Loops are a way to repeat a block of code. It is used to remove repetitive code and to make code more readable. It is a way to write iterative code.

There are three types of loops in Java.

• For
• While
• Do-While

For Loops

For loops are used to iterate over a range of numbers. In this loop, the range is defined by the for keyword. It needs a variable to store the current value and a condition to check if the loop should continue or not and a block of code to be executed for each value in the range of numbers (from the first to the last).

Semantics

    // syntax

    for (initialization; condition; update){
        // block of code
    }

FlowChart

Ok so let's see how to use for loops in writing a program.

    //program to find sum upto n numbers
    public class <ClassName> {
        public static void main(String[] args) {
            int sum = 0;
            for (int i = 0; i < 10; i++) {
                sum += i;
            }
            System.out.println("Sum of numbers upto 10 is " + sum);
        }
    }

so let's analyze the code.

• we have initialized the variable sum to 0.

• then we used for loop:

  • In the initialization, we have initialized the variable i to 0(int i=0;). This step is completely optional. You can also do the initialization before the loop. In that that would be look like:

      int sum = 0;
      int i = 0;
      for (; i < 10; i++) {
          ....

  or something like that.

      int i;
      for (i = 0; i < 10; i++) {

  • In the condition, it checks if i is less than 10. The condition checks if the loop should continue or not and this is not optional, you have to provide condition. In this case we have given a sigle condition i < 10. we can also have multiple conditions like i < 10 && i > 0. for multiple conditions, we need to use logical operator to combine them.

        int sum = 0;
        for(int i=0; i<10 && i%2==0; i++) {

  • In the update, it updates the value of i by 1. This is optional too. you can have the update like: i++, i--, i+=2, i-=2 etc.

        int sum = 0;
        for(int i=0; i<10;) {
            sum += i;
            i++;
        }
  

• Okay so these are the variation of for loops. let's understand how this for loop works. So i=0;, i is assigned to 0. now it checks is i(0) is less than 10, the answer is yes(true). if condition comes true, it executes the block of code. so inside the body, sum += i;, expression is evaluated. sum is assigned to 0 as sum = sum + i(0). so sum is assigned to 0. This is the first iteration.

• Now i get updated to 1. now it checks if i is less than 10, the answer is yes(true). if condition comes true, it executes the block of code. so inside the body, sum += i;, expression is evaluated. sum is assigned to 0 as sum = sum + i(1). so sum is assigned to 1. This is the second iteration.

• This iteration continues until i is less than 10. As i reaches 10, the condition becomes false and the loop stops.

• So the final value of sum is 10, which will be printed on the console.

And that's all. You can see that the for loop is very simple and easy to understand. You can practise some questions based on for loops in the Assignment Section.

While Loops

While Loops are used to iterate over a range of numbers. In this loop, the range is defined by the while keyword. It needs a condition to check if the loop should continue or not and a block of code to be executed for each value in the range of numbers (from the first to the last).

Semantics

    // syntax
    while(condition) {
        // block of code
    }

FlowChart

Let's see how to use while loops in writing a program.

    // Program to demonstrate while loop
    public class <ClassName> {
        public static void main(String[] args) {
            int i = 0;
            while (i < 10) {
                System.out.println("i is " + i);
                i++;
            }
        }
    }

let's analyze the code.

• In line six, we have used while to iterate over the range of numbers and we have put condition n > 0 because we want to iterate over the numbers from the first to the last.
• Inside the loop we have used n-- which is another way of writing n = n - 1, we're decrementing the value of n by 1 each time till it reaches zero and it reaches zero, condition will be false and the loop will break.
• Lastly we print the value of sum to the screen.

I think that's easy to understand. So let's move to our next loop.

Do-While Loops

Do-While loops are used to iterate over a range of numbers. In this loop, the range is defined by the do-while keyword. It needs a condition to check if the loop should continue or not and a block of code to be executed for each value in the range of numbers (from the first to the last). The main difference between do-while and while loop is that the do-while loop is iterates atleast once whether the condition is true or not it will execute the block of code atleast once which is not true in case of while loop.

While loop firstly checks the condition and then it works according to the condition but Do-while loop first execute the code inside the body of the loop then checks the condition.

Semantics

    // syntax
    do {
        // block of code
    } while(condition);

FlowChart

Let's see how to apply do-while loop in writing a program.

    // Program to demonstrate do-while loop
    public class <ClassName> {
        public static void main(String[] args) {
            int i = 0;
            do {
                System.out.println("i is " + i);
                i++;
            } while (i < 10);
        }
    }

Lets analyze the code.

• we have used do keyword to execute the block of code firstly. firstly this loop will start executing the block of code and print the value of i to the console.
• after that we're incrementing the value of i by 1 and then we check the condition i < 10 and if the condition is true then we execute the loop again and print the value of i to the console.
• and if the condition is false then we break the loop.
• so the conclusion is that we first, atleast once execute the block of code and then we'll check the condition and according to the condition we'll work further.

There may be a question when to use which loop?

Okay so it depends on the situation. It completey depends on the problem you are solving.

• for and while - you can use both loop as per your requirement, but there is one thing which recommended by all is you should use while loop when you don't know how many times you have to iterate and use a for loop when you know how many times you have to iterate. suppose you have to print numbers from 1 to 10 then you know how many times you have to run the loop so use for loop. And if you have given that keep taking input from a user till user doesn't press x, here you don't know how many times you have to iterate so use while loop.

• while and do-while - so in situations like, when you atleast once want to run the loop, irrespective of the condition, then you should use do-while loop and rest in all other cases either use while loop or for loop.