Program to Display Prime Numbers Between Intervals

// C Program to Display Prime
// Numbers Between Intervals
#include <stdio.h>

// Driver code
int main()
{
	// Declare the variables
	int a, b, i, j, flag;

	// Ask user to enter lower value
	// of interval
	printf("Enter lower bound of the interval: ");

	// Take input
	scanf("%d", &a);

	// Ask user to enter upper value
	// of interval
	printf("Enter upper bound of the interval: ");

	// Take input
	scanf("%d", &b);

	// Print display message
	printf("Prime numbers between %d and %d are: ",
			a, b);

	// Traverse each number in the interval
	// with the help of for loop
	for (i = a; i <= b; i++)
	{
		// Skip 0 and 1 as they are
		// neither prime nor composite
		if (i == 1 || i == 0)
			continue;

		// flag variable to tell
		// if i is prime or not
		flag = 1;

		for (j = 2; j <= i / 2; ++j)
		{
			if (i % j == 0)
			{
				flag = 0;
				break;
			}
		}

		// flag = 1 means i is prime
		// and flag = 0 means i is
		// not prime
		if (flag == 1)
			printf("%d ", i);
	}

	return 0;
}

----------------------------------------------------
Approach 3: (Using Sieve of Eratosthenes Method)
----------------------------------------------------

// C program to find the prime numbers
// between a given interval using Sieve of Eratosthenes
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

void SieveOfEratosthenes(int srt, int n)
{
	// Create a boolean array "prime[srt to n]" and
	// initialize all entries it as true. A value in
	// prime[i] will finally be false if i is Not a prime,
	// else true.
	bool prime[n + 1];
	memset(prime, true, sizeof(prime));
	prime[0] = false;
	prime[1] = false;

	for (int p = 2; p * p <= n; p++) {
		// If prime[p] is not changed, then it is a prime
		if (prime[p] == true) {
			// Update all multiples of p greater than or
			// equal to the square of it numbers which are
			// multiple of p and are less than p^2 are
			// already been marked.
			for (int i = p * p; i <= n; i += p)
				prime[i] = false;
		}
	}

	// Print all prime numbers
	for (int p = srt; p <= n; p++)
		if (prime[p])
			printf("%d ", p);
}

// Driver Code
int main()
{
	int srt = 1;
	int end = 10;
	SieveOfEratosthenes(srt, end);
	return 0;
}

// This code is contributed by Susobhan Akhuli