Recursive Programming for Text Predictive text

Question

I want to make a program that takes a set of numbers like 234, and at the moment, print out every combination of letters possible that are on a mobile phone keypad.

(1 - nothing, 2 - abc, 3- def and so on)

I currently have:

import java.util.*;

public class testCombo {
    static String str="217";
    static ArrayList<String> list=new ArrayList<String>();

    static void addLet(String seg){
        if(seg.length()==str.length()){
            list.add(seg);
            return;
        }
        char currentChar=str.charAt(seg.length());
        if(currentChar==1 || currentChar==0)
        {
            String str1=seg+" ";
            addLet(str1);
        }
        if(currentChar=='2'){
            addLet(seg+"a");
            addLet(seg+"b");
            addLet(seg+"c");
        }
        else if(currentChar=='3'){
            addLet(seg+"d");
            addLet(seg+"e");
            addLet(seg+"f");
        }   
        else if(currentChar=='4'){
            addLet(seg+"g");
            addLet(seg+"h");
            addLet(seg+"i");
        }   
        else if(currentChar=='5'){
            addLet(seg+"j");
            addLet(seg+"k");
            addLet(seg+"l");
        }   
        else if(currentChar=='6'){
            addLet(seg+"m");
            addLet(seg+"n");
            addLet(seg+"o");
        }   
        else if(currentChar=='7'){
            addLet(seg+"p");
            addLet(seg+"q");
            addLet(seg+"r");
            addLet(seg+"s");
        }   
        else if(currentChar=='8'){
            addLet(seg+"t");
            addLet(seg+"u");
            addLet(seg+"v");
        }   
        else if(currentChar=='9'){
            addLet(seg+"w");
            addLet(seg+"x");
            addLet(seg+"y");
            addLet(seg+"z");
        }   
    }

    public static void main(String[] args){
        addLet("");
        for(String str:list) //Sets str to each value in list during each iteration
            System.out.println(str);
    }
}

as my code as we are supposed to be using recursive programming, but I can't get it to work for 1s and 0s. (this is only a practice class, I have another one which allows input from user and it already verifies that it only contains digits)

Would this way of finding then printing out every combination count as recursive?

Answer 1

Yes it's recursive (it works by calling itself), but it's unnecessarily verbose. You can skip the temporary variables, thus saving a lot of space, and making it more readable. It took me a few moments to grok why you had several string variables in each case:

    if(currentChar==1 || currentChar==0)
    {
        addLet(seg+" ");
    }
    if(currentChar=='2'){
        addLet(seg+"a");
        addLet(seg+"b");
        addLet(seg+"c");
    } ...

WRT 1's and 0's, you should be comparing currentChar to '1' and '0', not 1 and 0.

Answer 2

You can simplify the code and reduce chances of error by using a mapping of digits to candidate letters:

import java.util.*;
import static java.util.Arrays.asList;

public class TestCombo {

    private static Map<Character, List<Character>> lettersByDigit = 
        new HashMap<Character, List<Character>>() {{
           put('0', asList(' '));
           put('1', asList(' '));
           put('2', asList('a', 'b', 'c'));
           put('3', asList('d', 'e', 'f'));

           // and so on - add all candidates per digit in this fashion
        }};

    private static List<String> candidates = new Vector<String>();

    private static void enumerate(String digits, String prefix) {
        if (prefix.length() == digits.length()) {
          candidates.add(prefix);
          return;
        }

        char nextDigit = digits.charAt(prefix.length());

        for (Character nextLetter : lettersByDigit.get(nextDigit)) {
            enumerate(digits, prefix + nextLetter.toString());
        }
    }

    public static void main(String[] args){
        enumerate("217", "");
        for(String candidate : candidates) {
            System.out.println(candidate);
        }
    }
}

Note that this is not tested, but hopefully you get the idea.