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Program: Array07.java
// Learning Object Array07
// arrays of arrayspublic class Array07 {
public static void main(/*String[] args*/) {
int[][] matrix = new int[2][2]; for (int i = 0; i < matrix.length; i++)
for (int j = 0; j < matrix[i].length; j++) matrix[i][j] = i*matrix.length + j;
int[] vector = matrix[1]; }
}
This program creates a matrix and then assigns the second row to a variable of type one-dimensional array.
matrix .
The variable
matrix contains one reference for each row and the rows are allocated as separate objects. Note that Jeliot displays rows from top to bottom as it does for all objects!matrix.length is used to get the number of rows and
matrix[i].length to get the number of columns in row
i , which
is the same for all rows in this program.vector of type one-dimensional array is declared and initialized
with the second row of the matrix,
matrix[1] .
Exercise Write a program to rotate the rows of the array
matrix . That is, row 0 becomes row 1 and row 1 becomes row 0. Now do this for an array of size
: row 0 becomes row 1, row 1 becomes row 2 and row 2 becomes row 0.
Concept A two-dimensional array is really an array of arrays; that is, each element of the array contains a reference to another array. However, the two-dimensional array need not be a square matrix, and each row can have a different number of elements. By using only one index a one-dimensional array is obtained and these arrays need not all be of the same size.
Program: Array08.java
// Learning Object Array08
// ragged arrayspublic class Array08 {
static int addElements(int[][] a) { int sum = 0;
for (int i = 0; i < a.length; i++)
for (int j = 0; j < a[i].length; j++) sum = sum + a[i][j];
return sum; }
public static void main(/*String[] args*/) { int[][] matrix = new int[3][];
int[] row0 = {0, 1, 2};
int[] row1 = {3, 4};
int[] row2 = {5};
matrix[0] = row0;
matrix[1] = row1;
matrix[2] = row2;
int sum = addElements(matrix); }
}
Here we create the upper-left triangle of a matrix: row 0 of length 3, row 1 of length 2 and row 2 of length 1. Then we add the elements of the “ragged” array.
matrix is allocated, but since the size of the rows is not given, it is allocated as a one-dimensional array whose elements are references to one-dimensional arrays of integers. The default value for the elements is
null .matrix .matrix is passed to the method
addElements which adds the elements of the array and returns the value.matrix.length is used to get the number of rows and
matrix[i].length to get the number of columns in row
i ; these
are different for each row.
Exercise Simplify the allocation of the array
matrix . First, show how the variables
row can be eliminated. Then find out how to write an initializer for a two-dimensional array so that the array can be initialized in one declaration. (Note: initializers for two-dimensional arrays are not supported in Jeliot.)
Concept Arrays can contain references to arbitrary objects. There is no difference between these arrays and arrays whose values are of primitive type, except that an individual element can be of any type.
Program: Array09.java
// Learning Object Array09
// arrays of objectsclass Access {
String name; int level;
Access(String u, int l) { name = u; level = l;
}}
public class Array09 {
static void swap(Access[] a, int i, int j) {
Access temp = a[i];
a[i] = a[j]; a[j] = temp; }
public static void main(/*String[] args*/) {
Access[] accesses = new Access[2]; accesses[0] = new Access("Bob", 3); accesses[1] = new Access("Alice", 4); swap(accesses, 0, 1);
}}
Objects of class
Access contain name of a bank customer and the access level permitted for that customer. This program creates two objects, assigns the their references to elements of the
Access and then swaps the elements of the array.
accessess of type
Access[] is allocated and contains null references.Access are allocated and initialized by its constructor; a reference to the object is stored in the first element of the array
accessess .accessess is passed to the method
swap along with the
indices 0 and 1.a[i] = a[j] ,
both elements of the array point to the second object (
Alice,4 ),
while a reference to the first object (
Bob,3 ) is saved in the variable
temp .
Exercise Modify the program so that the initialization of the array
accessess is done in one statement instead of three.
Exercise Explain what happens if the method
swap is replaced by:
static void swap(Access a, Access b) {
Access temp = a; a = b;
b = temp;}
and the call by
swap(accesses[0], accesses[1]);
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