1. 1 One-Dimensional (1-D) Array Overview l Why do we need 1-D array l 1-D array declaration and Initialization l Accessing elements of a 1-D array l Passing Array as parameter l Some Examples of Array processing l Preview: More on 1-D Arrays

2. 2 Why do we need data 1-D array? l There are many applications where several data values are needed in the memory at same time. l Example, counting number of students who perform below average in a class of 30. l Obviously we need to scan through the list of values two times; » first to compute the average and »second to count those below average. l To solve this problem, do we need to declare 30 variables or do we read the values the second time? l Both of these solutions are not convenient l This is even more so when data size is expected to grow very larger. l One convenient solution to this and similar problems is array. l An array is a contiguous list of memory cells that can be identified using a single variable.

3. 3 Array Declaration l An array is declared as shown below int [] grades = new int[10]; l Another way to declare array, which is not as readable as above is: int grades[] = new int[10]; l Each of the above methods declares an int array of size 10 which cab be shown as a diagram as below, but we discourage the use of the second method: l Other examples of array declaration are shown below. Notice that the last example where two arrays are declared at the same time is discouraged. float[] prices = new float[500]; boolean[] flags; flags = new boolean[20]; char[] codes = new char[1750]; int[] first = new int[100], second = new int [200]; grades Array object reference Index0123456789

4. 4 Bounds Checking Once an array is created, it has a fixed size Each array object has a public instance variable called length that stores the size of the array The length of an array object is referenced through the array name  just like any other object: grades.length An index used in an array reference must specify a valid element -- must be in the range 0 to N-1 for an array of N elements, otherwise, the exception ArrayIndexOutOfBoundsExecption is thrown. Thus, the Java interpreter is said to exhibit automatic bounds checking.

5. 5 Initializer List An initializer list can be used to instantiate and initialize an array in one step The values are delimited by braces and separated by commas as shown below int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; char[] letterGrades = {'A', 'B', ‘C’ 'D', 'F'}; Notice that in the above examples, it is actually the compiler that fills the gap. Thus, in the first example, the compiler would do the following: int[] units = new int[10]; units[0]=147; units[1]=323;...; units[9]=476; Observe that when an initializer list is used: the new operator is not used the size is not specified

6. 6 Accessing elements of an Array A particular cell in an array can be referenced using the array name followed by the index of the cell in brackets. For example, the following statement stores 20 in the cell whose index is 4 (cell number 5) grades[4] = 20; The following example demonstrates array index processing. Each cell is initialized to zero. class FillOneDArray { public static void main (String args[]) { int[] grades = new int[10]; for (int i=0; i < grades.length; i++) grades[i] = 0; } Notice that the only modification needed for the above function to handle a different array size, say 20, is in line 3. The loop remains the same. grades Array object reference20 Index0123456789

7. 7 Passing Array as a parameter l An entire array can be passed to a method as a parameter l Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other l Changing an array element in the method changes the original class ParameterPassingExample { public static void main(String [] args){ int i; int [] a = {1, 2, 3, 4}; double d = 5.5; System.out.println("After initialization"); for (i = 0; i < a.lenght; i++) System.out.println("a["+i+"]="+a[i]); System.out.println("d = " + d); System.out.println("Calling tryToChange."); tryToChange(a, d); System.out.println("Back from tryToChange"); for (i = 0; i < a.length; i++) System.out.println("a["+i+"]="+a[i]); System.out.println("d = " + d); }

8. 8 Passing Array as a parameter (cont’d) private static void tryToChange(int [] arr, double dd) { int i; dd = 3.14; arr[0] = 55; arr[3] = 100; for (i = 0; i < 4; i++) System.out.println("arr["+i+"]="+arr[i]); System.out.println("dd = " + dd); } After initialization a[0]= 1 a[1]= 2 a[2]= 3 a[3]= 4 d = 5.5 Calling tryToChange arr[0]= 55 arr[1]= 2 arr[2]= 3 arr[3]= 100 dd = 3.14 Back from tryToChange a[0]= 55 a[1]= 2 a[2]= 3 a[3]= 100 d = 5.5

9. 9 More examples: What if size is unknown? l It is important to note that the size of the array must be determined before it can be created. l Thus, if we do not know the exact size of data, the only option is create an array of reasonably large size and hope that the actual data will not be up to that. l In such case, we need to have an accompanying variable that keep count of the actual data stored in the array. l The following example reads grades from the user until –1 is entered and prints the average grade and those grades below average. import java.io.*; class ArrayGrades { public static void main(String[] args) throws IOException { BufferedReader stdin = new BufferedReader( new InputStreamReader(System.in)); int numGrades=0; double[] grades = new double[100]; double sum = 0, average, grade;

10. 10 More examples: What if size is unknown? do { System.out.print("Enter Next Grade: "); grade = Double.parseDouble(stdin.readLine()); if (grade >= 0) { grades[numGrades] = grade; sum += grade; numGrades++; } } while (grade >= 0); if (numGrades > 0) { average = sum / numGrades; System.out.println("The average = " + average); System.out.println("Those below average are: "); for (int i=0; i<numGrades; i++) if (grades[i] < average) System.out.println(grades[i]); }