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Thread: Prime number problem (please help)

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    Default Prime number problem (please help)

    the prime numbers from 1 to 2500 can be obtained as follows. From a list of the numbers of 1 to 2500,cross out al multiples of 2(but not 2 itself)Then, find the next number (n, say) that is not crossed out and cross out all multiples of n (but not
    including n). Repeat this last step provided that n has not exceeded 50 (the square
    root of 2500). The numbers remaining in the list (except 1) are prime. Write a
    program which uses this method to print all primes from 1 to 2500. Store your
    output in a file called primes.out.


    Can someone please tell me how to go about doing this?


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    Default Re: Prime number problem (please help)

    There are several algorithms on the 'net that describe how to find prime numbers. Google 'java prime numbers', 'sieve of Eratosthenes', 'euler prime numbers', or similar.

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    Default Re: Prime number problem (please help)

    I am not insulting you here but here are just a few questions:

    1.) Do you how to figure how prime numbers?
    2.) Figure it out on a piece of paper first, do 1-25 for example.
    3.) Post some code and we will help you out, but I am feeling nice and will give you a nice push in the right direction....


    //checks whether an int is prime or not.
    boolean isPrime(int n) {
        for(int i=2;2*i<n;i++) {
            if(n%i==0)
                return false;
        }
        return true;
    }

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by jocdrew21 View Post
    I am not insulting you here but here are just a few questions:

    1.) Do you how to figure how prime numbers?
    2.) Figure it out on a piece of paper first, do 1-25 for example.
    3.) Post some code and we will help you out, but I am feeling nice and will give you a nice push in the right direction....


    //checks whether an int is prime or not.
    boolean isPrime(int n) {
        for(int i=2;2*i<n;i++) {
            if(n%i==0)
                return false;
        }
        return true;
    }
    yes i already have that piece of code.I dont know how to get the list of numbers from 1 - 2500. And i dont understand the part about n exceeding 50 from the question.

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    Lightbulb Re: Prime number problem (please help)

    This method is called Sieve of Eratosthenes.

    1 2 3 4 5 6 7 8 9 10
    11 12 13 14 15 16 17 18 19 20
    21 22 23 24 25 26 27 28 29 30
    31 32 33 34 35 36 37 38 39 40
    41 42 43 44 45 46 47 48 49 50
    51 52 53 54 55 56 57 58 59 60
    61 62 63 64 65 66 67 68 69 70
    71 72 73 74 75 76 77 78 79 80
    81 82 83 84 85 86 87 88 89 90
    91 92 93 94 95 96 97 98 99 100

    The ones marked red are prime. For this method, an array is used (not this one, single dimensional). First, starting from after 2, every 2nd no. is crossed out. Then find out the smallest remaining no. >2, which is 3. So cross out every 3rd no. Now find smallest remaining no. >3 which is 5. So cross out every 5th no. This goes on till you have crossed out every no. divisible by [root of N] where [] stands for floor function. Use a 1D array for this. Good luck.

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    Default Re: Prime number problem (please help)

    how do i get 2500 numbers into the array?

    --- Update ---

    Quote Originally Posted by Abhilash View Post
    This method is called Sieve of Eratosthenes.

    1 2 3 4 5 6 7 8 9 10
    11 12 13 14 15 16 17 18 19 20
    21 22 23 24 25 26 27 28 29 30
    31 32 33 34 35 36 37 38 39 40
    41 42 43 44 45 46 47 48 49 50
    51 52 53 54 55 56 57 58 59 60
    61 62 63 64 65 66 67 68 69 70
    71 72 73 74 75 76 77 78 79 80
    81 82 83 84 85 86 87 88 89 90
    91 92 93 94 95 96 97 98 99 100

    The ones marked red are prime. For this method, an array is used (not this one, single dimensional). First, starting from after 2, every 2nd no. is crossed out. Then find out the smallest remaining no. >2, which is 3. So cross out every 3rd no. Now find smallest remaining no. >3 which is 5. So cross out every 5th no. This goes on till you have crossed out every no. divisible by [root of N] where [] stands for floor function. Use a 1D array for this. Good luck.

    this is what i have so far

     import java.util.*;
    public class  Prime{
    	public static void main (String [] args){
    		Scanner in = new Scanner (System.in);
     
    int [] primes = new int [2500];
     
    	int num=1,i=0;
    	while(num <= 2500){
    		primes[i] = num;
    		num ++;
    		System.out.printf("%d\n",primes[i]);
    	}	
    }//end main
     
    public static void sieve(int [] primes)
     
     
     
    	}

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    Default Populate an array?

    How can i populate an array with numbers 1 to 2500???

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    Default Re: Populate an array?

    Use a for loop containing an assignment statement.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Threads merged.

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    Default Re: Populate an array?

    Quote Originally Posted by Norm View Post
    Use a for loop containing an assignment statement.
    for (int i=0;i <=2500; array[i] ++)

    is that it??

