Java Basics - Lecture 3 - Loops in Java

Loops in Java

Loops are used to perform a task repeatedly. There are 3 types of loops

1. while loop
2. for loop
3. do-while loop

Suppose if you want to print first 5 digits

You need to write 5 print statements

System.out.println(1);
System.out.println(2);
System.out.println(3);
System.out.println(4);
System.out.println(5);

Instead of writing repeated code, we use loops

While loop :

A control flow statement which is used to perform the repeated tasks based on a given Boolean condition. In while loop number of iterations is not fixed. Loop will be executed only if the condition is true. It is also called the entry control loop.

Flow Chart Diagram

Disadvantage: It generates an infinite loop if we don't give an incremental part

For Loop:

A control flow statement which is used to perform repeated tasks. But in for loop number of iterations are fixed.

Flow chart Diagram

Parts of for loop (Initialization, Conditional part, Incremental part)

Initialization: In this part, the variable is initialized. It is a starting point of the loop

Condition: In this part condition is checked, it returns a boolean. In the case of the true, the loop body is executed.

Increment/Decrement: Variable is incremented or decremented here for next iteration

Loop termination: When the condition becomes false, then the loop is getting terminated.

For Loop Example

public class ForLoop {
public static void main(String[] args)  {
  for(int j=1;j<=10;j++){
  System.out.println("Values of ==> " + j);
  }
 }
}

Output:

value of j ==> 1
value of j ==> 2
value of j ==> 3
value of j ==> 4
value of j ==> 5
value of j ==> 6
value of j ==> 7
value of j ==> 8
value of j ==> 9
value of j ==> 10

If you want to print 10 digits in reverse order then below code will work fine.

public class ForLoop{
 public static void main(String[] args){
  for(int k=1;k<=10;j--){
  System.out.println("Values of ==> " + k);
    }
  }
}

Post Increment

An operator that is used to increment the value of the variable after the expression is executed in which post-increment is used. The value is first used in an expression and then incremented.

public class PostIncrement{
 public static void main(String[] args){
 int i = 1;
 int j = i++; //post increment

   }
}

Pre Increment

An operator that is used to increment the value of the variable before the expression is executed in which post-increment is used. The value is first incremented in an expression and then used.  

public class PreIncrement{
 public static void main(String[] args){
 int a = 1;
 int b = ++a; //pre-increment
   } 
}

Post Decrement

An operator that is used to decrement the value of the variable before the expression is executed in which post-increment is used. The value is first decremented in an expression and then used.  

public class PostDecrement{
 public static void main(String[] args){
 int i = 2;
 int j = i--; //post decrement
   }
}

Pre Decrement

An operator that is used to decrements the value of the variable before the expression is executed in which post decrement is used. The value is first decremented in an expression and then used.  

public class PreDecrement{
 public static void main(String[] args){
 int a = 2;
 int b = --a; //pre-decrement
 System.out.println("Value of a ==> "+a);
 System.out.println("Value of b ==> "+b); 
  }
}

do-while loop :

The do-While loop is similar to while loop but the difference is that the body of the do-while loop is executed once and then the condition is verified. A control flow statement that executes a piece of code at least once.

Flow Chart Diagram

If the test expression is true, the body of the loop is executed again and the test expression is evaluated.

The first loop body is executed then the condition is checked, this process continues until the condition is false.

This process goes on until the test expression becomes false.

If the test expression is false, the loop ends.

public class DoWhileLoop{
 public static void main(String[] args){
 int counter = 5;
 int factorial = 1;
 do{
    factorial *= counter--;
 }while (counter > 0);
 System.out.println("result " +factorial);
  }
}

Output:

 result ==> 120

Array in Java

Array: Stores similar data type values in an array variable.

1. A collection of similar data types
2. The lowest index is always “0”
3. The highest index is always n-1 (n is the size of the array)

Int array
Integer array stores the integer data type. 

 int arr[] = new int[4];
 arr[0] = 10;
 arr[1] = 20;
 arr[2] = 30;
 arr[3] = 40;
 System.out.println(arr[2]);
 System.out.println(arr[3]);

Output:

value of arr[2] ==> 30
value of arr[3] ==> 40

If you will try to find the arr[4], it will give exception

When you go beyond the limit, an exception will raise “ArrayIndexOutOfBoundsException”

Size of array

The size of the array is determined by using length() method.

System.out.println(arr.length)

Print all the values of the array: use for loop

 for(int i=0; i < arr.length; i++){
 System.out.println(arr[i]);
 }

Double array

Double array stores the double data type. 

double d[] = new double[3];
d[0] = 3;
d[1] = 14.33;
d[2] = 44.89;

Character array
Character array stores the character data type. 

char c[] = new char[3];
c[0] = 'a';
c[1] = '3';
c[2] = '$';

String array

String array stores the string data type. 

String s[] = new String[3];
s[0] = "test";
s[1] = "World";
s[2] = "Hello";

Disadvantages:

1. The array size is fixed. Therefore it is called a static array
2. Stores only similar data types

To overcome size problem we use Collections like ArrayList, HashTable

To overcome data type problem we use Object array

Object array(Object is a class) – It is used to store different data types

Object array

 Object obj[] = new Object[6];
 obj[0] = "Tom";
 obj[1] = 25;
 obj[2] = "M";
 obj[3] = 12.33;
 obj[4] = "1/10/1998";
 obj[5] = "London";

Find all elements of Object array

for(int i=0; i<obj.length; i++) {
 System.out.println(obj[i]);
}

Output

value of obj[0] ==> Tom
value of obj[1] ==> 25
value of obj[2] ==> M
value of obj[3] ==> 12.33
value of obj[4] ==> 1/10/1998
value of obj[5] ==> London

2D Array in Java

Collection of data in the form of cells

Represented as a matrix with a number of rows and columns

String x[][] = new String[3][5];

Output:

System.out.println(x.length);
Number of row ==> 3
System.out.println(x[0].length);
Number of columns ==> 5

//1st row

x[0][0] = "A";
x[0][1] = "B";
x[0][2] = "C";
x[0][3] = "D";
x[0][4] = "E";

//2nd row

x[1][0] = "A1";
x[1][1] = "B1";
x[1][2] = "C1";
x[1][3] = "D1";
x[1][4] = "E1";

//3rd row

x[2][0] = "A2";
x[2][1] = "B2";
x[2][2] = "C2";
x[2][3] = "D2";
x[2][4] = "E2";

Output

System.out.println(x[1][2]);
value of x[1][2] ==> C1</
System.out.println(x[2][2]);
value of x[1][2] ==> C2
System.out.println(x[0][4]);
value of x[1][2] ==> E

Print all the values of 2D array

Two for loops are required to get array values

//row = 0, col = 0 to 4

//row = 1, col = 0 to 4

//row = 2, col = 0 to 4

for(int i = 0; i < x.length; i++){
  for(int j = 0; j < x[0].length; j++){
  System.out.println(x[i][j]);
   }
}

Output:

  value of x[0][0] ==> A value of x[0][1] ==> B
  value of x[0][2] ==> C value of x[0][3] ==> D
  value of x[0][4] ==> E

  value of x[1][0] ==> A1 value of x[1][1] ==> B1
  value of x[1][2] ==> C1 value of x[1][3] ==> D1
  value of x[1][4] ==> E1

  value of x[2][0] ==> A2 value of x[2][1] ==> B2
  value of x[2][2] ==> C2 value of x[2][3] ==> D2
  value of x[2][4] ==> E2