not existing while loop (java code)

Question

public class Q3
{
public static void main(String args[]){
int i, j;
int Max = 1000;
    //It's obvious that the first fifty prime numbers are less than 1000.
int counter = 1;
while (counter <= 50){
    for (i = 2; i < Max; i++){
        for (j = 2; j < i; j++){
            if ( i % j == 0){
            break;
            }
        }
        if (j >= i){
            System.out.printf("%s ", i);
            counter++;
        }
        if(counter % 10 == 0){
        System.out.print("\n");
        }       
    }
}

}
}

This is a program that I wrote to list first 50 prime numbers, ten of them per line. However, it's not working properly because of the while loop. After execution, this program lists all prime numbers less than 1000. It seems that the while loop is not functioning at all. Can anyone tell me the reason? Many thanks.

Just for the record. There shouldn't be a while loop at all. Only use while loops when it is not sure that the loop will be executed at all. In your case you will definitely enter the loop. In this case, use do-while. — cyphorious
– cyphorious, Commented Sep 25, 2013 at 8:28
Many thanks guys. But I may need to rephrase my question.(Sorry for my English, it's not my mother tongue.) I need that while loop to keep track of the prime numbers I've listed. The algorithm is that I check every integer one by one. The value of counter is only incremented when the number checked is primitive. Unfortunately, the code doesn't work that way. Again, many thanks — Ryan Zhao
– Ryan Zhao, Commented Sep 25, 2013 at 8:36
@user2814227 why don't you try the solutions without the while? They do just that. — Henry
– Henry, Commented Sep 25, 2013 at 8:47

Henry · Accepted Answer · 2013-09-25 08:24:17Z

1

The primes are generated by the first for loop. The while body is only executed once.

You could remove the while and instead use a different condition on the for:

for (i = 2; counter <= 50; i++){

answered Sep 25, 2013 at 8:24

Henry

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Comments

Alexia Wang · Accepted Answer · 2013-09-25 08:32:25Z

0

you have a big problem, the true code is following:

int i, j;
int Max = 1000;
//It's obvious that the first fifty prime numbers are less than 1000.
int counter = 0;
for (i = 2; i < Max && counter < 50; i++){
    for (j = 2; j < i; j++){
        if ( i % j == 0){
        break;
        }
    }
    if (j >= i){
        printf("%d ", i);
        counter++;
         if(counter % 10 == 0){
             printf("\n");
         }  
    }    
}

the output is: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229

answered Sep 25, 2013 at 8:32

Alexia Wang

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2 Comments

Ryan Zhao Over a year ago

Thanks a lot. It works, and this is what I want. Could you plz explain to me what the big problem is and the reason why while loop is not working. BTW, I'm also Chinese.(Judging from ur surname.)

Alexia Wang Over a year ago

the first problem is that only when "for (i = 2; i < Max; i++)" ends, "while (counter <= 50)" just starts judging if counter<=50; the second one is "if(counter % 10 == 0){ System.out.print("\n"); } " if counter first adds to 10,print a new space line, if the next N numbers are not prime so counter is still 10, so the statement also satisfies, it will print N space lines. the third one is that if counter initializes 1, there will be 9 not 10. My English is not great, so I donnot know if you understand me.

Silviu Burcea · Accepted Answer · 2013-09-25 08:42:04Z

0

Why are you not writing a boolean isPrime(int number) function? You will have to check if it is true, and if it is, increase the counter and print the number. Here is a naive implementation, I have seen some other better implementations:

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

Inside your for:

for (i = 2; i < max; i++) {
  if (isPrime(i)) {
    counter++;
    // print number, print new line if needed
  }
}

answered Sep 25, 2013 at 8:42

Silviu Burcea

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3 Comments

Ryan Zhao Over a year ago

yup this is an elegant way of doing it. But i only wanted to get things done easily. Many thanks, anyway.

Silviu Burcea Over a year ago

Elegant way is usually the way of doing things easily.

Ryan Zhao Over a year ago

agreed, will try later

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