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Thread: Issue when returning an array

  1. #1
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    Question Issue when returning an array

    I'm writing my own mergeSort method from scratch for a CS course, but somewhere a long the way it seems to lose it's way...

    public static int[] mergeSort(int[] array) {
     
    		if (array.length <= 1){
    			return array;
    		}
     
    		int[] left;
    		int[] right;
     
    		if (array.length%2 == 1){
    			left = new int[array.length/2+1];
    		}else {
    			left = new int[array.length/2];
    		}
    		right = new int[array.length/2];
     
    		System.arraycopy(array, 0, left, 0, left.length);
    		System.arraycopy(array, left.length, right, 0, right.length);
     
    		mergeSort(left);
    		System.out.println("A left value returned with result: " + printArray(left));
    		mergeSort(right);
    		System.out.println("A right value returned with result: " + printArray(right));
     
     
    		//printArray(merge(left, right));
    		return merge(left, right);
    	}
     
    	public static int[] merge(int[] left, int[] right) {
     
    		int[] result = new int[left.length+right.length];
     
    		int count = 0;
     
    		while (left.length > 0 || right.length >0) {
    			if (left.length > 0 && right.length > 0) {
    				if (left[0] <= right[0]){
    					System.out.println("Left most of " + printArray(left) + " is less than left most of " +printArray(right));
    					result[count] = left[0];
    					left = helperFunction(left);
    				}
    				else {
    					System.out.println("Left most of " + printArray(right) + " is less than left most of " +printArray(left));
    					result[count] = right[0];
    					right = helperFunction(right);
    					}
    				}
    			else if (left.length > 0) {
    				System.out.println("Left most of " + printArray(left) + " is the next to add.");
    				result[count] = left[0];
    				left = helperFunction(left);
    			}
    			else if (right.length > 0) {
    				System.out.println("Left most of " + printArray(right) + " is the next to add.");
    				result[count] = right[0];
    				right = helperFunction(right);
    			}
    			count++;
    		}
    		System.out.println("The result is: "+printArray(result));
    		return result;
    	}
     
    	public static int[] helperFunction(int[] array) {
    		int[] helperArray;
    		helperArray = new int[array.length-1];
    		System.arraycopy(array, 1, helperArray, 0, array.length-1);
    		return helperArray;
    	}
     
    	public static String printArray(int[] array) {
    		String result = "[";
    		for(int i=0;i<array.length;i++){
    			result += (array[i]);
    			if (i!=array.length-1){
    				result += ", ";
    			}
    		}
    		result += "]";
     
    		return result;
    	}

    Those are the methods I'm using. helperMethod just removes the first element of an array, the printArray method is there for testing purposes, along with the extensive amount of line printing.

    When I call mergeSort on the array {3, 2, 1} the out put is:

    [3, 2, 1]
    A left value returned with result: [3]
    A right value returned with result: [2]
    Left most of [2] is less than left most of [3]
    Left most of [3] is the next to add.
    The result is: [2, 3]
    A left value returned with result: [3, 2]

    A right value returned with result: [1]
    Left most of [1] is less than left most of [3, 2]
    Left most of [3, 2] is the next to add.
    Left most of [2] is the next to add.
    The result is: [1, 3, 2]
    If you take a look at the lines in red, I call printArray on the "result" right before I return it and it's fine and sorted. Then after it's been returned to mergeSort, I have a println that should in theory be printing the same array, but it's been flipped.

    What gives?

    If anyone can explain what's going on I would greatly appreciate it.
    Last edited by Broxxar; January 21st, 2012 at 07:57 PM.


  2. #2
    Super Moderator pbrockway2's Avatar
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    Default Re: Issue when returning an array

    mergeSort(left);
    System.out.println("A left value returned with result: " + printArray(left));
    Your println() might be a bit confusing here. It is true that mergeSort(left) does return *something* but that's not what you print: you print left. Only under rather special conditions does mergeSort() return the same array it was passed. To see the difference try:

    int[] returnedArr = mergeSort(left);
    System.out.println("A left value returned with result: " + printArray(returnedArr));
    System.out.println("While left, itself, contains: " + printArray(left));

    You need to be clear in your own mind about the distinction between left and the array returned by mergeSort(left). And which you intend sending on to merge().

  3. #3
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    Default Re: Issue when returning an array

    Just noticed it (in the wikipedia pseudocode algorithm) right as you replied pbrockway2.

    You are correct, the left being sent on is not what I want it to be:

    mergeSort(left);
    should instead be:
    left = mergeSort(left);

    Problem solved!

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Issue when returning an array

    Too late. Deleted.

  5. #5
    Super Moderator pbrockway2's Avatar
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    Default Re: Issue when returning an array

    Quote Originally Posted by Broxxar View Post
    Problem solved!
    Great - I'm glad you've got it figured out.

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