Java中数组的部分副本

Question

[[-6, 3, 9], [-7, 2, 9], [-3, 2, 5], ... , [3, 4, 1]]

The array that I'm using is structured like the one above.

My goal is to divide this array based upon a certain position that has been previously determined.

I've attempted Arrays.copyOf , Arrays.copyOfRange , and System.arraycopy - but have not experienced success, which is why I wrote my own method for this; it also didn't work.

partitionResult is an instance (variable) array of type int structured just like arrayOfVals

arrayOfVals seems to become initialized with the entire partitionResult array despite my intention of only copying only a portion. I have tested ie System.out.println (partitionResult[begin+i][j]) , and the values printed are as desired.

 private int[][] copyArray(int begin, int end)
    {
        int SUBARRAY_SIZE = 2;
        // below the '+1' is due to zero-indexing
        int[][] arrayOfVals = new int[end-begin+1][SUBARRAY_SIZE+1];
        end -= begin;

        for (int i = 0; i <= end; i++) {
            for (int j = 0; j <= SUBARRAY_SIZE; j++) {
                arrayOfVals[begin][j] = partitionResult[begin+i][j];
            }
        }
        return arrayOfVals;
    }

Why can I not do the following as desired?

private void foo(int begin)
{
    int[][] arrayOne = copyArray(0, begin);
    int[][] arrayTwo = copyArray(begin+1, partitionResult.length -1);
    ...

}

Edit:

[[-6, 3, 9], [-7, 2, 9], [-3, 2, 5], [3, 4, 1], [0, 5, 5], [2, 3, 1], [3, 4, 1]]

This is my test array. I would like to split this array using the copyArray method at the defined position begin .

When I print the values that I'd like copied, partitionResult[begin+i][j] , the result is exactly as it should be; however, display the final arrayOfVals - the output is not what I printed, it is the entire partitionResult array.

I want arrayOne to equal [[-6, 3, 9], [-7, 2, 9], [-3, 2, 5]]

and arrayTwo to equal [[3, 4, 1], [0, 5, 5], [2, 3, 1], [3, 4, 1]]

Edit2: The problem was not with the method copyArray but with another method. The toString method that I wrote was displaying the values used by the instance variable partitionResult rather than the array that I passed to it - this made it seem as if nothing was being copied. The mistake should have been obvious to me. I greatly appreciate the advice.

Though, one small bug was found by @Andrea.

Answer 1

The error should be in

 arrayOfVals[begin][j] = partitionResult[begin+i][j];

change it to

 arrayOfVals[i][j] = partitionResult[begin+i][j];

because your newly created array has to start inserting values from 0.

Answer 2

This should be simple enough, you're just making it hard for yourself by mutating end , making it hard to understand your loop's progression. Just copy the values between begin and end (inclusive), but make sure to clone each subarray. (The cloning effectively replaces your inner loop.)

private int[][] copyArray(int begin, int end) {
    // Calculate the size of the output
    // below the '+1' is due to zero-indexing
    int size = end - begin + 1;
    int[][] arrayOfVals = new int[size][];
    for (int i = 0; i < size; i++) {
        // Clone each subarray to make sure changes to the copy
        // don't affect the internal array
        // (A shallow .clone() suffices for integer arrays)
        arrayOfVals[i] = partitionResult[begin + i].clone();
    }
    return arrayOfVals;
}

This gives the expected output for your sample input when calling foo(2) .

Answer 3

if fromArray is the input array and index is the index at which you want to break input array, you could do:

    System.arraycopy(fromArray, 0, arrayOne, 0, index);
    System.arraycopy(fromArray, index+1, arrayTwo, 0, fromArray.length-index);

Answer 4

What you have, when you create a 2d array, is an array of arrays. Look at the problem like this:

[0,0]  [1,0]  [2,0] ... [n,0]
[0,1]  [1,1]  [2,1] ... [n,1]
[0,2]  [1,2]  [2,2] ... [n,2]  

[[-6, 3, 9], [-7, 2, 9], [-3, 2, 5], ... , [3, 4, 1]]

We can represent your example data set as follows:

[-6]  [-7]  [-3] ... [ 3]
[ 3]  [ 2]  [ 2] ... [ 4]
[ 9]  [ 9]  [ 5] ... [ 1]