将锯齿状2D阵列中的元素组合成一个新的锯齿状2D阵列（深层复制问题）

Question

Given two jagged arrays: a & b where a + b will always have the same # of rows:

int[][] a = { {1,2}, {4,5,6} };
int[][] b = { {7}, {8,9,0} };

how exactly can I manipulate a new jagged array c to return: { {1,2,7}, {4,5,6,8,9,0} } ?

Here's what I have so far:

int[][] c = null;    
for(int i = 0; i<a.length; i++){
        c = new int[a.length][a[i].length + b[i].length];
}

//rest of my code for assigning the values into the appropriate position works.

The trouble arises, as you all can see, that I am performing a deep copy, which, on the second iteration of the for-loop, is setting ALL rows to a length of the length of the current row on the step of the iteration.

Answer 1

Flaw in your approach

You are creating a new 2D array object each iteration of your loop. Each time through, you are reassigning c , thus throwing out all of your previous work. Additionally, placing a number in both set of brackets at the same time results in each row having the same length .

Using your example, the first time through the loop, c is assigned to a 2D array with two rows, each of length three. The second time through the loop, you throw out your previous 2D array and create a new one having two rows, each of length six.

But what you need to be doing is creating a new row each time through the loop, not the entire 2D array.

Solution

First, we create a 2D array called c and specify that it has a.length rows. We don't put a value in the second bracket, because that would indicate that all of the rows are of the same length. So at this point, c does not know about row length. It just knows how many rows it can have. Keep in mind: c doesn't actually have any rows yet, just a capacity for a.length rows.

Next, we must create the rows and assign a length/capacity to them. We set up our loop to run as many times as there are rows. The current row index is denoted by i , and therefore, c[i] refers to a specific row in the 2D c array. We use new int[] to create each individual row/array, but inside the brackets, we must specify the length of the current row. For any row c[i] , its length is given by the sum of the lengths of a[i] and b[i] ; that is, a[i].length + b[i].length .

What we are left with is an array c that contains rows/arrays, each with a set length/capacity that matches the sum of the corresponding rows lengths in a and b .

Keep in mind that c still does not contain any integer values, only containers that are of the correct size to hold the values in a and b . As you mentioned, you already have code to populate your array with values.

int[][] c = new int[a.length][];     

for (int i = 0; i < a.length; i++) {
    c[i] = new int[a[i].length + b[i].length];
}

Answer 2

When initialize Java 2D array, lets consider it as a table; you only have to give the number of rows and in each row of your table can have different number of columns.

Eg. Say we have a 2D array call c defined as follows,

int[][] c = new int[10][];

It says you defined c contains 10 of int[] elements. But in order to use it you have to define the number of columns each row has.

Eg. Say we have 3 columns in the second row

int c[1] = new int[3];

So in this example you have to add the column values of 2D arrays a and b to calculate the resultant array which is c .

c[i] = new int[a[i].length + b[i].length];

This will give you what you expected.

    int[][] a = { {1,2}, {4,5,6} };
    int[][] b = { {7}, {8,9,0} };
    int[][] c = new int[a.length][];    
    for(int i = 0; i<a.length; i++){
            c[i] = new int[a[i].length + b[i].length];
            for (int j=0;j< a[i].length; j++) {
                c[i][j] = a[i][j];
            }
            int length = a[i].length;
            for (int j=0;j< b[i].length; j++) {
                c[i][length+j] = b[i][j];
            }
    }

Answer 3

尝试c[i] = new int[a[i].length + b[i].length]

Answer 4

int[][] c = new int[a.length][];    
for(int i = 0; i < c.length; i++){
    c[i] = new int[a[i].length + b[i].length];

    int x = 0;
    for (int num : a[i]) {
        c[i][x] = num;
        x++;
    }
    for (int num : b[i]) {
        c[i][x] = num;
        x++;
    }
}

or even simpler...

int[][] c = new int[a.length][];    
for(int i = 0; i < c.length; i++){
    c[i] = new int[a[i].length + b[i].length];

    System.arraycopy(a[i], 0, c[i], 0, a[i].length);
    System.arraycopy(b[i], 0, c[i], a[i].length, b[i].length);
}

Answer 5

Try this one:

int[][] c = new int[a.length][];
for(int i = 0; i<a.length; i++){ 
    c[i] = new int [a[i].length + b[i].length];
    int j;
    for(j=0; i < a[i].length; j++){
        c[i][j] = a[i][j];
    }
    for(int k=0; i < b[i].length; k++){ 
        c[i][j+k] = b[i][j]; 
    }
} 

Answer 6

public static void main(String [] args) {

 int[][] a = { {1,2}, {4,5,6} };
 int[][] b = { {7}, {8,9,0} };
 int[][] c = null;    
 for(int i = 0; i<a.length; i++){
         c = new int[a.length][a[i].length + b[i].length];
 }

 for(int i = 0; i<a.length; i++){
    for (int j = 0; j < a[i].length+b[i].length; j++) {


    if(j< a[i].length){
        c[i][j]=a[i][j];
    }

    if(j< a[i].length+b[i].length && j>= a[i].length){
        c[i][j]=b[i][j-a[i].length];
    }


    }

 }


 for(int i = 0; i<a.length; i++){
        for (int j = 0; j < a[i].length+b[i].length; j++) {
  System.out.print(c[i][j]);

        }
        System.out.println();
 }

}

This works in my system ...........