Multidimensional Arrays in Java

1. Multidimensional Arrays in Java Vidhu S. Kapadia

2. The Basic Definitions • What is an Array? • An array is a fixed size sequential collection of elements of identical types. • They are created with either a new operator or array initializes • The storage space for arrays is allocated from the garbage-collected heap. • The element in an array are indexed by the integers 0 to n- 1, where n is the size of the array

3. Declaring an Array • Using the newoperator  new Type [ ] Here the size of the array must be specified, and all the elements in the array are initialized to the default initial values based on the types of elements. • int ia1 = new int [3]; //creates an integer array of size 3; all elements are initialized to 0 • Point pa [ ] = new Point [10]; //creates an array of points of size 10; all elements are initialized to null

4. Declaring an Array • Using the one-dimensional array initializer {v0, v1, …..,vn-1} The values forv0, v1, …..,vn-1 are the initial values of the elements in the array. The size of the array is determined by the number of initial values provided in the array initializer • Int ia2 [ ] = {1, 2, 3}; // Creates an array of size 3; elements are initialized to 1, 2, and 3 respectively

5. What are Multidimensional Arrays • A multidimensional array is treated as an array of arrays. • Let a be a k-dimensional array; the elements of a can be accessed using the following syntax: a [ i1 ] [ i2 ]…[ ik ] where n1, n2, ….., nk

6. Multidimensional Arrays • Applying what we learned about simple arrays to multidimensionalarray, we get the following: A k-dimensional array can be created with either of the following methods: • Using the new operator • Using the k-dimensional initializer

7. Using the new operator new Type [ n1] [n2]…[nk] Size of each dimension is n1, n2, …,nk, respectively. All the elements in the array are initialized to default initial values based on the type of the elements Using k-dimensional array initializer {I1, I2, ...., Ik} where eachI1, I2, ...., Ikis a (k-1)- dimensional array initializer. Creating k-dimensional Array

8. Using the new operator, the following example is of creating and initializing a two-dimensional array. double mat1[ ][ ] = new double[4][5]; This creates a 4 x 5 two-dimensional array. All elements are initialized to 0.0. Using k-dimensional array initializer, the following example is of creating and initializing a two-dimensional array. int mat2[ ] [ ] = {{1, 2, 3}, {4, 5, 6}}; This creates a 2 x 3 two-dimensional array. The elements are initialized as: mat2[0][0]=1 mat2[1][0]= 4 mat2[0][1]=2 mat2[1][1]= 5 mat2[0][2]=3 mat2[1][2]= 6 Examples…

9. Illustrating Two-Dimensional Array

10. Processing Multidimensional Arrays • Multidimensional arrays are often processed using for statements. To process all the items in a two-dimensional array, a for statement needs to be nested inside another. int[ ][ ] A = new int[3][4]; The following loop stores 0 into each location in two dimensional array A : for (int row = 0; row < 3; row++) { for (int column = 0; column < 4; column++) { A [row][column] = 0; } }

11. An Example: • Using a mathematical function in the Java Math class and handling of two–dimensional arrays, the following example calculates the max – min value.

12. public class MaxMin{ public static void main(String[]args) { double mat[ ] [ ]= { {2.3, 5.1, 9.9}, {8.3, 4.5, 7.7}, { 5.2, 6.1, 2.8}} int n = mat.length; int m= mat[0].length; double maxmin = 0.0; for (int j = 0; j < m; j++){ double min = mat[j][0]; for (int i = 1; I <n ; i ++){ min = Math.min(min,mat[i][j]); } if (j==0){ maxmin = min; }else{ maxmin = Math.max(maxmin, min); } } System.out.println(“The max-min value is” + maxmin); } } • Running the program produces the following result: The max-min value is 4.5

13. Keep in mind that… • Computer memory is linear – • In accessing a multidimensional array you are really accessing a one-dimensional array in memory. • You can simulate multidimensional arrays by packing them into a one-dimensional array. • The two manners of packing the one-dimensional array are row-major order, where the array is filled one row at a time; and column-major order, where columns are placed into the array.

14. Interesting Tidbits about Arrays in Java • One of the main differences between the arrays in Java and those in C/C++ is that the Java arrays are ALWAYS bounds-checked at run time. • Array bound-checking will automatically detect a major source of faults in programs, that is exceeding the bounds of arrays while accessing or manipulating their elements. • There is no limit on the number of dimensions that an array type can have. However, arrays of dimension three or higher are fairly uncommon.

15. References • Jia, Xiaoping. Object Oriented Software Development using Java- 2nd edition, Adison Wesley 2003 • The historical collection classes – Arrays, IBM. April, 16, 2006, http://www-128.ibm.com/developerworks/java/library/j-arrays/  • Array initialization, The Code Guru. April, 16, 2006, http://codeguru.earthweb.com/java/tij/tij0053.shtml