递归方法的输出错误

Question

    public static int getIndexOf(char ch, String str) {
    if (str == null || str.equals("")) {
        return 0;
     //base case
    }else{
        char first = str.charAt(0);
        if (ch != first) {
            return -1;
     //returns -1 when the character cannot be found within the string
        }else{
            int rest = str.length() - 1;
            if (str.charAt(rest) == ch) {
                return rest; 
            }
            return lastIndexOf(ch, str.substring(0, rest));
            //recursive case
        }
    }
}

This my method which returns the index of the input character of the input string. However, when I run it in the interaction plane, it returns wrong number. For example, when I enter 'a' and "peach", it is supposed to return 2, but it returns -1. This method should return -1 only when the character cannot be found in the string. Can anyone tell me how to deal with it? Thank you!

Answer 1

Well why don't you step through the logic and see what happens.

getIndexOf('a', "peach")

Method goes in, string isn't null or empty so it falls through to the next line of code.

char first = str.charAt(0); // this is 'p'

if (ch != first) {  // is 'p' equal to 'a'? no. so return -1
        return -1;

And then the rest of your logic will never execute. Can you see how to fix this problem?

Answer 2

your following portion of code means that it will check for the first character of the string if it is matching otherwise it will return -1.

char first = str.charAt(0);
        if (ch != first) {
            return -1;

this says that if character at 0th index doesn't match then send -1 so as 'p' in "peach" isn't matching with 'a' so it is return -1.

did you get it?

Answer 3

The output's not wrong, the implementation is!

Think in words first. If the desired character is the first character in the string, then the result is zero. Otherwise it's (1 + the index in the string that remains after cutting off the first character). Now code the words:

return (str.charAt(0) == ch) ? 0 : 1 + getIndexOf(ch, str.substring(1));

This doesn't yet handle the case where the character is not in the string at all. Here the charAt(0) call will eventually throw IndexOutOfBoundsException because str doesn't have one!

The cleanest way to handle this case is to catch the exception. So you's have two functions: mustGetIndexOf , which is the recursive relation above, and getIndexOf , which calls the one above inside a try {} catch() {} , returning -1 in that case.

Of course if you don't want to allow the exception, you can test the recursive call's result with if for the special case of -1 . The code is uglier, but it will work. Whenever a -1 is seen, return -1 again. This propagates the -1 all the way back to the caller. The exception "unwinds" the recursive calls on the stack in a similar manner, just with one chop instead of the gradual call-by-call way your if statements will do it.

I won't give you full code so you can continue to learn.