Printing integers in the ascending order recursively

Question

Today, I have taken an exam, and there was a question:

Write a method which prints integer numbers in the ascending order recursively from 1 to n:

public class PrintIntegersAscendingOrder {

    static int counter = 0;
    public static void PrintIntegersAscendingOrder (int n)
    {
        if (n == 1)
        {

            System.out.printf("%d\n", ++counter);
        }
        else
        {
            System.out.printf("%d ", ++counter);
            PrintIntegersAscendingOrder(n-1);
        }

    }
    public static void main (String args[])
    {
        PrintIntegersAscendingOrder(5);
    }
}

Although this method worked now, the initial question didn't ask for the class definition, but the method. There, I couldn't be able to fit counter (I have written counter inside the if on the paper, but it gives an error in the program). How can I write the method precisely and correctly without counter variable?

Answer 1

You can do it like this:

public class IntegerAscendingOrder {
    public static void main(String[] args) throws Exception {
        printIntegersAscendingOrder(n);
    }

    private static void printIntegersAscendingOrder(int i) {
        if (i < 1) {
            return;
        }

        printIntegersAscendingOrder(i-1);
        System.out.println(i);
    }
}

You don't need the counter variable in the class, using recursion you can limit the method call within the method itself.

Notice the if (i < 1) {return;} line, this will terminate the recursive method call(s).

This article should help you Getting started with recursion

Answer 2

Do it as follows:

public class Main {
    public static void printIntegersAscendingOrder(int n) {
        if (n == 0) {
            return;
        }
        printIntegersAscendingOrder(n - 1);
        System.out.printf("%d ", n);
    }

    public static void main(String args[]) {
        printIntegersAscendingOrder(5);
    }
}

Output:

1 2 3 4 5 

As @RobOhRob has already pointed out, the counter defeats the purpose of recursion in your code. When you are calling a function recursively, you need to analyse three important things:

1. When to stop the recursive call
2. Processing before making the recursive call
3. Processing before making the recursive call

Since you are already decreasing the parameter by 1 and passing it to the method to call it recursively, you can simply make use of this parameter instead of creating an additional variable (eg counter ).

Answer 3

In your code you have defined in your class a method PrintIntegersAscendingOrder having the same name of the class PrintIntegersAscendingOrder containing it. This is an error that can be avoided for example renaming the including class to PrintIntegers . Below the code of class without the error and with the recursive method:

public class PrintIntegers {
    public static void PrintIntegersAscendingOrder(int n) {
        if (n > 0) {
            PrintIntegersAscendingOrder(n - 1);
            System.out.printf("%d\n", n);   
        }
    }
    public static void main (String args[]) {
        PrintIntegersAscendingOrder(5);
    }
}