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Thread: N queens problem! Please help !I don't really know what the f... is going on ?

  1. #1
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    Default N queens problem! Please help !I don't really know what the f... is going on ?

    I need to solve the N queens problem (place n queens on a chess board size nxn so that the queens don't attack each other). Consist of 4 class :

    Queen.java : hold the row and column of the queen

    Stack.java : a data structure stack to hold the queen.

    Nqueens.java : to solve the problem

    testN.java : to test the solution

    the error I got is <identifier> expected in the testN.java, I don't really know what is going on, everything seems normal so far.

    Here are the codes:

    public class Queen{
     
      int row;
      int column;
     
      public Queen(int r,int c){
      row=r;
      column=c;
      } 
     
      public int getRow(){
      return row;
      }
     
      public int getCol(){
      return column;
      }
     
      public boolean isConflict(Queen q)
      {
      boolean ans= false;
      //row and collum conflict
      if(row==q.row||column==q.column) ans=true;
      //same diagonal if the vector creats by two queens has same absolute value for x and y coordinator.
      else if(Math.abs(row-q.row)==Math.abs(column-q.column)) 
        ans=true;
      return ans;
      }
     
      public String toString(){
      String ans="";
      ans=ans+"( "+row+" , "+column+" )";
      return ans;
      }
     
    }


    public class Stack
    {
    private Queen[] data;
    private int count;
     
    public  Stack(int n){
      data= new Queen[n];
     }
     
    public void push(Queen item)
    {
      if(count==data.length) increaseSize();
    data[count]=item;
    count++;
    }
     
    public int getSize(){
    return count;
    }
     
    public Queen pop(){
    Queen ans= null;
    if(count !=0)ans =data[count-1];
    count--;
    return ans;
    }
     
    public Queen lookitem(int i){
    Queen ans=null;
    if(count !=0)ans =data[i];
    return ans;
    }
     
    public Queen peek()
    {
    Queen ans= null;
    if(count !=0)ans =data[count-1];
    return ans;
    }
     
    public void increaseSize(){
    Queen[] temp=new Queen[2*data.length+1]; 
     
     
    for(int i=0;i<data.length;i++)
    {
    temp[i]=data[i];
    }
     
    data=temp;
    }
     
     
    }


    public class Nqueens{
     
    private Stack s;
    private int N;
     
    public Nqueens(int size)
    {
    s=new Stack(size); 
    N=size;  
    }
     
    //Solve the nqueens problem
    public void solution()
    {
     
    s.push(new Queen(1,1));s.push(new Queen(2,1));
     
    boolean solve=false;
     
    while(s.getSize()!=0&&solve==false)
    {
    boolean conflict=false;int i=0;
    while(i<s.getSize()-1&&conflict==false)
    {
    conflict=s.peek().isConflict(s.lookitem(i));  
    i++;      
    }
    if (conflict=false)
    {
    while(s.getSize()!=0&&s.peek().getRow()==N)
    {
    s.pop();
    }
    if(s.getSize()!=0)
    {
    Queen qq=new Queen(s.peek().getRow(),s.peek().getCol()+1);  
    s.pop();
    s.push(qq);
    }
    }
    else if(s.peek().getRow()==N) solve=true;
    else s.push(new Queen(s.peek().getRow()+1,1));
    }
     
    }
     
    public String toString(){
      String ans="";
        for(int i=0;i<s.getSize();i++){
        ans=ans+s.lookitem(i).toString()+"  ";
     
    }
    return ans;
    }}


    public class testN{
      public static void main(String arg[]){
     
        Queen q1=new Queen(1,1);
        Queen q2=new Queen(2,2);
        Queen q3=new Queen(2,1);
        Queen q4=new Queen(4,3);
        System.out.println(q1);
        System.out.println(q2);
        System.out.println(q1.isConflict(q4));
         System.out.println(q4.isConflict(q3));
      Stack s=new Stack(0);
      s.push(q1);s.push(q3);
      s.push(new Queen(3,6));
      System.out.println("Element on s "+s.peek());
      } 
     
      Nqueens qq = new Nqueens(4);
    qq.solution();
     
      System.out.println(qq);  
     
    }


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: N queens problem! Please help !I don't really know what the f... is going on ?

    You have some code in your testN class which isn't inside of any method (last 3 lines).

  3. The Following User Says Thank You to helloworld922 For This Useful Post:

    bruce88 (October 12th, 2012)

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    Default Re: N queens problem! Please help !I don't really know what the f... is going on ?

    Quote Originally Posted by helloworld922 View Post
    You have some code in your testN class which isn't inside of any method (last 3 lines).
    Sorry I don't know what you mean ?

    I created a object qq which belongs to the Nqueens class. Then use object qq to activate the method solution ( method solution in the Nqueens class).

    Then I want to print out the result using toString() method.

  5. #4
    Super Moderator jps's Avatar
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    Default Re: N queens problem! Please help !I don't really know what the f... is going on ?

    public class Sample extends Nothing {
       int myIntVar;
       double myDoubleVar = 4.0;
     
       public static void main(String[] args) {
          doSomeWork();
          doSomeOtherWork();
       }
     
       //random lines of code do not belong here!!
    }
    But you can have other methods....
    public class Sample extends Nothing {
       int myIntVar;
       double myDoubleVar = 4.0;
     
       public static void main(String[] args) {
          doSomeWork();
          doSomeOtherWork();
       }
     
       //random lines of code do not belong here!!
     
       public static void someLegalCode() {
          doDifferentWork();
       }
    }
    Last edited by jps; October 12th, 2012 at 02:21 PM. Reason: Fixed code missing }

  6. #5
    Super Moderator helloworld922's Avatar
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    Default Re: N queens problem! Please help !I don't really know what the f... is going on ?

      }  // <-- end of main method
     
      Nqueens qq = new Nqueens(4); // valid declaration and initialization of object field, but I don't think this is what you wanted
    qq.solution(); // code isn't inside of a method, Java doesn't allow this
     
      System.out.println(qq);   // code isn't inside of a method, Java doesn't allow this
     
    }

    The solution would be to move your code inside the main method.

  7. The Following User Says Thank You to helloworld922 For This Useful Post:

    bruce88 (October 12th, 2012)

  8. #6
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    Default Re: N queens problem! Please help !I don't really know what the f... is going on ?

    ok i fixed it , thanks a lot. i know what you mean now.

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