How to fix my simple Java recursion method?

Question

I am trying to write a simple recursion program that will print out the canonical sum of all of the integers preceding the input and the input itself. For example, entering 5 should print out "1 + 2 + 3 + 4 + 5". The input must be greater than zero. A bump in the right direction would be appreciated.

import java.util.Scanner;

public class Quiz10 
{
    public static void main (String[] args)
    {
        int input;
        System.out.println("Please enter an integer greater than one: ");
        Scanner scan = new Scanner(System.in);
        input = scan.nextInt();
        sumReverse(input);
    }
    public static void sumReverse(int n)
    {
        int x = n;

        if(x == 1)
            System.out.print(x);
        else if(x > 0)
        {
            System.out.print(x + " + " + (x-1));
        }
        x--;
        sumReverse(x);
    }
}

Edit: with an input of 5 I am currently getting: "5 + 44 + 33 + 22 + 11Exception in thread "main" java.lang.StackOverflowError..."

I'm assuming you're using '5' as input, yes? What output are you getting? — Refugnic Eternium
– Refugnic Eternium, Commented Mar 31, 2013 at 8:31
With an input of 5 I get "5 + 44 + 33 + 22 + 11Exception in thread "main" java.lang.StackOverflowError" — Johnny
– Johnny, Commented Mar 31, 2013 at 8:33
Your recursive call is inconditional and that leads to a loop. — PeterMmm
– PeterMmm, Commented Mar 31, 2013 at 8:33

Refugnic Eternium · Accepted Answer · 2013-03-31 08:34:28Z

1

You're missing the termination condition. Try this:

public static void sumReverse(int n)
{
    if(n == 1) {
        System.out.print(n);
        return;
    }
    else if(n > 0)
    {
        System.out.print(n + " + " + (n-1));
    } else return;
    sumReverse(--n);
}

This function will stop once n hits 1 or if 1 is lower or equal than zero.

An alternative would be:

public static void sumReverse(int n)
{
    if(n == 1) System.out.print(n);
    else if(n > 0)
    {
        System.out.print(n + " + " + (n-1));
        sumReverse(--n);
    }        
}

This has the same effect.

answered Mar 31, 2013 at 8:34

Refugnic Eternium

4,3651 gold badge17 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Refugnic Eternium Over a year ago

No, see the second function for an alternative. Also, you may want to remove the (n - 1), because it will lead to the 5 + 44 effect.

Steph · Accepted Answer · 2013-03-31 08:34:37Z

0

You need to recurse only in the good case. Here you recurse everytime so you get an infinite loop.

    else if(x > 0)
    {
        System.out.print(x + " + ");
        x--;
        sumReverse(x);
    }

Notice I also deleted the + (x - 1) as it will be printed in the next recursion.

answered Mar 31, 2013 at 8:34

Steph

2,06915 silver badges21 bronze badges

2 Comments

Steph Over a year ago

By the way, is recursion a requirement? If not, a simple for (int x = n; x > 0; x--) would do the trick in a cleaner way

Johnny Over a year ago

Yeah, recursion was a requirement or I would have just used a loop.

Denys Séguret · Accepted Answer · 2013-03-31 08:34:18Z

0

As you don't accumulate the sum, you can't compute it. The best would be to return it : to define a function summing until n and call it recursively.

I'll give it to you in pseudo-code as I understand you'd prefer to learn rather than to have your work done :

function sum(int n) -> int {
     if x==1 : return 1
     else : return n + sum(n-1)
}

Then you would print sum(n).

answered Mar 31, 2013 at 8:34

Denys Séguret

384k90 gold badges813 silver badges780 bronze badges

Comments

Jayyrus · Accepted Answer · 2013-03-31 08:35:48Z

0

public static void sumReverse(int n){
    if(n==0)
        return;

    if(n == 1){
        System.out.print(n);
    }else if(n > 0)
    {
        System.out.print(n + " + ");
        sumReverse(n-1);
    }

}

answered Mar 31, 2013 at 8:35

Jayyrus

13.1k41 gold badges136 silver badges215 bronze badges

Comments

Maroun · Accepted Answer · 2013-03-31 08:36:00Z

0

You are not stopping the recursion.
You should add return after the recursion stop condition.
Another thing, remove the + (n-1) from the print in the else and you should be fine.
Plus, it's redundant to assign n to x. You can work directly on n.

Try to draw the recursion calls on paper and you'll better understand how it works.

public static void sumReverse(int n)
{
     if(n == 1) {
         System.out.print(n);
         return;
     }
     else if(n > 0)
     {
         System.out.print(n + " + ");
     }
     sumReverse(n-1);
}

answered Mar 31, 2013 at 8:36

Maroun

96.3k30 gold badges195 silver badges249 bronze badges

Comments

prashant · Accepted Answer · 2013-03-31 08:47:21Z

Would like to point out a few things: 1. This problem is not a usual candidate for recursion. 2. Your best bet if you intend to use recursion is to hold the return values as String and print finally in your main method as below. 3. Declaring a new variable x is really unnecessary.

public static void main (String[] args)
    {
        int input;
        System.out.println("Please enter an integer greater than one: ");
        Scanner scan = new Scanner(System.in);
        input = scan.nextInt();
        System.out.println(sumReverse(input));
    }
    public static String sumReverse(int n)
    {
        String a = "";
        if(n > 1)
        {
            return (n + "+" + sumReverse(n-1));
        }
        return "1";
    }

Alya'a Gamal · Accepted Answer · 2013-03-31 08:47:30Z

0

public class Main {

    public static String s = "";

    public static void main(String[] args) {

        int input;
        System.out.println("Please enter an integer greater than one: ");
        Scanner scan = new Scanner(System.in);
        input = scan.nextInt();
        String b=sumReverse(input);
        System.out.println(b);
    }

    public static String sumReverse(int n) {
        int x = n;

         if (x == 1) {
          s = "1" + s;
          return s;
        } else if (x > 0) {
           s ="+"+  Integer.toString(x) +s ;
           sumReverse(x - 1);
    }
        return s;

    }
}

The Output:

Please enter an integer greater than one: 
10 
1+2+3+4+5+6+7+8+9+10

edited Mar 31, 2013 at 8:47

answered Mar 31, 2013 at 8:38

Alya'a Gamal

5,64823 silver badges34 bronze badges

Comments

gkiko · Accepted Answer · 2013-03-31 08:49:08Z

0

public static void sum(int n){
  sumReverse(n-1);
  System.out.print(n);
}

public static int sumReverse(int n){
  if(n==1){
    return n;
  }
  System.out.print(sumReverse(n-1)+"+");
  return n;
}

answered Mar 31, 2013 at 8:49

gkiko

2,2873 gold badges31 silver badges51 bronze badges

Collectives™ on Stack Overflow

How to fix my simple Java recursion method?

8 Answers 8

1 Comment

2 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

1 Comment

2 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related