我正在尝试从arraylist中删除所有素数，但是什么都没打印？

Question

I'm a bit of a newbie, so I apologize for any silly errors.

Here's the code:

    public static void keepOnlyCompositeNumbers( List<Integer> nums ){

    int num=0;
    boolean isPrime=true;
    for(int i=0; i<nums.size(); i++){
        num=nums.get(i);
        {
            for (int o = 2; o < num; ++i)
            {
                if (num % o == 0){
                    isPrime=false;
                    o=2;        
                    if(isPrime==true){
                                nums.remove(num);
                            }
                }


            }


}

And the runner class:

    Integer[] nums = {2,6,8,9,10,12,13,15,17,24,55,66,78,77,79};
    List<Integer> list = new ArrayList<Integer>( Arrays.asList(nums) );

    System.out.println( list );
    ArrayListFunHouse.keepOnlyCompositeNumbers( list );
    System.out.println( list );

Nothing's printing. Can somebody please tell me what I'm doing wrong, please? I have been stumped for quite some time. Any help would be appreciated. Thank you!

Answer 1

You can use like this:

public static void keepOnlyCompositeNumbers( List<Integer> nums ){
  boolean isPrime=false;
  for(int i=0;i<nums.size();i++){
  isPrime = checkPrime(nums.get(i));
    if(isPrime){
        nums.remove(i);
    }
  System.out.println(nums);
}
boolean checkPrime(int n) {
        for(int i=2;i<n;i++) {
            if(n%i==0)
                return false;
        }
        return true;
}

Fetch each number in for loop and check in separate method for prime if return value is true which is prime number than remove it.

You'r not getting output because of infinite loop

for (int o = 2; o < num; ++i)

problem is you are increment i instead of o, and loop will get infinite when it get input which is greater than 2

Answer 2

More efficent checkPrime

boolean checkPrime(int n) {
        for(int i=2;i<=n/2;i++) {
            if(n%i==0)
                return false;
        }
        return true;
}

Answer 3

Please have a look

public static void main(String[] args) throws IOException {
        Integer[] nums = { 2, 6, 8, 9, 10, 12, 13, 15, 17, 24, 55, 66, 78, 77, 79 };
        List<Integer> list = new ArrayList<Integer>(Arrays.asList(nums));

        System.out.println("Original List => " + list);
        keepOnlyCompositeNumbers(list);
        System.out.println("FInal List => " + list);
    }

    public static void keepOnlyCompositeNumbers(List<Integer> nums) {

        for (int i = 0 ; i < nums.size() ; i++) {
            if (isPrime(nums.get(i))) {
                nums.remove(i);
            }
        }
    }

    public static boolean isPrime(int number) {
        for (int i = 2 ; i < number ; i++) {
            if (number % i == 0) {
                return false; // number is divisible so its not prime
            }
        }
        return true; // number is prime now
    }

output

Original List => [2, 6, 8, 9, 10, 12, 13, 15, 17, 24, 55, 66, 78, 77, 79]

FInal List => [6, 8, 9, 10, 12, 15, 24, 55, 66, 78, 77]

Answer 4

Working from Sarz's answer, something like this should work a little better for checkPrimes:

public static void keepOnlyCompositeNumbers( List<Integer> nums ){
  boolean isPrime=false;
  for(int i=0;i<nums.size();i++){
  if ( checkPrime(nums.get(i), nums)) 
        nums.remove(i);
}
boolean checkPrime(int n, List <Integer> primes) {
    sqrtn = Math.sqrt(n) ;
    for (Integer p : primes) {
        if (p > sqrtn) 
             return true;
        if (n % p == 0)
             return false
    }
}

Answer 5

There are a number of mistakes here. There may be others.

   num=nums.get(i);
    {
        for (int o = 2; o < num; ++i)
        {
            if (num % o == 0){
                isPrime=false;
                o=2;        
                if(isPrime==true){
                            nums.remove(num);
                        }
            }


        }

1) for (int o = 2; o < num; ++i) seems like a typo. The loop will exit when o becomes num or greater. But you're not incrementing o , you're incrementing i . Understandable since they're right next to each other on the keyboard, and the compiler didn't catch it since i exists as a variable.

2) Even if you change it to o++ , you sabotage your loop by setting o=2 if the number is non-prime. This means that o < num will never be true, because you keep setting it back to 2, and the loop will go on forever.

It appears that you're trying to say "this number is composite, so let's go to the next number". But you're not. nums.remove(num) removes num from the list, but it does not change num . You haven't done anything to tell it to go to the next num . There is nothing that changes num inside the for loop. There's a statement that assigns to num outside the for loop, but that doesn't do you any good as long as you're stuck in the for loop.

3) The isPrime==true test is in the wrong place. It will never succeed, because you've just set it to false . The right place to test isPrime==true is after you've checked every divisor. That means you have to move it outside the for loop, after the closing curly brace. (Also, you don't need to say if (isPrime==true) ; the usual way is just if (isPrime) .)

4) There's one more subtle error that trips up lots of people. If you have an index that goes through a list, and in the middle of that loop you remove an item from the list, you will miss numbers in the list. Consider this:

List<Integer> list = new ArrayList<>(Arrays.asList(10, 20, 30, 40));
for (int i = 0; i < list.size(); i++) {
    int num = list.get(i);
    if (needToDelete(num)) {
        list.remove(i);
    }
}

The first time through the loop, i is 0 and num is 10. Say we decide 10 needs to be deleted. So we remove it. The list is now [20, 30, 40]. Then we go back, increment i to 1, and set num to list.get(1) --which is now 30! So we never looked at the element 20.

There are a number of ways to write this differently. I don't like changing indexes in the middle of a for loop, so I'd do this:

int i = 0;
while (i < list.size()) {
    num = list.get(i);
    if (needToDelete(num)) {
        list.remove(i);
    } else {
        i++;
    }
}

Note that I increment i only when I don't remove an element from the list. If I do, i stays the same and will now be the index of the next element, since I've removed the earlier one.

Answer 6

1) Put boolean isPrime = true; inside for loop. 
2) in for(int o = 2; o < num; ++i) // change ++i to ++o 
3) comment out o=2;  //o=2 and put a break;
4) put nums.remove(i); outside for loop

public static void keepOnlyCompositeNumbers(List<Integer> nums) {
int num = 0;
for (int i = 0; i < nums.size(); i++) {
    boolean isPrime = true;
    num = nums.get(i);
    {
    for (int o = 2; o < num; ++o) {
        if (num % o == 0) {
        isPrime = false;
        // o=2;
        break;
        }

    }
    if (isPrime == true) {
        nums.remove(i);
    }

    }
}
}