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KOTLIN - DO - WHILE LOOP

The 'do - while' loop is just like the while loop with a little difference.

This loop will execute the code block at least once, because it gets into the loop first, executes the block inside the loop. Then it checks for the conditional part i.e. while(num <= 5 ).

Example :

fun main(){
    var x = 1
    do {
        println("Hello World")
        x = x + 1
    } while (x <= 5)
}

Output :

Hello World
Hello World
Hello World
Hello World
Hello World

Now if you see the output, 'Hello World' is printed 5 times. So, how is it different from 'while' loop.

Let us understand with the below example. Where we will provide a 'while' loop and a 'do - while' loop, with a false condition.

Sounds tough?

Let us simplify it.

Comparison of 'while' loop and 'do - while' loop

At first, let us write the code with 'while' loop.

Example :

fun main(){
    var x = 1
    while (x >= 5) {
        println("Hello World")
        x = x + 1
    }
}

Output :

Now, if you look at the output, 'Hello World' is NOT printed on the screen.

Because, the initial value of 'x' is '1'.

And the condition in the 'while' loop is,

while (x >= 5)

Which says enter the loop if the value of 'x' is greater than or equal to '5'. And in this case the value of 'x' is '1'. So, it won't enter the loop.

You can consider the conditional part of the 'while' loop as a security guard. Who will only let you enter, if the condition is satisfied.

And in this case the security guard will not let you enter, because the value of 'x' is '1'.And the condition says, only let them for whom the value of 'x' is greater than(or equal to) '5'.

Fair enough.

Now, let us write the same example using 'do - while' loop.

Example :

fun main(){
    var x = 1
    do {
        println("Hello World")
        x = x + 1
    } while (x >= 5)
}

Output :

Hello World

Now, if you look at the output, 'Hello World' is printed one time on the screen.

This is because of the 'do - while' loop.

    
do {
    println("Hello World")
    x = x + 1
} while (x >= 5)

In 'do - while' loop, the conditional part is at the end of the loop.

Once again, let us consider the conditional part of the 'do - while' loop as a security guard. Who will only let you enter, if the condition is satisfied.

But the security guard is not at the entry point. He is at the exit point of the loop.

Now, since there is no restriction at the entry point, you enter the 'do - while' loop.

    
do {
--> println("Hello World")
    x = x + 1
} while (x >= 5)

And 'Hello World' is printed.

    
do {
    println("Hello World")
--> x = x + 1
} while (x >= 5)

Then the value of 'x' is incremented. And you come to the end of the loop.

    
    do {
        println("Hello World")
        x = x + 1
--> } while (x >= 5)

And this is where the security catches you. He finds out the value of 'x' is '2', which is less than '2'.

So the condition is not satisfied. And he puts you out of the loop.

So, in simple words, since there is no restriction at the beginning of the 'do - while' loop,no matter what the condition is, the control gets into the 'do - while' loop at least once.

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