# Prime Numbers using For Loop and While Loop in Java

Prime numbers are an important concept in mathematics and programming. In this tutorial post, we will explore how to generate prime numbers using the for loop and while loop in Java, using the BlueJ programming language. We will cover the syntax, examples, and differences between these two looping constructs.

## Understanding Prime Numbers

Before diving into the code, let’s briefly discuss what prime numbers are. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 2, 3, 5, 7, 11, and 13 are all prime numbers.

## Generating Prime Numbers using a For Loop

The for loop is a commonly used looping construct that allows us to iterate over a fixed range of values. Here’s how we can use the for loop to generate prime numbers:

// Example: Generating Prime Numbers using a For Loop

int n = 100; // Maximum number to check for prime
boolean isPrime;

System.out.println(“Prime numbers between 1 and ” + n + “:”);
for (int i = 2; i <= n; i++) {
isPrime = true;
for (int j = 2; j < i; j++) {
if (i % j == 0) {
isPrime = false;
break;
}
}
if (isPrime) {
System.out.println(i);
}
}

Explanation

• We start by initializing the variable `n` with the maximum number we want to check for prime.
• We then use a for loop with the variable `i` to iterate from 2 (the first prime number) up to `n`.
• Inside the outer loop, we set the boolean variable `isPrime` to `true` initially for each number `i`.
• Next, we use another for loop with the variable `j` to check if `i` is divisible by any number between 2 and `i-1`.
• If `i` is divisible by any number between 2 and `i-1`, we set `isPrime` to `false` and break out of the inner loop.
• Finally, we check the value of `isPrime` and if it is `true`, we print the prime number.

## Generating Prime Numbers using a while Loop

The while loop is another looping construct in Java that allows us to repeatedly execute a block of code until a specified condition becomes false. Here’s how we can use the while loop to generate prime numbers:

// Example: Generating Prime Numbers using a While Loop

int n = 100; // Maximum number to check for prime
int i = 2;

System.out.println(“Prime numbers between 1 and ” + n + “:”);
while (i <= n) {
boolean isPrime = true;
int j = 2;
while (j < i) {
if (i % j == 0) {
isPrime = false;
break;
}
j++;
}
if (isPrime) {
System.out.println(i);
}
i++;
}

Explanation

• Similar to the for loop example, we initialize the variable `n` with the maximum number we want to check for prime.
• We initialize the variable `i` to 2, which is the first prime number.
• Inside the while loop, we perform the same prime number checking logic as in the for loop example.
• The loop continues until `i` reaches `n`.
• We increment `i` at the end of each iteration to move to the next number.

## Java program that checks whether a given number is prime or not

import java.util.Scanner;

``````public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter a number: ");
int number = scanner.nextInt();

if (number <= 1) {
System.out.println(number + " is not a prime number.");
} else {
boolean isPrime = true;

for (int i = 2; i < number; i++) {
if (number % i == 0) {
isPrime = false;
break;
}
}

if (isPrime) {
System.out.println(number + " is a prime number.");
} else {
System.out.println(number + " is not a prime number.");
}
}
}``````

}

Explanation

1. The program prompts the user to enter a number.
2. If the entered number is less than or equal to 1, it is not prime, so the program prints that it is not a prime number.
3. Otherwise, a boolean variable `isPrime` is initialized as `true` to assume the number is prime.
4. The program uses a `for` loop to iterate from 2 to one less than the entered number.
5. Inside the loop, it checks if the number is divisible by any of these values. If it is, the `isPrime` variable is set to `false`, and the loop is terminated using the `break` statement.
6. After the loop, the program checks the value of `isPrime`. If it is still `true`, the number is considered prime and the program prints that it is a prime number. Otherwise, it prints that it is not a prime number.

## Conclusion

In this tutorial, we explored how to generate prime numbers using both the for loop and while loop in Java, specifically using the BlueJ programming language. We discussed the syntax, provided examples, and highlighted the differences between these two looping constructs. Understanding prime numbers and how to generate them programmatically is a valuable skill for any aspiring programmer or mathematician.

Remember, generating prime numbers efficiently is a complex problem, and there are more advanced algorithms available. However, the examples provided here offer a basic understanding of how to generate prime numbers using simple looping constructs.