Redacting everything in a string except a particular word

Question

I am working through the Coding Bat exercises for Java. Here is one I have just completed:

Given a string and a non-empty word string, return a version of the original String where all chars have been replaced by pluses ("+"), except for appearances of the word string which are preserved unchanged.

And here is my solution:

public String plusOut(String str, String word){

    String s = "";
    String n = str + "  ";

    int i = 0;
    while(i < str.length()) {

        if (!(n.substring(i, i + word.length()).equals(word))) {
            s += "+";
            i++;
        } else {
            s += word;
            i += word.length();
        }
    }

    return s;
}

As you can see, I concatenated two spaces on the original string and used that to test the substrings. This was to avoid out of bounds exceptions.

What I want to do is optimise my solution, because I don't like how I added two spaces just to avoid errors. I want to change the if statement so that it tests the substrings of str rather than n, and get rid of n entirely. I have tried but I can't seem to find a way of doing it. How can I do this? What else needs optimising?

Answer 1

Manipulating Strings directly is a bad idea, especially when you are doing it repeatedly. Java strings are immutable, so every time you do s += …, you are making a copy of the entire provisional result. Anytime you need to do a substantial amount of editing on a string, use a StringBuilder.

To look for the next occurrence of word, use String.indexOf() or StringBuilder.indexOf(). It's a lot easier than extracting substrings and checking for equality. When extracting the substring, your code crashes if word is longer than three characters. Using .indexOf() is one way to eliminate that problem.

Here is an example of a more efficient solution:

public String plusOut(String str, String word) {
    StringBuilder s = new StringBuilder(str);

    // Find substrings to redact, from index i (inclusive)
    // to index j (exclusive)
    for (int i = 0; i < s.length(); i += word.length()) {
        int j = s.indexOf(word, i);
        if (j < 0) {
            // word not found; redact the rest of the string
            j = s.length();
        }

        while (i < j) {
            s.setCharAt(i++, '+');
        }
    }
    return s.toString();
}

Answer 2

As @mklinker proposed, you could try to use the word lengths. I tried to come up with an example:

public String plusOut(String str, String word){
   String result = "";
   int s = str.length();
   int w = word.length();

   for (int i = 0; i < s; i++) {
      if (i <= s - w) {
        if (str.substring(i,i+w).equals(word)) {
          result += word;
          i += w-1;
        }
        else {
          result += "+";
        }
      }
      else {
        result += "+";
      }
  }    
  return result;
}

Answer 3

At its core, this is a conditional string replacement problem, so it might make sense to use string replacement techniques.

public String plusOut(String str, String word) {
    StringBuffer result = new StringBuffer(str.length());
    Matcher m = Pattern.compile(Pattern.quote(word) + "|.").matcher(str);
    while (m.find()) {
        // Replace word with itself, and any other character with "+"
        m.appendReplacement(result, word.equals(m.group()) ? "$0" : "+");
    }
    return m.appendTail(result).toString();
}

At every point in str, it looks for either the word (which, if found, is replaced by itself) or a single character (which is replaced by a "+").

Pros

• This solution is based on a standard recipe.
• The thinking is at a higher level of abstraction, without headache-inducing indexes like i or the possibility of StringIndexOutOfBoundsExceptions. All of those details are taken care of by code within Matcher.

Cons

• This solution is appealing to those who know regular expressions (which is really a separate skill from knowing Java), and probably befuddling to those who don't.

• There are subtle pitfalls that could cause one to write this solution wrong:

  • The alternation in the pattern has to be (?>word)|., not .|(?>word), due to the leftmost-longest rule of pattern matching.
  • The word in the pattern has to be quoted in case word contains characters with special significance in regexes.
  • The replacement text has to be "$0", not word, in case word contains a $ character.
• It is less efficient than a well-implemented low-level StringBuilder-based solution. Still, it scales better than your original code (which is O(n²) due to inefficient string concatenation).

Answer 4

200_success and DJanssens both make good points.

I want to add that, again, Regular Expressions are often useful for text manipulation. This work requires a complicated pattern if you want to keep the groups, but if you can ignore the groups, it is easier.

Note also, that StringBuilders are also more efficient than String concatenation.

Two traditional Java versions. One using a simple split, the other using a more complex one:

Simple split

public static String plusOut(String str, String word) {
    String[] parts = str.split(Pattern.quote(word));
    StringBuilder sb = new StringBuilder(str.length());
    for (int i = 0; i < parts.length; i++) {
        if (i > 0) {
            sb.append(word);
        }
        sb.append(parts[i].replaceAll(".", "+"));
    }
    return sb.toString();
}

Note the use of Pattern.quote(...) in case the input word is something like Boo(hoo)? ... we don't want the word to be interpreted as a regex pattern itself.

Complex split:

public static String plusOut(String str, String word) {
    String[] parts = str.split("(?=\\Q" + word + "\\E)|(?<=\\Q" + word + "\\E)");
    StringBuilder sb = new StringBuilder(str.length());
    for (int i = 0; i < parts.length; i++) {
        sb.append(i % 2 == 0 ? parts[i].replaceAll(".", "+") : parts[i]);
    }
    return sb.toString();
}

Java 8

In Java 8 I would consider the following:

public static String plusOut8(String str, String word) {
    return Stream.of(str.split("\\Q" + word + "\\E"))
        .map(fill -> fill.replaceAll(".", "+"))
        .collect(Collectors.joining(word));
}

See these all working in Ideone