    scanf("%d", &user_input_number);

    for (outer_loop_counter = 1; outer_loop_counter <= user_input_number; outer_loop_counter++) {
        // reset the variable after every loop
        is_prime = 1;

        for (i = 2; i <= outer_loop_counter / 2; i++) {
            if (outer_loop_counter % i == 0) {
                is_prime = 0;
            }
        }

        if (is_prime == 1) {
            sum = sum + outer_loop_counter;
            count++;

            printf("%d is prime.\n", outer_loop_counter);
        } else {
            printf("%d is not prime.\n", outer_loop_counter);
        }
    }

    printf("Sum of all the prime numbers within 1...%d is: %d\n", user_input_number, sum);
    printf("Count of prime numbers: %d\n", count);

    return 0;
}

array_1_basics.c

#include <stdio.h>

/**
 * Array basics
 */

void main() {
    /*
     * An array holding 5 integer values
     * this is equivalent to declaring 5 separate variables like
     * int a, b, c, d, e;
     */
    int my_array[ 5 ];

    /*
     * Arrays have 0 based index (position). So, first item is at
     * index (position) 0, second item is at 1 and so forth
     *
     * SYNTAX: my_array[ index/position ]
     */
    my_array[0] = 5; // we're setting the value of first position's value to 5
    my_array[1] = 3;
    my_array[2] = 6;
    my_array[3] = 10;
    my_array[4] = 20;

    printf( "Value at position 3 is: %d\n",  my_array[3] );
}

array_2_using_loops.c

#include <stdio.h>

/**
 * Accessing array using a loop
 */

void main() {
    // This is another way of declaring an array
    // AND also initializing the values at the same time
    int x[10] = { 20, -40, 30, 60, 80, 43, 85, 81, 99, -102 };
    int i;

    // we can take advantage of a loop to access a large array!
    // let us assume i will hold the index of the array
	
    for (i = 0; i < 10; i++) {
        // print the index
        printf("i: %d\n", i);

        // print the value at that index
        printf("x[%d]: %d\n\n", i, x[i]);
        // OUTPUT will be similar to:
        // x[3]: 60
    }
}

array_3_largest_number.c

#include <stdio.h>

/**
 * Find largest number in an array
 */

void main() {
    // this time, let's take the value of 5 items from the user
    int x[5];
    int i;
    int largest_number;

    // take 5 inputs from the user by looping 5 times and
    // set the values
    for (i = 0; i < 5; i++) {
        printf("Input value x[%d]: ", i);
        scanf( "%d", &x[i] );
    }

    // let us assume, the largest number is at position 0
    largest_number = x[0];

    for (i = 0; i < 5; i++) {
        // if the largest number is less than the value of x[i]
        // then x[i] is the NEW largest number
        if (largest_number < x[i]) {
            largest_number = x[i];
        }
    }

    printf("Largest number is: %d\n", largest_number);
    
    
    /**
     *  Sample OUTPUT:
        
        Input value x[0]: 20
        Input value x[1]: -23
        Input value x[2]: 450
        Input value x[3]: 23
        Input value x[4]: 89
        Largest number is: 450
     */
}

array_4_second_largest_number.c

#include <stdio.h>

/**
 * Find second largest number in an array
 */

void main() {
    // Here, 54 is the SECOND LARGEST and 823 is the LARGEST
    int x[5] = { 823, 12, 53, 23, 54 };
    int i;
    int largest_number;
    int second_largest_number;

    // let us assume, largest number is at position 0
    // and second largest number is at position 1
    // WARNING: DO NOT SET THE VALUE OF BOTH TO THE SAME POSITION
    largest_number = x[0];
    second_largest_number = x[1];

    // find the largest number
    for (i = 0; i < 5; i++) {
        if (largest_number < x[i]) {
            // change the value of largest_number to the new largest number
            largest_number = x[i];
        }
    }

