Defining 2-dimensional arrays as arguments to a method

  • 2 dimensional arrays can be used as parameters to a method as follows: 

        dataType[][] arrayName

  • Example: a method that computes the sum of all elements in a 2 dimensional array: 

        public static int sum(int[][] m) 
    {
        int total = 0;

	for (int i = 0; i < m.length; i++) 
	    for (int j = 0; j < m[i].length; j++)
	        total = total + m[i][j];

        return total;
    }

Passing 2-dimensional arrays as arguments to a method

  • 2-dimensional arrays can be passed as arguments to methods as follows: 

      int[][] myGrid = ...  // initialize the 2-dim array

  int s = sum(myGrid);  // Call sum wiith argument myGrid

    Example: 

        public static void main(String[] args)
    {
        int[][] myGrid = { {1,2,3},
                           {4,5},
                           {6,7,8,9} };
        int s;
       
        s = sum(myGrid);
        System.out.println( s );
    }

DEMO: demo/09-multi-dim-array/04-array-argument/SumArray.java

Remember:    Java passes an array reference to a method

  • In Java, an array is passed to a method in the following manner:

      • When you invoke a method with an array argument, the array reference in the argument is copied (= assigned) to the parameter variable.

  • That means:   the parameter variable will point to the same array object:

    Schematically:

            

Review:   In Java, a method can update the elements in an array parameter

In Java, methods can change/update the elements in an array parameter: 

    public static void main(String[] args)
    {
        int[][] myGrid = { {1,2,3}, 
                           {4,5},   
                           {6,7,8,9} };

        increment(myGrid);  // This method will update elements in myGrid

        System.out.println( Arrays.toString(myGrid[0]) ); // [2,3,4]    changed !
        System.out.println( Arrays.toString(myGrid[1]) ); // [5,6]      changed !
        System.out.println( Arrays.toString(myGrid[2]) ); // [7,8,9,10] changed !
    }

    public static void increment(int[][] m) 
    {

	for (int i = 0; i < m.length; i++) 
	    for (int j = 0; j < m[i].length; j++)
	        m[i][j]++;;
    }

DEMO: demo/09-multi-dim-array/04-array-argument/UpdateArray.java

Methods that return a 2-dimensional array

  • Methods can return a 2-dimensional array

  • It works the same way as methods that return an 1-dimensional array

    Except:   you must define the return type as datatype[][] 

          public static datatype[][] methodName( ... )
    {
        // (1) Create a 2-dimensional array to store the result
        datatype[][] result = new datatype[M][N];

	.... // fill result[][] with desired output values

        // (2) Return the 2-dimensional array
        return result;
    }

  • We will see an example in the next slide when we discuss how 2-dimensional arrays are used in Mathematics