Java - for Loop

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Java for Loop

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to be executed a specific number of times.

A for loop is useful when you know how many times a task is to be repeated. Just like the while loop, the for loop is also an entry control loop where the given condition executes first.

Syntax of for Loop

The syntax of a for loop is −

for(initialization; Boolean_expression; update) {
   // Statements
}

Parts of Java For Loop

In Java, the for loop is constructed (implemented) using three parts. The following are the parts of a for loop in Java -

• Initialization - Contains the initialization statement (s) of the loop counter.
• Boolean expression - Contains the condition to be tested.
• Body - Contains the statements to be iterated till the given Boolean expression is true, also to update the loop counter.

Execution Process of a for Loop

Here is the flow of control in a for loop −

• The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables and this step ends with a semi colon (;).

• Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop will not be executed and control jumps to the next statement past the for loop.

• After the body of the for loop gets executed, the control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank with a semicolon at the end.

• The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats (body of loop, then update step, then Boolean expression). After the Boolean expression is false, the for loop terminates.

Flow Diagram

The following diagram shows the flow diagram (execution process) of a for loop in Java -

Java for Loop Examples

Example 1: Printing Numbers in a Range Using for Loop

In this example, we're showing the use of a for loop to print numbers starting from 10 to 19. Here we've initialized an int variable x with a value of 10 within initialization blook of for loop. Then in expression block, we're checking x as less than 20, and in the end under update block, we're incrementing x by 1. Within body of for loop, we're printing the value of x. For loop will run until x becomes 20. Once x is 20, loop will stop execution and program exits.

public class Test {

   public static void main(String args[]) {

      for(int x = 10; x < 20; x = x + 1) {
         System.out.print("value of x : " + x );
         System.out.print("\n");
      }
   }
}

Output

value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19

Example 2: Printing Array Elements Using for Loop

In this example, we're showing the use of a for loop to print contents of an array. Here we're creating an array of integers as numbers and initialized it some values. We've created a variable named index to represent index of the array within for loop, check it against size of the array and incremented it by 1. Within for loop body, we're printing element of the array using index notation. Once index becomes same as array size, for loop exits and program quits.

public class Test {

   public static void main(String args[]) {
      int [] numbers = {10, 20, 30, 40, 50};

      for(int index = 0; index < numbers.length; index++) {
         System.out.print("value of item : " + numbers[index] );
         System.out.print("\n");
      }
   }
}

Output

value of item : 10
value of item : 20
value of item : 30
value of item : 40
value of item : 50

Java Infinite for Loop

An infinite loop never ends unless you stop manually by pressing CTRL + C. To implement an infinite for loop either use such a condition that is always true or directly use true as the condition.

Example: Implementing Infinite for Loop

In this example, we're showing the infinite loop using for loop. It will keep printing the numbers until you press ctrl+c to terminate the program.

public class Test {

   public static void main(String args[]) {
      int x = 10;

      for( ;; ) {
         System.out.print("value of x : " + x );
         x++;
         System.out.print("\n");
      }
   }
}

Output

value of item : 10
value of item : 11
value of item : 12
value of item : 13
value of item : 14
...
ctrl+c

Nested for Loop in Java

A nested for loop is a for loop containing another for loop inside it.

Example: Print Tables from 1 to 10 Using Nested for Loop

In this example, we are printing tables of the numbers from 1 to 10.

public class Main {
  public static void main(String[] args) {
    // Implementing nested for loop
    // Initializing loop counters 
    int num = 1;
    int i = 1;

    // outer for loop
    for (num = 1; num <= 10; num++) {
      //inner for loop
      System.out.print("Table of " + num + " is : ");
      for (i = 1; i <= 10; i++) {
        // printing table
        System.out.print(num * i + " ");
      }
      // printing a new line
      System.out.println();
    }
  }
}

Output

Table of 1 is : 1 2 3 4 5 6 7 8 9 10 
Table of 2 is : 2 4 6 8 10 12 14 16 18 20 
Table of 3 is : 3 6 9 12 15 18 21 24 27 30 
Table of 4 is : 4 8 12 16 20 24 28 32 36 40 
Table of 5 is : 5 10 15 20 25 30 35 40 45 50 
Table of 6 is : 6 12 18 24 30 36 42 48 54 60 
Table of 7 is : 7 14 21 28 35 42 49 56 63 70 
Table of 8 is : 8 16 24 32 40 48 56 64 72 80 
Table of 9 is : 9 18 27 36 45 54 63 72 81 90 
Table of 10 is : 10 20 30 40 50 60 70 80 90 100