Separate text in k-shingles without Scanner.class in Java

Question

I am trying to separate a text in k-shingles, sadly I cannot use scanner. If the last shingle is too short, I want to fill up with "_". I came this far:

public class Projektarbeit {

    public static void main(String[] args) {
        testKShingling(7, "ddssggeezzfff");
    }

    public static void testKShingling(int k, String source) {
        //first eliminate whitespace and then fill up with withespaces to match target.length%shingle.length() == 0
        String txt = source.replaceAll("\\s", "");

        //get shingles
        ArrayList<String> shingles = new ArrayList<String>();
        int i;
        int l = txt.length();
        String shingle = "";

        if (k == 1) {
            for(i = 0; i < l; i++){
                shingle = txt.substring(i, i + k);
                shingles.add(shingle);
            };
        }
        else {
            for(i = 0; i < l; i += k - 1){
                try {
                    shingle = txt.substring(i, i + k);
                    shingles.add(shingle);
                }
                catch(Exception e) {
                    txt = txt.concat("_");
                    i -= k - 1;
                };
            };
        }
        System.out.println(shingles);
    }
}

Output: [ddssgge, eezzfff, f______]

It works almost, but in the with the given parameters in the example the last shingle is not necessary (it should be [ddssgge, eezzfff]

Any idea how to do this more beautiful?

Answer 1

To make the code posted work you only need to add break and the end of the catch block:

catch(Exception e) {
     txt = txt.concat("_");
     i -= k - 1;
      break;
};

Having said that I wouldn't use an Exception to control the program. Exception are just that: should be used for run time errors. Avoid StringIndexOutOfBoundsException by controlling the loop parameters:

public static void main(String[] args) {
    testKShingling(3, "ddssggeezzfff");
}

public static void testKShingling(int substringLength, String source) {

    //todo validate input
    String txt = source.replaceAll("\\s", "");
    //get shingles
    ArrayList<String> shingles = new ArrayList<>();
    int stringLength = txt.length();

    if (substringLength == 1) {
        for(int index = 0; index < stringLength; index++){
            String shingle = txt.substring(index, index + substringLength);
            shingles.add(shingle);
        };
    }
    else {
        for(int index = 0; index < stringLength -1 ; index += substringLength - 1){
            int endIndex = Math.min(index + substringLength, stringLength);
            String shingle = txt.substring(index, endIndex);
            if(shingle.length() < substringLength){
                shingle = extend(shingle, substringLength);
            }
            shingles.add(shingle);

        };
    }
    System.out.println(shingles);
}

private static String extend(String shingle, int toLength) {

    String s = shingle;
    for(int index = 0; index < toLength - shingle.length(); index ++){
        s = s.concat("_");
    }
    return s;
}

An alternative implementation of testKShingling:

public static void testKShingling(int substringLength, String source) {

    //todo validate input
    String txt = source.replaceAll("\\s", "");
    ArrayList<String> shingles = new ArrayList<>();

    if (substringLength == 1) {
        for(char c : txt.toCharArray()){
            shingles.add(Character.toString(c));
        };
    }
    else {
        while(txt.length() > substringLength) {
            String shingle = txt.substring(0, substringLength); 
            shingles.add(shingle);
            txt = txt.substring(substringLength - 1); //remove first substringLength - 1 chars 
        }

        if(txt.length() < substringLength){  //check the length of what's left 
            txt = extend(txt, substringLength); 
        }
        shingles.add(txt); //add what's left 
    }
    System.out.println(shingles);
}