help with recursion methods

**hoosierfan24** · 11-05-2010, 05:28 PM

Hello for my java programing class i have to do this lab that counts how many vowels are in the word and it can not use loops or *
It has to use recursive methods and i have no idea where to start please help.

**Zack** · 11-05-2010, 06:05 PM

Well... I'd probably have a method that finds the index of the next vowel. If it's greater than or equal to zero, then increment a counter which is returned from each method. Otherwise, return the existing count and stop recursing. Something like this:

Java Code:

String x, "sequoias"
int count = find_vowels(x, 0, 0)

public int find_vowels(str, index, count)
    // we need our original string, a starting index to find the NEXT vowel, and a maintained count
    store nextA, nextE, nextI, nextO, and nextU based on their indices from indexOf
    find lowest value of nextA, nextE, nextI, nextO, and nextU
    if (all indices are < 0) return count
    return find_vowels(str,index+1,count+1) // increment search index and total count

PS: Just put this together with actual code and the concept works.

**hoosierfan24** · 11-06-2010, 04:01 PM

ok that kind of makes sense but the framework required to do this is

Java Code:

System.out.println( countVowels( "coconut" ) ); 

public static int countVowels( String s )
{



}

**hoosierfan24** · 11-06-2010, 04:08 PM

i know how to calculate the vowels in a for loop but i cant understand how to do it with recursion here is how i would do it in a loop

Java Code:

import java.lang.String;
import java.io.*;
import java.util.*;

public class CountVowels{

public static void main(String args[])throws IOException{
  BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
  System.out.println("Enter the String:");
  String text = bf.readLine();
  int count = 0; 
    for (int i = 0; i < text.length(); i++) {
        char c = text.charAt(i);
        if (c=='a' || c=='e' || c=='i' || c=='o' || c=='u') {
        count++;
        }
    }
    System.out.println("There are" + " " + count + " " + "vowels");
}
}

**hoosierfan24** · 11-06-2010, 04:39 PM

i figured it out and used this as my code

Java Code:

  public static int countVowels( String s )
  {
    if ( s.length() == 0 )
      return 0;

    if ( "aeiouAEIOU".indexOf( s.substring( 0, 1 ) ) < 0 )
      return countVowels( s.substring( 1 ) );
    else
      return 1 + countVowels( s.substring( 1 ) );
  }

**JosAH** · 11-06-2010, 05:19 PM

Originally Posted by hoosierfan24

i figured it out and used this as my code

Java Code:

  public static int countVowels( String s )
  {
    if ( s.length() == 0 )
      return 0;

    if ( "aeiouAEIOU".indexOf( s.substring( 0, 1 ) ) < 0 )
      return countVowels( s.substring( 1 ) );
    else
      return 1 + countVowels( s.substring( 1 ) );
  }

Well done but change that first substring(0, 1) to charAt(0) (I know, it's a minor nitpick but I am like that ;-)

kind regards,

Jos

**mophiejoe** · 11-16-2012, 04:35 AM

why did you put ( on line 6)
< 0??

and why do you have to have ( on line 7)
s.substring( 1 )

I am just trying to learn, and dont understand.

thank you!

Originally Posted by hoosierfan24

i figured it out and used this as my code

Java Code:

  public static int countVowels( String s )
  {
    if ( s.length() == 0 )
      return 0;

    if ( "aeiouAEIOU".indexOf( s.substring( 0, 1 ) ) < 0 )
      return countVowels( s.substring( 1 ) );
    else
      return 1 + countVowels( s.substring( 1 ) );
  }