Prime Number Sequence Java

Question

So what my program does is that it reads user input and prints out a series of prime numbers that leads up to the input. So for example, if I input 20 the sequence should print out 2 5 7... etc. until it reaches 19 (since 20 is not a prime number) The only issue is when I run the program 1 keeps on printing out. Here is my method for figuring out whether a number is prime or not.

 public boolean isPrime(int number)
{  
    int prime;
    for(prime = 2; prime < number; prime++)
    { 
      if (number % prime == 0)
      { 
          if(number == 1)
          { 
              return false; 
          }
       return false;
      }
    }
    return true;
}

I have read multiple forums and solutions and none seem to help. I am completely stuck.

Answer 1

Putting

if (number < 2) return false;

as the first statement into your method should do the trick.

Answer 2

Your loop is never entered when number is 1 (your loop would start at 2 ). Move the test before the loop, and you never use prime after the loop - so I would make it local to the loop.

public boolean isPrime(int number) 
{
    if (number == 1) 
    {
        return false;
    }
    for (int prime = 2; prime < number; prime++)
    {
        if (number % prime == 0) 
        {
            return false;
        }
    }
    return true;
}

You might optimize things a bit though, for example - eliminate all values less than 2 (as @luk2302 suggested) and a little math to eliminate even values and only test up to the square root.

public boolean isPrime(int number) {
    if (number < 2) {
        return false;
    } else if (number == 2 || number == 3) {
        return true;
    } else if (number % 2 == 0) {
        return false;
    }
    double sq = Math.sqrt(number);
    for (int prime = 3; prime <= sq; prime += 2) {
        if (number % prime == 0) {
            return false;
        }
    }
    return true;
}

Answer 3

This is simple way of finding prime numbers between 1 to n. Hope it's clear.

int  n=20;
System.out.println("All Prime Numbers Between 1 to "+n);
boolean isPrime;

for (int i = 2; i <= n; i++) {
    isPrime = true;
    for (int j = 2; j <=Math.sqrt(i); j++) {
        if (i % j == 0) {
            isPrime = false;
            break;
        }
    }

    if (isPrime)
        System.out.print(i + " ");
}

Ouput-

All Prime Numbers Between 1 to 20
2 3 5 7 11 13 17 19 

Answer 4

This is what i did:

package CH10;

public class PrimeSequence implements Sequence {
private int n = 2;

public int next() {
    if (isPrime(n)) {
        int primeN = n;
        n++;
        return primeN;
    } else {
        n = nextPrime(n);
        int primeN = n;
        n++;
        return primeN;
    }
}

private boolean isPrime(int n) {
    boolean test = true;
    if (n == 2 || n == 3) {
        test = true;
    } else if (n % 2 == 0 || n % 3 == 0 || isPerfectSquare(n)) {
        test = false;
    } else if (isMultipleOfPrime(n)) {
        test = false;
    }
    return test;
}

private int nextPrime(int n) {
    while (!isPrime(n)) {
        n++;
        nextPrime(n);
    }
    return n;
}

private boolean isPerfectSquare(int n) {
    int sqrt = (int) Math.sqrt(n);

    if (Math.pow(sqrt, 2) == n) {
        return true;
    }
    return false;
}

private boolean isMultipleOfPrime(int n) {
    boolean test = false;
    for (int i = (int) Math.sqrt(n); i > 3; i--) {
        if (n % i == 0) {
            test = true;
        }
    }
    return test;
}
}