“Introduction to Programming With Java”

“Introduction to Programming With Java” Lecture - 6 UMESHPATIL (umesh@cse.iitb.ac.in) nlp-ai@cse.iitb

Contents for Today’s Lecture • ‘switch’ construct • Introduction to Arrays nlp-ai@cse.iitb

‘if-else’ Program class print_num { public static void main(String args[]) { int a_number; a_number = 0; if(a_number = = 1) System.out.println("One"); else { if(a_number = = 2) System.out.println("Two"); else { if(a_number = = 3) System.out.println("Three"); else System.out.println("No. is out of the range"); } } }} nlp-ai@cse.iitb

‘switch’ Program class print_num_switch { public static void main(String args[]) { int a_number; a_number = 3; switch(a_number){ case 1:System.out.println("One"); break; case 2:System.out.println("Two"); break; case 3:System.out.println("Three"); break; default:System.out.println("No. is out of the range"); } }} nlp-ai@cse.iitb

Syntax switch(expression){ case value1:statements; break; case value2:statements; break; . . case valueN:statements; break; default:statements; } nlp-ai@cse.iitb

Note • The casevalues can be compared only for equality with the switchexpression • The expression must be of type int, char, short, byte • Each casevalue must be a constant, not a variable • Duplicate casevalues are not allowed • The default part is optional nlp-ai@cse.iitb

Arrays • Definition: • An array is a group/collection of variables of the same type that are referred to by a common name and an index • Examples: • Collection of numbers • Collection of names • Collection of suffixes nlp-ai@cse.iitb

Examples Array of numbers: Array of names: Array of suffixes: nlp-ai@cse.iitb

Analogy Array is like a pen box with fixed no. of slots of same size. nlp-ai@cse.iitb

Syntax Declaration of array variable: data-type variable-name[]; eg. int marks[];This will declare an array named ‘marks’ of type ‘int’. But no memory is allocated to the array.Allocation of memory:variable-name = new data-type[size]; eg. marks = new int[5];This will allocate memory of 5 integers to the array ‘marks’ and it can store upto 5 integers in it. ‘new’ is a special operator that allocates memory. nlp-ai@cse.iitb

Syntax… Accessing elements in the array: Specific element in the array is accessed by specifying name of the array followed the index of the element. All array indexes in Java start at zero.variable-name[index] = value; eg. marks[0] = 10;This will assign the value 10 to the 1st element in the array. Andmarks[2] = 863;This will assign the value 863 to the 3rd element in the array. nlp-ai@cse.iitb

Example STEP 1 : (Declaration)int marks[];marks  null STEP 2: (Memory Allocation)marks = new int[5]; marks marks[0] marks[1] marks[2] marks[3] marks[4] STEP 3: (Accessing Elements)marks[0] = 10; marks  marks[0] marks[1] marks[2] marks[3] marks[4] nlp-ai@cse.iitb

Program class try_array { public static void main(String args[]) { int marks[]; marks = new int[3]; marks[0] = 10; marks[1] = 35; marks[2] = 84; System.out.println(“Marks obtained by 2nd student=” + marks[1]); } } nlp-ai@cse.iitb

Note • Arrays can store elements of the same data type. Hence an int array CAN NOT store an element which is not an int. Though an element of a compatible type can be converted to int and stored into the int array. eg. marks[2] = (int) 22.5;This will convert ‘22.5’ into the int part ‘22’ and store it into the 3rd place in the int array ‘marks’. • Array indexes start from zero. Hence ‘marks[index]’ refers to the (index+1)th element in the array and ‘marks[size-1] refers to last element in the array.eg. marks[0] refers to 1st element, marks[1] refers to 2nd element… etc. etc. nlp-ai@cse.iitb

Alternative Syntax Combined declaration & memory allocation:data-type variable-name[] = new data-type[size]; eg. int marks[] = new int[5];This will declare an int array ‘marks’ and will also allocate memory of 5 integers to it. Combined declaration, allocation & assignment:data-type variable-name[] = {comma-separated values}; eg. int marks[] = {10, 35, 84, 23, 5};This will declare an int array ‘marks’, will allocate memory of 5 integers to it and will also assign the values as- marks marks[0] marks[1] marks[2] marks[3] marks[4] nlp-ai@cse.iitb

Assignment • Write a program that uses two arrays of size 5. One string array for storing names of students and one int array for storing marks obtained by the students. Assign values to the elements in the arrays. Print names and marks of students who have scored more than 35 marks. • Write a program same as the above, but don’t use numbers (like 0, 1, 2…etc) to access the elements in the array. Declare an int variable ‘i’ and use it as an index into the arrays. nlp-ai@cse.iitb

End Thank you… nlp-ai@cse.iitb

“Introduction to Programming With Java”