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    Default Re: Prime number problem (please help)

    Try it and see what happens. Print the contents of the array using:
    System.out.println("an ID "+ java.util.Arrays.toString(theArrayNameHere));
    Recommend doing it for 25 elements vs 2500 for testing.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    Try it and see what happens. Print the contents of the array using:
    System.out.println("an ID "+ java.util.Arrays.toString(theArrayNameHere));
    Recommend doing it for 25 elements vs 2500 for testing.

    ok im making some progress with the problem.But im trying to output the results to a file however im getting an error.
    	if(isPrime[i - 1]){
     			out.printf("%d",i);
     			out.println();

    everything worked well before i tried to output the results to a file.
    im getting an error at the two out.printf lines that says cannot find symbol.

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    Default Re: Prime number problem (please help)

    cannot find symbol.
    What symbol is not found?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    What symbol is not found?
    idk it just says symbol not found. I made sure to close the file. However im not getting my data from an input file.It is populated into an array.Everything worked b4 i tried to print to a file. So im stuck

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    Default Re: Prime number problem (please help)

    symbol not found
    That's a compiler error which means your code is not ready for execution. You need to find out what symbol is not found. Try using the javac command to compile the file. It gives nice error messages:
    The error message shows the source with a ^ under the location of the error.
    Here is a sample from the javac compiler:
    TestSorts.java:138: cannot find symbol
    symbol  : variable var
    location: class TestSorts
             var = 2;
             ^
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    That's a compiler error which means your code is not ready for execution. You need to find out what symbol is not found. Try using the javac command to compile the file. It gives nice error messages:
    The error message shows the source with a ^ under the location of the error.
    Here is a sample from the javac compiler:
    TestSorts.java:138: cannot find symbol
    symbol  : variable var
    location: class TestSorts
             var = 2;
             ^
    public static void Sieve(int n)throws IOException{
     
     	boolean [] isPrime = new boolean [n];
     	isPrime [0] = false;
     
     	for(int c=1;c <n; c++){
     		isPrime[c] = true;
     	}//end for
     	for(int i=2;i <=n;i++){
     		if(isPrime[i - 1]){
     			out.printf("%d",i);
     			out.println();
     		for(int j= i * i;j <=n;j+=i){
     			isPrime[j-1]=false;
     		}
     		}
     	}	
     }//end method
    }//end class

    thats my method. it says cannot find symbol
    out.println();
    ^
    symbol: variable out
    location: class
    2 errors

    Process completed.

    --- Update ---

    the method works without the implementation of the output file. How can someone help me plz.How can i send the data to an output file

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    Default Re: Prime number problem (please help)

    Where is the variable: out defined? Is it in scope where the error happens?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    Where is the variable: out defined? Is it in scope where the error happens?
    no its defined in the main .
    still getting the same error

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    Default Re: Prime number problem (please help)

    its defined in the main
    If it is defined inside of a method, then it is OUT OF SCOPE in any other method. Move its definition out of the method to the class level so all methods can see it.
    A variable can be defined in one place and given a value at another place.
    String var;
    ...

    var = "AS";
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    If it is defined inside of a method, then it is OUT OF SCOPE in any other method. Move its definition out of the method to the class level so all methods can see it.
    A variable can be defined in one place and given a value at another place.
    String var;
    ...

    var = "AS";
    im still not getting it. Here's the entire thing.
    [
    Last edited by javaman1; June 20th, 2014 at 09:44 PM.

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    Default Re: Prime number problem (please help)

    Please read post#19
    Where is out defined?
    class A_Class {
       int classVar; // define at class level;  in Scope for all methods
     
       void meth1() {
           int aVar;    // define aVar; in Scope only within meth1
           //  classVar in Scope here
           //...
       } // end meth1()
     
       void meth2() {
           //  aVar in NOT in SCOPE here
           //  classVar in Scope here
           //...
       } //  end meth2()
    } // end class
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    Please read post#19
    Where is out defined?
    class A_Class {
       int classVar; // define at class level;  in Scope for all methods
     
       void meth1() {
           int aVar;    // define aVar; in Scope only within meth1
           //  classVar in Scope here
           //...
       } // end meth1()
     
       void meth2() {
           //  aVar in NOT in SCOPE here
           //  classVar in Scope here
           //...
       } //  end meth2()
    } // end class
    Ok i did that and its saying expected. I dont know what else to do

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    Default Re: Prime number problem (please help)

    its saying ...
    You need to copy the full text of the error message and paste it here. I can't see your code from here.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Prime number problem (please help)

    Quote Originally Posted by Norm View Post
    You need to copy the full text of the error message and paste it here. I can't see your code from here.
    --------------------Configuration: <Default>--------------------
    C:\Users\Desktop\Primes.java:32: error: <identifier> expected
    out.close();
    ^
    1 error

    Process completed.

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    Default Re: Prime number problem (please help)

    Please post the whole class where that statement is.

    The error message says that statement is in the wrong place in the code.

    Do you know how to define a variable?
    If you don't understand my answer, don't ignore it, ask a question.

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