    // find the second largest number
    for (i = 0; i < 5; i++) {
        // check whether the value of x[i] is NOT EQUALS to the previously
        // calculated largest number
        if ( (second_largest_number < x[i]) && (x[i] != largest_number) ) {
            second_largest_number = x[i];
        }
    }

    printf("Largest number is: %d\n", largest_number);
    printf("Second largest number is: %d\n", second_largest_number);
}

array_5_reverse.c

#include <stdio.h>

// change this value to the size of the array you're working with
#define ARRAY_SIZE 11

/**
 * Reverse an array
 * @return 0
 */
void main() {
    int x[ARRAY_SIZE] = {0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100};

    // The swapping will continue till half of the array is reached
    int exchange_upto = ARRAY_SIZE / 2;

    for (int i = 0; i < exchange_upto; i++) {
        // as x[i] will soon be used to hold another value in the next line of code,
        // let's store the current value in a temporary variable
        int temp = x[i];

        // if i = 4, then
        // = ((ARRAY_SIZE - 1) - i)
        // = ((    11     - 1) - 4)
        // = (10 - 4)
        // = 6

        // Store the value of position 6 in 4
        x[i] = x[ (ARRAY_SIZE - 1) - i ];

        // Store the value of position 4 in 6
        // here, 'temp' contains the original value of x[i] before
        // the swapping happened
        x[ (ARRAY_SIZE - 1) - i ] = temp;

        // The following line shows how the swapping works, you can remove it
        printf("x[%d] <-> x[%d]\n", i, (ARRAY_SIZE - 1) - i);
    }

    // show the result
    for (int i = 0; i < ARRAY_SIZE; ++i) {
        printf("%d\n", x[i]);
    }
}

ARRAY_PROBLEMS.c

#include <stdio.h>

void main() {
	int x[7];
	int i;

	// ----- START OF 1 ----- //
	printf("\n\n1. TAKE INPUT AND SHOW THE RESULT ------------------\n\n");

	// TAKE INPUT FROM THE USER
	for (i = 0; i < 7; i++) {
		printf("Input %d: ", i);
		scanf("%d", &x[i]);
	}

	printf("\nOutput:\n");

	// DISPLAY THE RESULT
	for (i = 0; i < 7; i++) {
		printf("x[%d]: %d\n", i, x[i]);
	}



	// ----- START OF 2 ----- //
	printf("\n\n2. FIND MAX ------------------\n\n");
	int max = x[0];
	for (i = 0; i < 7; i++) {
		if (max < x[i])
			max = x[i];
	}

	printf("MAX: %d\n", max);



	// ----- START OF 3 ----- //
	printf("\n\n3. FIND MIN ------------------\n\n");
	int min = x[0];
	for (i = 0; i < 7; i++) {
		if (min > x[i])
			min = x[i];
	}

	printf("MIN: %d\n", min);

	

	// ----- START OF 4 ----- //
	printf("\n\n4. FIND AVERAGE ------------------\n\n");
	int average;
	int sum = 0;
	for (i = 0; i < 7; i++) {
		sum = sum + x[i];
	}
	average = sum / 7;
	printf("Average: %d\n", average);


	
	// ----- START OF 5 ----- //
	printf("\n\n5. FIND MAX INDEX ------------------\n\n");
	int max_index;
	for (i = 0; i < 7; i++) {
		if (max == x[i])
			max_index = i;
	}
	printf("Max index: %d\n", max_index);


	
	// ----- START OF 16----- //
	printf("\n\n6. FIND MIN INDEX ------------------\n\n");
	int min_index;
	for (i = 0; i < 7; i++) {
		if (min == x[i])
	 		min_index = i;
	}
	printf("Min index: %d\n", min_index);

	
	
	// ----- START OF 7 ----- //
	printf("\n\n7. SEARCH INPUT NUMBER ------------------\n\n");
	int input_num;
	printf("Input a number: ");
	scanf("%d", &input_num);

	for (i = 0; i < 7; i++) {
		if (input_num == x[i]) {
			printf("Found at index: %d\n", i);
			break;
		} else if (i == 6) {
			printf("The given number is not present in the array.\n");
		}
	}


	
	// ----- START OF 8 ----- //
	printf("\n\n8. FIND SECOND MINIMUM ------------------\n\n");
	int second_min = x[max_index];
	for (i = 0; i < 7; i++) {
		if (second_min > x[i] && x[i] != min) {
			second_min = x[i];
		}
	}
	printf("Second minimum: %d\n", second_min);

