Question

I created this method deleteCharAt in order to remove a char from a string by its index:

public String deleteCharAt(String str, int i) {

    if (i == 0) {
        return str.substring(1);
    }
    else if (i == str.length()) {
        return str.substring(0, i-1);
    }

    String first = str.substring(0, i - 1);
    String second = str.substring(i+1);

    return first + second;
}

However it’s not working as expected, I think it may be because I’m not understanding how the substring function works.

Does this look correct? Will this code remove the i-th character from a string successfully? Or did I mess up the substring function?

Answer 1

It’s not quite right – this:

String first = str.substring(0, i - 1);

should be:

String first = str.substring(0, i);

(Think about a simple example – if i is 1, you want to take substring(0, 1) to get the first character; substring(0, 0) would give an empty string.)

because the second parameter of substring is already exclusive.

Likewise this optimization:

else if (i == str.length()) {
    return str.substring(0, i);
}

should be:

else if (i == str.length() - 1) {
    return str.substring(0, i);
}

You should also add argument validation.

After making those changes, this code:

public static void main(String[] args) {
    for (int i = 0; i < 10; i++) {
        System.out.println(deleteCharAt("0123456789", i));
    }
}

Gives this output:

123456789
023456789
013456789
012456789
012356789
012346789
012345789
012345689
012345679
012345678