JAVA: help fixing recursive function

Question

I have to use recursion for this problem, I managed to make it work using loops pretty quickly but I'm a bit stuck on this. My current code is

public static String ReverseR(String n){
    String finalstring="";
    int i = 0;
    int len = n.length();
    while (i < len) {
        finalstring += (n.charAt(len -  1));
        ReverseR(n.substring(0, len - 1));
        i++;
    }
    return finalstring;
}

When I input any string, the resulting string is the correct length, but only uses the last letter. Ex: ReverseR("Hello") = ooooo Any ideas?

Check out this link for help on recursion: http://stackoverflow.com/questions/19526274/java-help-fixing-recursive-function — James Cronen, Oct 22 '13 at 19:12

score 5 · Answer 1 · answered Oct 22 '13 at 19:12

5

Recursion is kind of like proof by induction.

Get rid of the while-loop
If you're inverting a 0-character string, that's easy: just return ""
if you're inverting a n-character string, invert [0..n-2] and prepend the last letter. Which you're already doing.

answered Oct 22 '13 at 19:12

iluxa

6,941
18
36

2

also "remember that Strings in java aren't mutable, you need to _return_ the modified string" – jtahlborn Oct 22 '13 at 19:14

score 1 · Answer 2 · edited May 23 '17 at 11:57

1

change n.charAt(len - 1)) to n.charAt(len - i))

you are always in the same place with len -1 ;)

[EDIT]

while (i < len){
    finalstring += (n.charAt(len - 1 - i));
    ReverseR(n.substring(0, len - 1 - i));
    i++;
}

this will fix your code, however you have to choose between while or ReverseR(...)

Duplicate question, check this Reversing a String with Recursion in Java

edited May 23 '17 at 11:57

Community

1
1

answered Oct 22 '13 at 19:11

X-Pippes

1,170
7
25

1

while this does solve the proble, it removes the need for the recursion :) (which may be required for the OPs homework). – jtahlborn Oct 22 '13 at 19:12
1

This will throw a `StringIndexOutOfBoundsException` – Alexis C. Oct 22 '13 at 19:15

score 0 · Answer 3 · answered Oct 22 '13 at 19:17

Here is a full working solution

public static String reverse(final String s) {
    if (s == null) {
        throw new NullPointerException();
    }

    final int length = s.length();
    if (length <= 1) {
        return s;
    } else {
        // s = start + lastChar
        final String start = s.substring(0, length - 1); 
        final char lastChar = s.charAt(length - 1);
        return lastChar + reverse(start);
    }
}

score 0 · Answer 4 · answered Oct 22 '13 at 19:39

Your recursive algorithm shouldn't require any loops, i.e. while and for loops. Any looping constructs can essentially be implemented through recursion without ever touching a loop. An example of basic recursive String reversal could be like this:

public static String reverseR(String n){
  if (n.length() <= 1) return n;
  return reverseR(n.substring(1))+n.charAt(0);
}

With this algorithm, you're basically saying: return "the reverse of every letter but the first"+"the first letter"

A good thing to help with writing recursive algorithms is making a lot of assumptions. Assume first that your reverse function works and then place it within itself wherever you want to reverse a portion of your string. Just remember to add a base case and you'll be golden. Languages like Haskell or Prolog will get you used to recursive algorithms since that is their main source of iteration.

duplicate code from here http://stackoverflow.com/questions/9723912/reversing-a-string-recursion-java — X-Pippes, Oct 22 '13 at 19:41
Ah, good find. Looks like this is a pretty common way of doing this! — user2466999, Oct 22 '13 at 19:52

JAVA: help fixing recursive function

4 Answers4