How to copy values from a 2d array to a new and bigger one in Java?

Question

How can I copy a 2d array to a new and bigger one with eye for adding more items in the new one while still having the original data? My code makes out of bounds failure - what is my mistake? I try to say that the copy array should be original array.length + x - but doesn't work.

import java.util.Scanner; // import scanner to the program

public class TestTest {

// Declaring Scanner object.

public static Scanner userInput = new Scanner(System.in); 

public static void main(String[] args) {
    
    // Variables
    
     int number = 1;
     
    // Declaring 2d Array
     
     int[][] myArray = new int [3][3];
     
     // Loop values into the array
     
     for(int i =0; i<myArray.length; i++) 
     {
         myArray[i][0]= number;
         number++;
         
         myArray[i][1]= (int) (Math.random () * 10) +1;
         myArray[i][2]= (int)(Math.random() * ((1000 - 100) + 1)) + 100;         
         
     }
     
     // Print the original array
     
     System.out.println("This is original array:");
     
     for(int i =0; i<myArray.length; i++) 
     {
         
         
      System.out.println(myArray[i][0]+" " + myArray[i][1]+" "+ myArray[i][2]);
             
         
     }
     
     // Make a copy of the original array and expand the length with 3.
     
      int[][] copyArray =new int[myArray.length+3][3];
      
      for (int i = 0; i < copyArray.length; ++i) 
      {
        copyArray[i] = new int[myArray[i].length];
         
        
         for (int j = 0; j < copyArray[i].length; ++j) 
         {
            copyArray[i][j] = myArray[i][j];
         }
         
      }
      
     
     
      
      
      // Print the copy
      
      System.out.println("This is the copy:");
      
      
      for (int i = 0; i < copyArray.length+3; ++i) 
      {
          
        System.out.println(copyArray[i][0]+" " + copyArray[i][1]+" "+ copyArray[i][2] ); 
          
         
      
      }
      
      
      
      
      
      
      
      
      
 

}

}

Answer 1

You're getting ArrayIndexOutOfBounds because array copyArray has greater length than your source array myArray .

Therefore, when index i exceeded the length of the source array, this line would cause an exception:

copyArray[i] = new int[myArray[i].length];

Because there would be no nested array referred by myArray[i] .

Answer 2

Try it like this

int[][] copyArray =new int[myArray.length+3][3];
      
for (int i = 0; i < myArray.length; ++i) { // length of myArray
    
    copyArray[i] = new int[myArray[0].length]; // 0 instead of i
    
    for (int j = 0; j < myArray[i].length; ++j) { // length of myArray
       copyArray[i][j] = myArray[i][j];
    }      
}

Also since copyArray has already increased size you don't need the +3 in the last for loop. Hope I helped, good luck

Answer 3

Note that as you declare both dimensions in int[][] copyArray = new int[myArray.length+3][3] the JVM pre-allocates these arrays:- one is new int[myArray.length+3][] plus there is a new int[3] for each row of that int[][] .

See this with following code:

int[][] copyArray =new int[myArray.length+3][3];
System.out.println("copy:"+Arrays.deepToString(copyArray));
// Prints:
// copy:[[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

Thus the line in your for loop copyArray[i] = new int[myArray[i].length] is totally unnecessary - it replaces those newly constructed arrays with yet another array. Either drop the second dimension value 3 or remove the assignment.

However there is better way to copy arrays, just call System.arraycopy which works for arrays of primitive types or objects. A solution using System.arraycopy is:

 int[][] copyArray = new int[myArray.length+3][3];
 for (int i = 0; i < myArray.length; ++i) {
     System.arraycopy(myArray[i], 0, copyArray[i], 0, myArray.length);
 }