Combining elements in jagged 2D arrays into one new jagged 2D array (Deep Copy issue)

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];
}