What is a Prime Number?
A Prime Number is a number which is greater than 1 and divisible by 1 and only itself. Some of the Prime Numbers are 2, 3, 5, 7, 11, 13, 17… In this Prime Number Program in Java, let’s take a look into the different prime number programs.
Is 0 a prime number?
0 is neither prime nor composite number, as per the definition, a prime number is a number with exactly two positive divisors, 1 and itself. Zero has an infinite number of divisors (we can divide 0 by all real numbers) Therefore, zero is not a Prime Number.
Is 1 a prime number?
1 is not considered as a Prime because it does not meet the criteria which is exactly two factors 1 and itself, whereas 1 has only one factor
Prime Number Program in Java using Scanner
We all know that the prime numbers can only be divided by itself and 1. Let’s understand the range to consider.
In general, a number cannot be divided by any number which is greater than itself and hence we can set the upper limit as to the number. Furthermore, we can limit the range by considering the fact that no number can have factors greater than the square root of the number (or) number by half (including the number itself).
For Example, Let’s take the number 19. It cannot be divided by any number greater than 19, 20 cannot divide 19 and range to consider is 19/2 which is 9.5 and hence we can consider the range between 2 to 9.
- Get the number to check from the user.
- Check whether the number is greater than 1, if the number is less than 1 then it cannot be a prime.
- In an iterative loop, divide the number between the range 2 to number/2, and check if the remainder is not zero, if zero then the number is not a prime.
package com.javainterviewpoint;
import java.util.Scanner;
public class PrimeNumber1
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.println("Enter a number check :");
int number = scanner.nextInt();
if(checkPrime(number))
System.out.println(number +" is a Prime Number");
else
System.out.println(number +" is not a Prime Number");
}
public static boolean checkPrime(int n)
{
if(n <= 1)
return false;
for(int i=2; i<= n/2; i++)
{
if((n % i) == 0)
return false;
}
return true;
}
}
- Using Scanner get the input from the user and store it in the variable “number”.
- The checkPrime() method, checks whether the number passed is prime or not. It returns boolean values based on the below criteria
- Returns false when the number is less than or equal to 1.
- Returns false when the remainder is zero.
- If the number is greater than 1 and it is not divisible any number within the range of 2 to number/2 then it returns true.
Prime Number Program in Java using While Loop
We can also use the while loop instead of for loop, let’s re-write the above code using while loop.
We just need to make some minor modifications like the initialization of “i” [i=2] happens just before the start of the loop, incrementation of “i” [i++] happens inside the loop and of course, we need to change for loop into while loop.
package com.javainterviewpoint;
import java.util.Scanner;
public class PrimeNumber2
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.println("Enter a number check :");
int number = scanner.nextInt();
if(checkPrime(number))
System.out.println(number +" is a Prime Number");
else
System.out.println(number +" is not a Prime Number");
}
public static boolean checkPrime(int n)
{
if(n <= 1)
return false;
int i=2;
while(i<= n/2)
{
if((n % i) == 0)
return false;
i++;
}
return true;
}
}
Java program to find all Prime Numbers from array
In this approach, let’s get the input from the user and store it in the array and find all the prime numbers from array.
- Get the size of the array from the user and create an array to store the input numbers
- Get the elements of the array from the user and store it in the array which is created in the previous step
- Finally, iterate the array and pass each element to the checkPrime() method and perform the prime validation
package com.javainterviewpoint;
import java.util.Arrays;
import java.util.Scanner;
public class PrimeNumber3
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.println("Enter the size of the array :");
int arraySize = scanner.nextInt();
int[] values = new int[arraySize];
System.out.println("Enter the elements of the array : ");
for (int i = 0; i < arraySize; i++)
{
values[i] = scanner.nextInt();
}
System.out.println("*** Printing the Prime Numbers ***");
for (int i = 0; i < arraySize; i++)
{
if (checkPrime(values[i]))
{
System.out.println(values[i]);
}
}
}
public static boolean checkPrime(int n)
{
if (n <= 1)
return false;
int i = 2;
while (i <= n / 2)
{
if ((n % i) == 0)
return false;
i++;
}
return true;
}
}
Output:
Enter the size of the array : 5 Enter the elements of the array : 1 2 3 4 5 *** Printing the Prime Numbers *** 2 3 5
Prime Numbers between 1 to 100 / List of Prime Numbers from 1 to 100
Let’s print all the prime numbers between 1 to 100
package com.javainterviewpoint;
public class PrimeNumber4
{
public static void main(String[] args)
{
System.out.println("*** Prime Numbers between 1 to 100 ***");
for (int i = 2; i < 100; i++)
{
if (checkPrime(i))
{
System.out.print(i+" ");
}
}
}
public static boolean checkPrime(int n)
{
if(n <= 1)
return false;
int i=2;
while(i <= n/2)
{
if((n % i) == 0)
return false;
i++;
}
return true;
}
}
Output:
*** Prime Numbers between 1 to 100 *** 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
Find all Prime Numbers between 1 to N
- Get the upper limit from the user and store it in the variable “N”
- Start the loop from 2 to N, for each iteration increment the loop by 1
- In the checkPrime() method, we have used a boolean flag. It will be set to false when the number is less than 1 or if the number is divisible by number/2.
- Validate the boolean returned by the checkPrime() and print the number if the boolean returned is true.
package com.javainterviewpoint;
import java.util.Scanner;
public class PrimeNumber5
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.println("Enter the Upper limit :");
int N = scanner.nextInt();
System.out.println("*** Prime Numbers between 1 to N ***");
for (int i = 2; i <= N; i++)
{
if (checkPrime(i))
{
System.out.print(i+" ");
}
}
}
public static boolean checkPrime(int n)
{
boolean flag = true;
if(n <= 1)
flag = false;
for(int i=2; i<= n/2; i++)
{
if((n % i) == 0)
{
flag = false;
break;
}
}
return flag;
}
}
Output:
Enter the Upper limit : 55 *** Prime Numbers between 1 to N *** 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53
Bonus – Prime Numbers Chart
Below table contains the list of Prime Numbers from 1 to 100. All the prime numbers are shaded with a green background.
Happy Learning!!
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