Groovy convert every alphabet of a string into its equivalent numeric form

import groovy.io.*;
class AlphabetToDigit {
    Scanner scan=new Scanner(System.in);
    def txt;
    def ch;
    def str="";
    def input(){
        try{
            printf "Enter A Word: ";
            txt=scan.nextLine();
            compute(txt);
        } catch(e){
            System.err.println "Error Occurred!\n$e"+e.getMessage();
            System.exit(0);
        }
    }
    def compute(txt){
        for(def i in 0..txt.length()-1){
            ch=txt.toUpperCase().charAt(i);
            switch(ch){
            case 'A': str+="01"; break;
            case 'B': str+="02"; break;
            case 'C': str+="03"; break;
            case 'D': str+="04"; break;
            case 'E': str+="05"; break;
            case 'F': str+="06"; break;
            case 'G': str+="07"; break;
            case 'H': str+="08"; break;
            case 'I': str+="09"; break;
            case 'J': str+="10"; break;
            case 'K': str+="11"; break;
            case 'L': str+="12"; break;
            case 'M': str+="13"; break;
            case 'N': str+="14"; break;
            case 'O': str+="15"; break;
            case 'P': str+="16"; break;
            case 'Q': str+="17"; break;
            case 'R': str+="18"; break;
            case 'S': str+="19"; break;
            case 'T': str+="20"; break;
            case 'U': str+="21"; break;
            case 'V': str+="22"; break;
            case 'W': str+="23"; break;
            case 'X': str+="24"; break;
            case 'Y': str+="25"; break;
            case 'Z': str+="26"; break;
            case ' ': str+=" "; break;
            default: str+=ch;
            }
        }
    }
    def display(){
        println "Digits: $str";
    }
    def String getDigit(str){
        compute(str);
        return this.str;
    }
    public static void main(args){
        AlphabetToDigit alf=new AlphabetToDigit();
        alf.input();
        alf.display();
    }
}