Initialising a multidimensional array in Java

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小鲜肉
小鲜肉 2020-11-22 14:20

What is the correct way to declare a multidimensional array and assign values to it?

This is what I have:

int x = 5;
int y = 5;

String[][] myStringA         


        
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  • Multidimensional Array in Java

    Returning a multidimensional array

    Java does not truely support multidimensional arrays. In Java, a two-dimensional array is simply an array of arrays, a three-dimensional array is an array of arrays of arrays, a four-dimensional array is an array of arrays of arrays of arrays, and so on...

    We can define a two-dimensional array as:

    1. int[ ] num[ ] = {{1,2}, {1,2}, {1,2}, {1,2}}

    2. int[ ][ ] num = new int[4][2]

      num[0][0] = 1;
      num[0][1] = 2;
      num[1][0] = 1;
      num[1][1] = 2;
      num[2][0] = 1;
      num[2][1] = 2;
      num[3][0] = 1;
      num[3][1] = 2;
      

      If you don't allocate, let's say num[2][1], it is not initialized and then it is automatically allocated 0, that is, automatically num[2][1] = 0;

    3. Below, num1.length gives you rows.

    4. While num1[0].length gives you the number of elements related to num1[0]. Here num1[0] has related arrays num1[0][0] and num[0][1] only.

    5. Here we used a for loop which helps us to calculate num1[i].length. Here i is incremented through a loop.

      class array
      {
          static int[][] add(int[][] num1,int[][] num2)
          {
              int[][] temp = new int[num1.length][num1[0].length];
              for(int i = 0; i<temp.length; i++)
              {
                  for(int j = 0; j<temp[i].length; j++)
                  {
                      temp[i][j] = num1[i][j]+num2[i][j];
                  }
              }
              return temp;
          }
      
          public static void main(String args[])
          {
              /* We can define a two-dimensional array as
                   1.  int[] num[] = {{1,2},{1,2},{1,2},{1,2}}
                   2.  int[][] num = new int[4][2]
                       num[0][0] = 1;
                       num[0][1] = 2;
                       num[1][0] = 1;
                       num[1][1] = 2;
                       num[2][0] = 1;
                       num[2][1] = 2;
                       num[3][0] = 1;
                       num[3][1] = 2;
      
                       If you don't allocate let's say num[2][1] is
                       not initialized, and then it is automatically
                       allocated 0, that is, automatically num[2][1] = 0;
                    3. Below num1.length gives you rows
                    4. While num1[0].length gives you number of elements
                       related to num1[0]. Here num1[0] has related arrays
                       num1[0][0] and num[0][1] only.
                    5. Here we used a 'for' loop which helps us to calculate
                       num1[i].length, and here i is incremented through a loop.
              */
              int num1[][] = {{1,2},{1,2},{1,2},{1,2}};
              int num2[][] = {{1,2},{1,2},{1,2},{1,2}};
      
              int num3[][] = add(num1,num2);
              for(int i = 0; i<num1.length; i++)
              {
                  for(int j = 0; j<num1[j].length; j++)
                      System.out.println("num3[" + i + "][" + j + "]=" + num3[i][j]);
              }
          }
      }
      
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  • 2020-11-22 14:42
     int[][] myNums = { {1, 2, 3, 4, 5, 6, 7}, {5, 6, 7, 8, 9, 10, 11} };
     for (int x = 0; x < myNums.length; ++x) {
        for(int y = 0; y < myNums[i].length; ++y) {
           System.out.print(myNums[x][y]);
        }
     }
    

    Output

    1 2 3 4 5 6 7 5 6 7 8 9 10 11

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  • 2020-11-22 14:44

    You can look at this to start off:

        int [][][] i = {                //third dimension curly brace
                         {               // second dimension curly brace
                            {            //first dimension curly brace
                               1,1,1    //elements
                            },           
                        {3,3,3},    
                        {2,2,2}     
                          },
                          {
                             {
                              1,1,1
                             },
                             {3,3,3},
                             {2,2,2}
                           }
                        };      
    
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  • 2020-11-22 14:46

    Java doesn't have "true" multidimensional arrays.

    For example, arr[i][j][k] is equivalent to ((arr[i])[j])[k]. In other words, arr is simply an array, of arrays, of arrays.

    So, if you know how arrays work, you know how multidimensional arrays work!


    Declaration:

    int[][][] threeDimArr = new int[4][5][6];
    

    or, with initialization:

    int[][][] threeDimArr = { { { 1, 2 }, { 3, 4 } }, { { 5, 6 }, { 7, 8 } } };
    

    Access:

    int x = threeDimArr[1][0][1];
    

    or

    int[][] row = threeDimArr[1];
    

    String representation:

    Arrays.deepToString(threeDimArr);
    

    yields

    "[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]"
    

    Useful articles

    • Java: Initializing a multidimensional array
    • Java: Matrices and Multidimensional Arrays
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  • 2020-11-22 14:46

    You can declare multi dimensional arrays like :

    // 4 x 5 String arrays, all Strings are null
    // [0] -> [null,null,null,null,null]
    // [1] -> [null,null,null,null,null]
    // [2] -> [null,null,null,null,null]
    // [3] -> [null,null,null,null,null]
    
    String[][] sa1 = new String[4][5];
    for(int i = 0; i < sa1.length; i++) {           // sa1.length == 4
        for (int j = 0; j < sa1[i].length; j++) {     //sa1[i].length == 5
            sa1[i][j] = "new String value";
        }
    }
    
    
    // 5 x 0  All String arrays are null
    // [null]
    // [null]
    // [null]
    // [null]
    // [null]
    String[][] sa2 = new String[5][];
    for(int i = 0; i < sa2.length; i++) {
        String[] anon = new String[ /* your number here */];
        // or String[] anon = new String[]{"I'm", "a", "new", "array"};
        sa2[i] = anon;
    }
    
    // [0] -> ["I'm","in","the", "0th", "array"]
    // [1] -> ["I'm", "in", "another"]
    String[][] sa3 = new String[][]{ {"I'm","in","the", "0th", "array"},{"I'm", "in", "another"}};
    
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  • 2020-11-22 14:48

    Try replacing the appropriate lines with:

    myStringArray[0][x-1] = "a string";
    myStringArray[0][y-1] = "another string";
    

    Your code is incorrect because the sub-arrays have a length of y, and indexing starts at 0. So setting to myStringArray[0][y] or myStringArray[0][x] will fail because the indices x and y are out of bounds.

    String[][] myStringArray = new String [x][y]; is the correct way to initialise a rectangular multidimensional array. If you want it to be jagged (each sub-array potentially has a different length) then you can use code similar to this answer. Note however that John's assertion that you have to create the sub-arrays manually is incorrect in the case where you want a perfectly rectangular multidimensional array.

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