reverse an array in java using recursion

Question

I am just starting to learn recursion and was able to use it write a simple factorial program without much of a problem. Now I am trying to write a recursive method that writes an array in reverse order but I can't figure out what I'm doing wrong. What am I missing? Thank you.

import java.io.*;

public class Recursion {
  public static void main(String[] args) throws IOException{
    int myArray[] = {1,2,3,4,5,6,7,8,9,10};
  }

  public static void reverseDisplay(int[] ary, int position){
    if(position > 0)
      System.out.print(ary[position]);
     reverseDisplay(ary, position - 1);
  }
}

Please clarify exactly what is going wrong, preferably with some examples. — Sam Estep
– Sam Estep, Commented Feb 12, 2016 at 17:47
Maybe System.out.print(ary[position-1]); ? The ending condition if(position > 0) seems to suggest that. — WhatsUp
– WhatsUp, Commented Feb 12, 2016 at 17:47
Well, you never actually call reverseDisplay, so that would be why. Could you try testing the function itself? — Sam Estep
– Sam Estep, Commented Feb 12, 2016 at 17:50

Marcinek · Accepted Answer · 2016-02-12 18:18:55Z

4

You do not call your recursion method.
You have an endless recursion because reverseDisplay() is always being executed due to missing enclosing brackets.
Also your stop condition has to be >= 0 because the first index of an array is 0.

Your method should read:

import java.io.IOException;

public class Recursion {
    public static void main(String[] args) throws IOException{
        int myArray[] = {1,2,3,4,5,6,7,8,9,10};

        reverseDisplay(myArray, myArray.length -1);
    }

    public static void reverseDisplay(int[] ary, int position){
        if(position >= 0) {
            System.out.print(ary[position]);
            reverseDisplay(ary, position - 1);
        }
    }
}

edited Feb 12, 2016 at 18:18

answered Feb 12, 2016 at 17:52

Marcinek

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1 Comment

catdog Over a year ago

Thank you very much. I understand what I did wrong now.

javaNinja · Accepted Answer · 2016-02-12 17:52:08Z

0

When doing recursion you need something called a base case. You need the base case to end the recursion or you will get a stack overflow.

You might try something like this. The return statement stops the recursion from being endless.

public static void reverseDisplay(int[] ary, int position){
     if(int == -1)
         return;
     System.out.print(ary[position]);
     reverseDisplay(ary, position - 1);
  }

answered Feb 12, 2016 at 17:52

javaNinja

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Comments

Martín Muñoz del Río · Accepted Answer · 2016-02-12 17:52:11Z

0

Try:

import java.io.*;

public class Recursion {
  public static void main(String[] args) throws IOException{
    int myArray[] = {1,2,3,4,5,6,7,8,9,10};

    reverseDisplay(myArray,0);
  }

  public static void reverseDisplay(int[] ary, int position){
    if(position == ary.length){
        return;
    }

    reverseDisplay(ary, position + 1);
    System.out.print(ary[position]);
  }
}

If position == ary.length you finish the recursion, else you call reverseDisplay recursively and after that print the current position of ary.

answered Feb 12, 2016 at 17:52

Martín Muñoz del Río

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