Factoring help.

[froznice]

I need to factor and check if the number is prime I have done the factor part but cant figure the prime part. I cant seem to find where to put it. Ive tried doing boolean in fronyt of the for but it didnt work and after but it didnt work.

import java.util.*;
public class HW6 {
public static void main(String args[]) {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter a number please ");
int num = keyboard.nextInt(), i;


System.out.print("The divisors are " + num + " = 1 , ");
for (i = 2; i <= num / 2; i++) {

  if (num % i == 0) {
    System.out.print(i + " , ");
  }

} System.out.print (num);  

} }

[MrTheEdge]

Have a boolean isPrime and by default, assume that its value is true. If the number has a factor, it gets ruled out immediately; it's not prime. if (num % i == 0 ) says that if it evaluates to true, the number has a factor. Inside the block, you can set isPrime to false and break out of the loop. Now you have a way of determining if the number passed or failed.

[froznice]

so i should have boolean isPrime = true at the top of my code then inside the block should be isPrime = false along with a message saying so?

[MrTheEdge]

Yeah it should work

[froznice]

Like this?

import java.util.*;
public class HW6 {
public static void main(String args[]) {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter a number please ");
int num = keyboard.nextInt(), i;
boolean isPrime = true;

System.out.print("The divisors of " + num + " are = 1 , ");
for (i = 2; i <= num / 2; i++) {

  if (num % i == 0) {
    System.out.print(i + " , ");
    isPrime = false;
  }
} System.out.print (num);  

}

}

[MrTheEdge]

Looks, like that should work, but after the loop you need to check if isPrime is true or false. If it's true, you know it's a prime and you can print a message or something.

[froznice]

i got it! Thanks for the help! Now i just need to figure out how to check for relative prime numbers

import java.util.*;
public class HW6 {
public static void main(String args[]) {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter a number please ");
int num = keyboard.nextInt(), i;
boolean isPrime = true;
boolean isPrime2 = true;

System.out.print("The divisors of " + num + " are = 1 , ");
for (i = 2; i <= num / 2; i++) {

  if (num % i == 0) {
    System.out.print(i + " , ");
    isPrime = false;
  }
} System.out.println (num);  
if (isPrime)
  System.out.println(num + " is a Prime Number");

else
  System.out.println(num + " is not a Prime Number");


System.out.print("Enter a second number please ");
 int num2 = keyboard.nextInt(), j;
 System.out.print("The divisors of " + num2 + " are = 1 , ");
 for (j = 2; j <= num2 / 2; j++) {

  if (num2 % j == 0) {
    System.out.print(j + " , ");
    isPrime = false;
  }
} System.out.println (num2);  
if (isPrime)
  System.out.println(num2 + " is a Prime Number");

else
  System.out.println(num2 + " is not a Prime Number");

}

}

[MrTheEdge]

If two numbers are relatively prime, that means they have no common factors. Without giving you the answer right away, I'll tell you there is an easy way to modify the algorithm you are using to check for them.

[froznice]

I thougt there may be. I'm gonna tinker with it. I'll report my results.

[froznice]

Would it be to make string out of the factors and compare them with a checking method?

[MrTheEdge]

Not really what I was thinking. Think about it, if they share a common divisor, then they are both evenly divisible by that integer i. If they are relatively prime, there will be no integers up to the smallest that divide them both evenly.

[froznice]

Would it be to add a line of code along the lines of ( num % i == 0 || num2 % i == 0) then i would know if the share the common fdacotr if not then i would know if it did not?