Java - counting letter occurrence in a string using IndexOf

Question

public static void countLetters (String s, char x){
    int length = s.length();
    int count  = 0;
    int index = 0;
    int z = 0;
    while (index < length) {
        int y = s.indexOf(x, z);
        z = z+1;

        if(s.charAt(y)==x){
            count = count + 1;
        }
        index = index + 1;
}
    System.out.println(count);

Hello everyone, I am a newbie with java so any little guidance would be greatly appreciated. I am trying to write a method to count letter occurrence in a given string and a char.

The current method does count but not quite there. I am asked to use indexOf method to solve this problem. Problem i am having is if I put a text "amazing" and asked to find char 'a' it goes through first 'a' and count updates but then when it comes to char 'm' count again updates which I don't want it.

I am using a book called think java by Allen Downey - Exercise 8.3. Its not any homework :).

Possible duplicate of [Simple way to count character occurrences in a string](http://stackoverflow.com/questions/6100712/simple-way-to-count-character-occurrences-in-a-string) — Unknown, Aug 10 '16 at 10:24
Possible duplicate of [Java: How do I count the number of occurrences of a char in a String?](http://stackoverflow.com/questions/275944/java-how-do-i-count-the-number-of-occurrences-of-a-char-in-a-string) — Aleksei Egorov, Aug 10 '16 at 10:34
@MohammadRezaKhatami It seems that the question states it need to use indexOf not charAt or any other form. — Smit, Aug 10 '16 at 10:43

Akintayo Jabar · Accepted Answer · 2016-08-11T08:30:32.383

You should use this :

    int length = s.length();
    int count  = 0;
    int z = 0;
    while (z < length) {
        int y = s.indexOf(x, z);
        if(y == -1){
            break;
        }
        z = (y == z) ? z + 1 : y + 1;

        if (s.charAt(y) == x) {
            count = count + 1;
        }
    }
    System.out.print(count);

basically when you use the s.indexOf(chr, fromIndex) it checks for the char from the specified index until it locates it that's why when it is at m it still checks till it finds a in the string. Also you should check for a -1 if the string is not found and break as that will be the end of the search since the string is not found from the index to the end of the string. This prevents indexOfBoundsException when doing the check if(s.charAt(y) == x)

The expression z = (y == z) ? z + 1 : y + 1; is more like an if else statement. the ? is a ternary operator and is a short form of:

if(y == z){
   z = z + 1;
}else{
  z = y + 1;
}

since it's a simple if an else statement that sets the value of a variable with an expression the ternary operator could be used to save lines of code.

Sohlowmawn, can you explain "z = (y == z) ? z + 1 : y + 1;" unfamiliar with that code. Thank you in advance for the answer and the explanation. — Daniel, Aug 10 '16 at 22:37

score 0 · Answer 2 · answered Aug 10 '16 at 10:40

This way

public static void countLetters(String string, char character) {
        int count = 0;
        int index = 0;
        while(true) {
            index = string.indexOf(character, index) + 1; // +1 is to continue search 
            if (index == 0)                               //from next character after match
                break;
            count++;
        }
        System.out.println(count);
    }

Calling it with countLetters("amazingaa", 'a'); gives 4

noscreenname · Answer 3 · 2016-08-10T11:07:12.607

You are iterating over the string one char at a time: z = z+1, but when you call s.indexOf(x, z), first occurrence of x (greater than z) is returned. So you iteration is going to go like following :

z=0, y=0 (index of first a in "amazing")
z=1, y=2 (index of second a in "amazing")
z=2, y=2 (index of second a in "amazing" again)

So instead of z = z+1, you should be doing z = y+1, and stop iterating once s.indexOf(x, z) return -1.

Java - counting letter occurrence in a string using IndexOf

3 Answers3