	
	// ----- START OF 9 ----- //
	printf("\n\n9. FIND SECOND LARGEST ------------------\n\n");
	int second_largest = x[min_index];
	printf("%d\n", max);
	for (i = 0; i < 7; i++) {
		if (second_largest < x[i] && x[i] != max) {
			second_largest = x[i];
		}
	}
	printf("Second largest: %d\n", second_largest);



	// ----- START OF 10 ----- //
	printf("\n\n10. FIND MINIMUM LENGTH ------------------\n\n");
	int length = max - min;
	int j;
	int comp;
	for (i = 0; i < 7; i++) {
		for (j = 0; j < 7; j++) {
			if (i == j) continue;

			if (x[i] > x[j])
				comp = x[i] - x[j];
			else
				comp = x[j] - x[i];

			if (length > comp) {
				length = comp;
			}
		}
	}
	printf("Minimum length: %d\n", length);

}

			max = x[i];
	}

	printf("MAX: %d\n", max);



	// ----- START OF 3 ----- //
	printf("3. FIND MIN ------------------\n\n");
	int min = x[0];
	for (i = 0; i < 7; i++) {
		if (min > x[i])
			min = x[i];
	}

	printf("MIN: %d\n", min);

	

	// ----- START OF 4 ----- //
	printf("4. FIND AVERAGE ------------------\n\n");
	int average;
	int sum = 0;
	for (i = 0; i < 7; i++) {
		sum = sum + x[i];
	}
	average = sum / 7;
	printf("Average: %d\n", average);


	
	// ----- START OF 5 ----- //
	printf("5. FIND MAX INDEX ------------------\n\n");
	int max_index;
	for (i = 0; i < 7; i++) {
		if (max == x[i])
			max_index = i;
	}
	printf("Max index: %d\n", max_index);


	
	// ----- START OF 16----- //
	printf("6. FIND MIN INDEX ------------------\n\n");
	int min_index;
	for (i = 0; i < 7; i++) {
		if (min == x[i])
	 		min_index = i;
	}
	printf("Min index: %d\n", min_index);

	
	
	// ----- START OF 7 ----- //
	printf("7. SEARCH INPUT NUMBER ------------------\n\n");
	int input_num;
	printf("Input a number: ");
	scanf("%d", &input_num);

	for (i = 0; i < 7; i++) {
		if (input_num == x[i]) {
			printf("Found at index: %d\n", i);
			break;
		} else if (i == 6) {
			printf("The given number is not present in the array.\n");
		}
	}


	
	// ----- START OF 8 ----- //
	printf("8. FIND SECOND MINIMUM ------------------\n\n");
	int second_min = x[max_index];
	for (i = 0; i < 7; i++) {
		if (second_min > x[i] && x[i] != min) {
			second_min = x[i];
		}
	}
	printf("Second minimum: %d\n", second_min);

	
	// ----- START OF 9 ----- //
	printf("9. FIND SECOND LARGEST ------------------\n\n");
	int second_largest = x[min_index];
	printf("%d\n", max);
	for (i = 0; i < 7; i++) {
		if (second_largest < x[i] && x[i] != max) {
			second_largest = x[i];
		}
	}
	printf("Second largest: %d\n", second_largest);



	// ----- START OF 10 ----- //
	printf("10. FIND MINIMUM LENGTH ------------------\n\n");
	int length = max - min;
	int j;
	int comp;
	for (i = 0; i < 7; i++) {
		for (j = 0; j < 7; j++) {
			if (i == j) continue;

			if (x[i] > x[j])
				comp = x[i] - x[j];
			else
				comp = x[j] - x[i];

			if (length > comp) {
				length = comp;
			}
		}
	}
	printf("Minimum length: %d\n", length);

}

check_for_3_7_divisibility.c

/*
 * Find if a number is divisible by both 3 AND 7
 */

#include <stdio.h>

int main()
{
    int input_number;
    int count = 0;

    printf("Input a number: ");
    scanf("%d", &input_number);

