2D-Arrays

1. 2D-Arrays Quratulain

2. Learning Objectives • Two-dimensional arrays • Declaration • Initialization • Applications

3. Introduction • Java builds multi-dimensional arrays from many one-dimensional arrays, the so-called "arrays of arrays" approach. • when created, arrays are automatically initialized with the default value of their type • String[] s = new String[100]; // default values: null • boolean[] b = new boolean[4]; // default values: false • int[] [] i= new int[10][10]; // default values: 0

4. Sorted array element inti, j, index, tmp; int n = arr.length; for (i = 0; i < n - 1; i++) {             index = i; for (j = i + 1; j < n; j++) if (arr[j] < arr[index]) index = j; if(index != i) { tmp = arr[i]; arr[i] = arr[index]; arr[index] = tmp;             } }

5. Introduction • The first dimension represents the rows, the second dimension, the columns curly braces {} may also be used to initialize two dimensional arrays. • Again they are only valid in array declaration statements. • int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} }; • you can initialize the row dimension without initializing the columns but not vice versa int[][] myArray = new int[5][]; • int[][] myArray = new int[][5]; // illegal • Now initialize columns in each row • myArray[0] = new int[3]; • myArray[1] = new int[4]; • myArray[2] = new int[5];

6. Demonstrate a two-dimensional array // Demonstrate a two-dimensional array. class TwoD { public static void main(String args[]) { int t, i; int table[][] = new int[3][4]; for(t=0; t < 3; ++t) { for(i=0; i < 4; ++i) { table[t][i] = (t*4)+i+1; System.out.print(table[t][i] + " "); } System.out.println(); } } }

7. Home work (Magic Square) • A magic square of order n is an arrangement of n2 numbers, usually distinct integers, in a square, such that the n numbers in all rows, all columns, and both diagonals sum to the same constant. • Develop code using 4-rules discussed in class. Here sum is 15.

8. Ragged Array • The use of irregular (or ragged) multidimensional arrays is not recommended for most applications, because it runs contrary to what people expect to find when a multidimensional array is encountered. • However, irregular arrays can be used effectively in some situations. • For example, if you need a very large two-dimensional array that is sparsely populated (that is, one in which not all of the elements will be used), an irregular array might be a perfect solution.

9. Example System.out.println("Riders per trip during the week:"); for(i=0; i < 5; i++) { for(j=0; j < 10; j++) System.out.print(riders[i][j] + " "); System.out.println(); } System.out.println(); System.out.println("Riders per trip on the weekend:"); for(i=5; i < 7; i++) { for(j=0; j < 2; j++) System.out.print(riders[i][j] + " "); System.out.println(); } } } // Manually allocate differing size second dimensions. class Ragged { public static void main(String args[]) { int riders[][] = new int[7][]; riders[0] = new int[10]; riders[1] = new int[10]; riders[2] = new int[10]; riders[3] = new int[10]; riders[4] = new int[10]; riders[5] = new int[2]; riders[6] = new int[2]; inti, j; // fabricate some fake data for(i=0; i < 5; i++) for(j=0; j < 10; j++) riders[i][j] = i + j + 10; for(i=5; i < 7; i++) for(j=0; j < 2; j++) riders[i][j] = i + j + 10;

10. Assign Array Reference • when you assign one array reference variable to another you are not causing a copy of the array to be made, nor are you causing the contents of one array to be copied to the other. int nums1[] = new int[10]; int nums2[] = new int[10]; for(i=0; i < 10; i++) nums1[i] = i; for(i=0; i < 10; i++) nums2[i] = -i; nums2 = nums1; // now nums2 refers to nums1