Download java_lect_06 - CSE, IIT Bombay

“Introduction to Programming
With Java”
Lecture - 6
UMESH PATIL ([email protected])
[email protected]
Contents for Today’s Lecture
• ‘switch’ construct
• Introduction to Arrays
[email protected]
‘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");
}
}
}
[email protected]
}
‘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");
}
}
}
[email protected]
Syntax
switch(expression){
case value1:
statements;
break;
case value2:
statements;
break;
.
.
case valueN:
statements;
break;
default:
statements;
}
[email protected]
Note
• The case values can be compared only for equality with the
switch expression
• The expression must be of type int, char, short, byte
• Each case value must be a constant, not a variable
• Duplicate case values are not allowed
• The default part is optional
[email protected]
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
[email protected]
Examples
Array of numbers:
10
23
863
8
229
Array of names:
Sholay
Shaan
Shakti
Array of suffixes:
ment
tion
ness
ves
[email protected]
Analogy
Array is like a pen box with fixed no. of slots of same size.
[email protected]
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.
[email protected]
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.
And
marks[2] = 863;
This will assign the value 863 to the 3rd element in the array.
[email protected]
Example
STEP 1 : (Declaration)
int marks[];
marks  null
STEP 2: (Memory Allocation)
marks = new int[5];
marks 
0
marks[0]
0
0
0
marks[1]
marks[2]
marks[3]
0
marks[4]
STEP 3: (Accessing Elements)
marks[0] = 10;
marks 
10
0
marks[0]
marks[1]
0
marks[2]
0
marks[3]
[email protected]
0
marks[4]
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]);
}
}
[email protected]
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.
[email protected]
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 asmarks 
10
marks[0]
35
84
23
marks[1]
marks[2]
marks[3]
[email protected]
5
marks[4]
Assignment
1. 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.
2. 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.
[email protected]
End
Thank you…
[email protected]