Csc 201-lecture 7

CSC 201 Lecture - 7

Scenario • Let us take a University scenario. I want to calculate the grade of all the students based on their marks in each course. How do I do?

Arrays in Java • An array is a group of variables of the same data type referred to by a common name • It is a contiguous block of memory locations referred by a common name. Ex : To store the marks of 1000 students, I can declare an array namely student_marks of size 1000 and store them.

How does an Array look? Student 0 Student 1 Student 2 Student 3 Each item in an array is called Element. The numbering begins with ‘0’.

Declaring an Array We can declare an array as below:1)datatype variable[] = new datatype[size]; Ex: int students[] = new int[20]; or datatype[] variable = new datatype[size]; 2) datatype[] variable; variable = new datatype[size]; Ex: int[] students; students = new int[20];

Creating Arrays • The length of an array is fixed at the time of creation. • A specific value in an array is accessed by placing the index value of desired element in square bracket. Ex : If I want to access the 5th student in an array, I would say student[4].

Structure of Arrays • Array contains the values which are referenced through indexes. • To access the stored values in an array, we use indexes. Suppose an array contains ‘n’ integers, the first element can be indexed with a ‘0’ value and last element by ‘n-1’ index value.

Advantages of using Arrays 1) We can refer to a large number of elements by just specifying the index number and the array name. 2) Arrays make it easy to do calculations in a loop. 3) An array has its size that is known as array length. Various types of arrays in Java:5) One – dimensional 6) Two – dimensional

Disadvantages of arrays 1) An array has fixed size. 2) Array holds only one type of data (including the primitive types).

Memory Allocation • New operator is used to allocate memory to an array. Ex: student_name = new String[10]; • Where else did we use a new operator previously?

Declaring simple arrays Public static void main(String[] args) { int[] a = new int[5]; Initializing an array! int[] b = {1,2,3,4,5}; String[] name = { “student1”, “student2”}; char[] c = {‘a’, ‘b’}; }

Simple program to print array elements public static void main(String[] args) { int [] a = {1,2,3,4,5}; for(int i =0; i < 5; i++) System.out.println(“Elements of array a are “+a[i]);}

Sample Program • What happens in the following scenario? public static void main(String[] args) { Array Index out of bounds? int [] a= {1,2,3,4,5}; for(int i =0; i < 6; i++) System.out.println(“Elements of array a are “+a[i]); }

public static void main(String[] args) { int [] a= {1,2,3,4,5}; for(int i =0; i < 5; i++) { System.out.println(“Elements of array a are “+a[i]);} System.out.println(a[2]); }

What gets printed here?

Program to calculate sum of numbers from 1 to 10 Public static void main(String[] args) { int a[] = new int[10]; int sum = 0; for(int k =0; k < 10; k++) Do you get the sum of first 10 numbers? { a[k] = k; sum = sum + a[k]; } System.out.println(“Sum of first 10 numbers is “+sum); }// How about k < 11? How about increasing size of array?

Calculate the sum public class sum { public static void main(String[] args) { int[] a = new int[11]; int sum = 0; for(int m=0; m < a.length ; m++) a[m] = m; for(int k =0; k< a.length; k++) sum += a[k]; System.out.println(sum); } } Output here ?

Example- Printing 10 random numbers import java.util.Random; public static void main(String[] args) { Random mynumber = new Random(); int[] student_marks = new int[10]; for(int i = 0; i < 10 ; i++) { student_marks[i] = mynumber.nextInt(10); System.out.println(“Marks for student “+i” is “student_marks[i]); } }

• What if you want to get an average of the marks of all the students in a class?

Program to search a number in an Array Public static void main(String[] args) { int a[] = {10,5,3,12,45,7}; int i=0, found=0; System.out.println(“Enter a number to search in the array:”); Scanner s = new Scanner(System.in); int num = s.nextInt(); while( i < a.length) { if(num == a[i]) {System.out.println(“Yes, it matched!”); found = 1; break; } i++; } If(found == 0) System.out.println(“Sorry, not found”); } What if you want the position of the element in the array if the number is found?

Two dimensional Arrays • 2-D arrays are defined as an Array of arrays. • An array of ints will have type int[], similarly we can have int[][], which means array of arrays of ints.

Print a matrix using a 2–D array Public static void main(String[] args) { int a[][] = new int[2][2]; for(int i=0; i< a.length;i++) { for(int j=0;j < a.length; j++) { a[i][j] = i; System.out.print(a[i][j]); } System.out.println(“ “); } }