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Thread: Problem with adding to a list in a loop

  1. #1
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    Question Problem with adding to a list in a loop

    Hi all,

    I wrote a code to check if an array is not in a list, then it should add it to another list. I used a linked list for this. But my problem is the program always add multiple copies of the current array and remove what's inside in the list before: My code is as follows:
    import java.util.Arrays;
    import java.util.LinkedList;
    import java.util.List;
     
    public class Trial {
     
    	/**
    	 * @param args
    	 */
     
    	public static void main(String[] args) {
    		// TODO Auto-generated method stub
     
    		List<int[]> G = new LinkedList<int []>(); 
    		List<int []> New = new LinkedList<int []>(); 
     
    		int [] f = new int [2];
     
    		for(int i=0; i<2; i++){
    			for(int j=0; j<2; j++){ 
    				f[0] = i;
    				f[1] = j;
    				//System.out.println("f is "+Arrays.toString(f));
    				if(!(G.contains(f))){
    					System.out.println("current f is "+Arrays.toString(f));  // I print here in       order to see what is f
    					New.add(f);
    					System.out.println("content of the list New");
    					// I print the list New to see its contents
    					for(int k=0; k<New.size(); k++){
    						System.out.println(Arrays.toString(New.get(k)));
    					}
    					System.out.println("finished printing the list New");
    				}
    			}
    		}
     
    	}
     
    }
    And this is the result I got after running:
    current f is [0, 0]
    content of the list New
    [0, 0]
    finished printing the list New
    current f is [0, 1]
    content of the list New
    [0, 1]
    [0, 1]
    finished printing the list New
    current f is [1, 0]
    content of the list New
    [1, 0]
    [1, 0]
    [1, 0]
    finished printing the list New
    current f is [1, 1]
    content of the list New
    [1, 1]
    [1, 1]
    [1, 1]
    [1, 1]
    finished printing the list New

    Please help!!!!
    Last edited by copeg; November 30th, 2010 at 09:08 AM.


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: Problem with adding to a list in a loop

    When posting code, make sure you use the code tags to preserve highlighting. Otherwise nobody will want to read it.

  3. #3
    Administrator copeg's Avatar
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    Default Re: Problem with adding to a list in a loop

    As mentioned, for future reference please flank your code with the [highlight=java][/highlight] tags. For now I have edited your post to reflect this.

    Your code changes the reference to the array, so any changes to said reference will be reflected in the references in the List. To add new values, you should create new references (eg place the int [] f = new int [2]; inside the double for loop). In general, its good practice to give variables meaningful names and don't even come close to conflicting with standard java statements (eg 'New').

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