计算素数的最后一列

Question

I'm getting the prime numbers with following code:

public static void printPrimeNumbers(int numberOfPrimes){
    final int NUMBER_OF_PRIMES_PER_LINE = 10;
    int count = 0;
    int number = 2;


    while (count < numberOfPrimes){
        //print the prime numbers and increase count
        if (isPrime(number)){
            count++;

            if (count % NUMBER_OF_PRIMES_PER_LINE == 0){
                System.out.printf("%-5s\n", number);
            }else {
                System.out.printf("%-5s", number);
            }

        }
        number++;
    }

}

private static boolean isPrime(int number) {
    for (int divisor = 2; divisor <= number / 2 ; divisor++) {
        if (number % divisor == 0){ // if true number is not prime
            return false;
        }
    }
    return true;
}

It's worked well, what i need to get the sum of last column primes, suppose i insert printPrimeNumbers(50) there will be 5 rows with 5 numbers in columns i want to get the sum of last column, how to achieve that?

Answer 1

What you need to do is include a new variable to hold the value of the sum. With that variable, you need to add the value of the number at the end of each column. Identifying where to go to a new line will show you when to add to the new sum variable. I've written in where you need to add variables/values but assigned these values to ?'s which you need to replace with the actual code to make this work.

public static void printPrimeNumbers(int numberOfPrimes){
    final int NUMBER_OF_PRIMES_PER_LINE = 10;
    int count = 0;
    int number = 2;
    int sum = ?; 


    while (count < numberOfPrimes){
        //print the prime numbers and increase count
        if (isPrime(number)){
            count++;

            if (count % NUMBER_OF_PRIMES_PER_LINE == 0){
                System.out.printf("%-5s\n", number);
                sum += ?;
            }else {
                System.out.printf("%-5s", number);
            }

        }
        number++;
    }

}

private static boolean isPrime(int number) {
    for (int divisor = 2; divisor <= number / 2 ; divisor++) {
        if (number % divisor == 0){ // if true number is not prime
            return false;
        }
    }
    return true;
}

Answer 2

You could edit your

printPrimeNumbers()

method to be:

public static void printPrimeNumbers(int numberOfPrimes){
    final int NUMBER_OF_PRIMES_PER_LINE = 10;
    int count = 0;
    int number = 2;
    **List<int> lastColumnPrimes = new ArrayList();**


    while (count < numberOfPrimes){
        //print the prime numbers and increase count
        if (isPrime(number)){
            count++;

            if (count % NUMBER_OF_PRIMES_PER_LINE == 0){
                System.out.printf("%-5s\n", number);
                l**astColumnPrimes.add(number);**
            }else {
                System.out.printf("%-5s", number);
            }

        }
        number++;
    }

    **int lastColumnSum = 0;
    for ( int n : lastColumnPrimes ) {
        lastColumnSum += n;
    }
    System.out.printf("Sum of numbers in last column is " + lastColumnSum);**

}

Notice the added lines by the double asterisks (**). Very few additions needed, just a list to keep track of the numbers in the last column, a loop to get the sum, and finally a print statement to report the sum. Viola!

If you don't need to keep track of the numbers in the last column, and just want the sum, then you can leave out the list and loop, and just use an int to tally the values instead of a List.

Answer 3

Create a new variable and initilize it to 0. Then you can just add to it in your if statement where you check for a new line. And then print out the total at the end.

int total = 0;


while (count < numberOfPrimes){
    //print the prime numbers and increase count
    if (isPrime(number)){
        count++;

        if (count % NUMBER_OF_PRIMES_PER_LINE == 0){
            System.out.printf("%-5s\n", number);
            total += number;
        }else {
            System.out.printf("%-5s", number);
        }

    }
    number++;
}
System.out.println("Total:" + total);