Replace every nth character with "-" in java without regex

Question

I am trying to replace every nth character in a string with some special character for e.g "-" so based on value of input (int n and String str), I want to replace all chars at n ( like 2, 4, 6) by this special characters so the length of string is maintained and this is what I have so far but it is not yielding correct output.

public String everyNth(String str, int n) {
      String result="";
      for(int i=0; i<str.length(); i+=n){
            result += " " + str.charAt(i);  
      } 
      return result;
}

Answer 1

What you could do is use a StringBuilder that will be initialized with your input String, then use setCharAt(int index, char ch) to modify a given char, this way you avoid building a new String at each iteration as you currently do with the operator += on String.

Something like:

public String everyNth(String str, int n) {
    if (n < 1) {
        throw new IllegalArgumentException("n must be greater than 0");
    }
    StringBuilder result = new StringBuilder(str);
    // This will replace every nth character with '-'
    for (int i = n - 1; i < str.length(); i += n) {
        result.setCharAt(i, '-');
    }
    return result.toString();
}

NB: Starts from n - 1 unless you want to replace the first character too if so starts from 0.

---

If you want to simply remove the characters, you should use the method append(CharSequence s, int start, int end) to build the content of your target String, as next:

public String everyNth(String str, int n) {
    if (n < 1) {
        throw new IllegalArgumentException("n must be greater than 0");
    }
    StringBuilder result = new StringBuilder();
    // The index of the previous match
    int previous = 0;
    for (int i = n-1; i < str.length(); i += n) {
        // Add substring from previous match to the current
        result.append(str, previous, i);
        // Set the new value of previous
        previous = i + 1;
    }
    // Add the remaining sub string
    result.append(str, previous, str.length());
    return result.toString();
}