Using recursion to find a character in a string

Question

I am trying to find the first occurrence of a letter in a string. For example, p in apple should return 1. Here is what I have:

// Returns the index of the          


        

Answer 1

first of all : Recursion has two pillars, Base Case and General Case.

Base Case (the termination point) is the one where Recursion terminates and General Case as the name implies is where the program keeps executing until it finds Base Case

you may try this example, where count is a global static variable

public static int indexOf(char ch, String str)
{
  // Returns the index of the of the character ch
  if (str == null || str.Equals(""))     //Base Case
  {
     if (count != 0)
     {
        if(str.Length == 0)
           return -1;  
        return count;
     }
     else
        return -1;          
  }
  else if (ch == str.CharAt(0))          //Base Case  
     return 1 + count; 
  count++;
  return indexOf(ch, str.Substring(1));  //General Case
}

Answer 2

Why not doing it straight forward?

public static void main(String[] args) {
    String str = "abcdef";
    for (int idx = 0; idx < str.length(); idx++) {
        System.out.printf("Expected %d, found %d\n", idx, indexOf(str.charAt(idx), str, 0));
    }
    System.out.printf("Expected -1, found %d\n", indexOf(str.charAt(0), null, 0));
}

public static int indexOf(char ch, String str, int index) {
    if (str == null || index >= str.length()) return -1;
    return str.charAt(index) == ch ? index : indexOf(ch, str, ++index);
}

OUTPUT:

Expected 0, found 0
Expected 1, found 1
Expected 2, found 2
Expected 3, found 3
Expected 4, found 4
Expected 5, found 5
Expected -1, found -1

Answer 3

Here's another variation. Instead of calling substring you could modify the function a bit to pass the next index to check. Notice that the recursion is initiated with index 0. (You could actually start on any index. There is also some error checking in case the letter isn't found. Looking for x in apple will return -1.)

public static void main(String []args){  
    System.out.println("Index: " + indexOf('e', "apple", 0));
    System.out.println("Index: " + indexOf('x', "apple", 0));
    System.out.println("Index: " + indexOf('p', "Mississippi", 3));
    System.out.println("Index: " + indexOf('p', "Mississippi", 908));
}

public static int indexOf(char ch, String str, int idx) {
    // check for garbage data and incorrect indices
    if (str == null || str.equals("") || idx > str.length()-1) 
        return -1;

    // check to see if we meet our condition
    if (ch == str.charAt(idx)) 
        return idx;

    // we don't match so we recurse to check the next character
    return indexOf(ch, str, idx+1);
}

Output:

Index: 4
Index: -1
Index: 8
Index: -1

Answer 4

Your attempt was good, but not quite there. Here is a correct implementation based off yours:

public static int indexOf(char ch, String str) {
    // Returns the index of the of the character ch

    if (str == null || str.equals("")) {
        // base case: no more string to search; return -1
        return -1;
    } else if (ch == str.charAt(0)) {
        // base case: ch is at the beginning of str; return 0
        return 0; 
    }

    // recursive step
    int subIndex = indexOf(ch, str.substring(1));

    return subIndex == -1 ? -1 : 1 + subIndex;
}

There were two problems with your attempt:

In the else if part, you had found the character, so the right thing to do was stop the recursion, but you were continuing it.

In your last return statement, you needed to be adding 1 to the recursive call (if the character was eventually found), as a way of accumulating the total index number.

Answer 5

Well if we must use recursion then try this:

class RecursiveFirstIndexOf {

public static void main(String[] args) {
    System.out.println(indexOf('p', "apple", 0));
}

static int indexOf(char c, String str, int currentIdx) {

    if (str == null || str.trim().isEmpty()) {
        return -1;
    }

    return str.charAt(0) == c ? currentIdx : indexOf(c, str.substring(1), ++currentIdx);

}}

Answer 6

I'll give you some hints:

1. When you've found the letter, you don't need to recurse further. Additionally, think about what you should be returning in this case.
2. When do you recurse, also think about what the function should return.
3. Is there anything special you need to do if the recursive call returns -1?