    // Replace '&&' with '||' in the line below,
    // to check for OR condition instead of AND
    
    if (input_number % 3 == 0   &&   input_number % 7 == 0) {
        printf("%d is divisible by both 3 and 7.\n", input_number);
        count++;
    } else {
        printf("%d is divisible NOT by both 3 and 7.\n", input_number);
    }
    
    printf("Count is: %d\n", count);

    return 0;
}

find_positive_negative_numbers.c

#include <stdio.h>

int main()
{
	// ask the user for how many input numbers to be taken
	int number_of_inputs;
	// loop counter
	int i;
	// this variable will hold user input in each looping step
	int n;

	// counter - for positive numbers
	int positive_count = 0;
	// counter - for negative numbers
	int negative_count = 0;

	printf("Number of inputs: ");
	scanf("%d", &number_of_inputs);

	for (i = 0; i < number_of_inputs; i++)
	{
		printf("Input #%d: ", i);
		scanf("%d", &n);

		if (n >= 0)
			positive_count++;
		else
			negative_count++;
	}

	printf("Positive count: %d\n", positive_count);
	printf("Negative count: %d\n", negative_count);

	return 0;
}

leap_year.c

/*
 * Find if a year is leap year or not
 */

#include <stdio.h>

int main()
{
    int year;

    printf("Input a year: ");
    scanf("%d", &year);

    if (year % 400 == 0)
        printf("%d is a leap year.\n", year);
    else if (year % 4 == 0 && year % 100 != 0)
        printf("%d is a leap year.\n", year);
    else
        printf("%d is NOT a leap year.\n", year);

    return 0;
}

prime_number.c

/*
 * Find if a number is prime or not
 */

#include <stdio.h>

int main()
{
    int input_number;
    int i;

    // is_prime = 1  -----  The number is prime
    // is_prime = 0  -----  The number is NOT prime
    int is_prime = 1;

    printf("Input a number: ");
    scanf("%d", &input_number);

    // i++  --->  i = i + 1
    for (i = 2; i < (input_number / 2); i++)
    {
        if (input_number % i == 0)
        {
            is_prime = 0;
            break;
        }
    }

    if (is_prime == 1)
        printf("The number is prime.\n");
    else
        printf("The number is not prime.\n");

    return 0;
}

reverse_a_number.c

/*
 * Find the reverse of a number
 * Also, find the sum of the digits of a number
 */

#include <stdio.h>

int main()
{
    int n;
    int reverse_number = 0;
    int sum = 0;
    int temp;
    int reminder;

    printf("Input a number: ");
    scanf("%d", &n);

    temp = n;
    while (n > 0)
    {
        reminder = n % 10;

        reverse_number = (reverse_number * 10) + reminder;
        sum += reminder;

        n = n / 10;
    }

    printf("The reverse of %d is: %d\n", temp, reverse_number);
    printf("The sum of the digits of %d is: %d\n", temp, sum);

    return 0;
}

sum_of_odd_numbers.c

/*
 * Sum of all the odd numbers from 1 to n
 */

#include <stdio.h>

int main()
{
    int sum = 0;
    int n;
    int i;

    printf("Input a number: ");
    scanf("%d", &n);

    // i += 2  --->  i = i + 2
    for (i = 1; i <= n; i += 2)
    {
        // sum = sum + i
        sum += i;
    }

    printf("Sum of all the odd numbers is: %d\n", sum);

    return 0;
}

switch_case-vowel_consonent.c

/*
 * Find if the input character is a vowel/consonant
 */

#include <stdio.h>

int main()
{
    char input_character;

    printf("Input a character: ");
    scanf("%c", &input_character);

    switch (input_character)
    {
        case 'a':
        case 'e':
        case 'i':
        case 'o':
        case 'u':
            printf("%c is a vowel.\n", input_character);
            break;
        default:
            printf("%c is a consonant.\n", input_character);
            break;
    }

    return 0;
}

Find the roots of a quadratic equation in the form of: ax^2 + bx + c = 0

https://gist.github.com/sazid/b9f94b56674c69682df7f7bad8ad7165

Great Samples!
Useful for all of us.....!