We Keep Coding

"unreachable statement" in a boolean recursion

I need to write a short recursive program for a class that checks if a string - t is transformation of another string - s. It simply needs to check if every character in s is also in t.
For ex:
"sddbs" is not a transformation of "sdb"
"sddbs" is a transformation of "sddb".
public static boolean isTrans (String s, String t)
{
if (t.indexOf(s.charAt(0)) != -1)
return true;
else
return false;
return isTrans(s.substring(1), t);
}
And still.. the code doesn't work as expected. "unreachable statement" in the last line of the code.

The reason is quite simple:
there is no possible way to execute this line:
return isTrans(s.substring(1), t);
why?:
you have a return IF this condition (t.indexOf(s.charAt(0)) != -1) is met and another if not....

It's because of the Law of Excluded Middle. You can treat return as exit function.
public static boolean isTrans (String s, String t) {
if (condition)
return true;
else
return false;
//the condition must have been true or false so the method has already exited.
return isTrans(s.substring(1), t);
}
If the condition is true, you return true, if it's false you return false. Otherwise you call recursively. There is no otherwise.

You final return statement is unreachable because your method body contains return statement for both if and else condition, hence assuring that the last return statement will never be reached.
But besides this I don't understand why you need to write a recursive function, though a non-recursive function using similar method calls will do the same thing:
public static boolean isTrans (String s, String t)
{
if (t.indexOf(s) > -1)
return true;
else
return false;
}
Edit:
As suggested by #Holger you can avoid unnecessary if else and replace your code with:
public static boolean isTrans (String s, String t)
{
return (t.indexOf(s) > -1) //returns true or false just like your if else
}
or even shorter:
public static boolean isTrans (String s, String t)
{
return t.contains(s); //internally contains() calls indexOf(), similar to what you are doing
}

According to your recursion method once you enter the if condition it would either return a true or false output. so your code never reaches the recursion statement.
I would like to suggest my way of implementing your transformation program using recursion.
import java.util.Scanner;
public class Recursion {
static int flag;
public void isTransformation(String str1, String str2) {
flag = str2.length();
char ch1[], ch2[];
ch1 = str1.toCharArray();
ch2 = str2.toCharArray();
if (ch1[0] == ch2[0]) {
flag--;
if (flag == 0) {
System.out.println("Transformation");
return;
}
isTransformation(str1.substring(1), str2.substring(1));
} else
System.out.println("Not a Transformation");
}
public static void main(String args[]) {
String str1, str2;
Scanner sc = new Scanner(System.in);
System.out.print("Enter string 1: ");
str1 = sc.nextLine();
System.out.print("Enter string 2: ");
str2 = sc.nextLine();
Recursion r = new Recursion();
if (str1.length() >= str2.length())
r.isTransformation(str1, str2);
else
r.isTransformation(str2, str1);
sc.close();
}
}

calculate the number of characters in a string recursively

I am trying to calculate the number of characters in a string by using recursive method. here is my code in java
public class MainClass {
public int length(String str) {
if (str == null) {
return 0;
} else {
return length(str.substring(1))+1; // in eclipse it says : at MainClass.length(MainClass.java:12)
}
}
public static void main(String args[]) {
MainClass m = new MainClass();
System.out.println(m.length("ali"));
}
}
This line does not work : return length(str.substring(1))+1;
How can I correct the code?
Thanks

You forgot the case when your String argument is an empty string, include that in your check:
if (str == null || str.length()==0) {
return 0;
} else {
[...]
Please note that the Exception that you get contains valuable information about what's going wrong, in this case it's probably a StringIndexOutOfBoundsException because you call substring(1) on an empty String object.

It should be
public int length(String str) {
if (str==null||str.isEmpty()) {
return 0;
}else {
return length(str.substring(1))+1
}

Validating input string with java

First off, I will admit that this is an assignment of mine. however, I am at my wits end. I tried need to validate that the user input a proper expression (ie: "7 + 5;") and I managed to do it with split methods but I was told that I can't do that. I'm sure there is a simple solution to the problem but I am not seeing it.
The code is rather lengthy so I won't post it but if I will if needed.
Thanks!
Edit to answer questions: I am writing in jGrasp, so they can do whatever is on the keyboard. I was told to "Find a creative way to use substrings" which I don't know what that means. The expression needs to be Number Space operand Space number semicolon
here is what I have for the validation... I am using arrays for each character in the expression
public static boolean validFormat(String expr)
{
String tokens[] = expr.substring()
if (tokens.length == 3)
{
if (tokens[0].equals(""))
{
return false;
}
if (tokens[1].equals(""))
{
return false;
}
if (tokens[2].length < 2)
{
return false;
}
else
{
if (tokens[2].endwith(";"));
{
return false;
}
else
{
return true;
}
}
}
return false;
}
I get an error with calling the substring as well as an "else without if" error

First, you should limits the input to 6 characters using an if statement. Then use the CharAt() method to return each character to check the condition.

I was told to "Find a creative way to use substrings".
As told, try using String.substring() for the same.
public class Demo {
public static void main (String[] args) {
String exp = "7 + 5;";
System.out.printf("%s\t%b%n", exp, validate(exp));
exp = "4 + d;";
System.out.printf("%s\t%b%n", exp, validate(exp));
}
public static boolean validate(String exp) {
String n1 = exp.substring(0,1);//operand
String n2 = exp.substring(4,5);//operand
String co = exp.substring(5);//semicolon
String s1 = exp.substring(1,2);//space
String s2 = exp.substring(3,4);//space
String op = exp.substring(2,3);//operator
return num(n1) && num(n2) && semi(co) && space(s1) && space(s2) && opt(op);
}
public static boolean num(String n) {
return "0123456789".contains(n);
}
public static boolean semi(String s) {
return ";".equals(s);
}
public static boolean space(String s) {
return " ".equals(s);
}
public static boolean opt(String s) {
return "-+*%/^".contains(s);
}
}
This solution uses RegExp:
public class Demo {
public static void main (String[] args) {
String exp = "7 + 5;";
System.out.printf("%s\t%b%n", exp, validate(exp));
exp = "4 + d;";
System.out.printf("%s\t%b%n", exp, validate(exp));
}
public static boolean validate(String exp) {
return exp.matches("\\d\\s(\\+|\\-|\\*|\\/|\\^|\\%)\\s\\d\\;");
}
}

Error in my Word program in Java

This is my code and it compiles fine but when I try to create a string it says
Error: cannot find symbol - variable racer
public class Word {
private String original;
public Word(String s) {
original = s;
}
public String reverse () {
String reverse= "";
int x = 1;
int length = original.length();
while (length - x >= 0) {
reverse = reverse + original.substring(length -x);
x++;
}
return reverse;
}
public boolean isPalindrome() {
if(original.equals(reverse()))
return true;
else
return false;
}
}

The stated problem is not in the code posted - my guess is irrelephant's comment is correct, ie change new Word(racer) --> new Word("racer").
But I offer this to eliminate any chance of any errors in your code by basically eliminating your code:
public class Word {
private String original;
public Word(String s) {
original = s;
}
public boolean isPalindrome()
return new StringBuilder(original).reverse().toString().equals(original);
}
}
or if you must expose a reverse() method:
public class Word {
private String original;
public Word(String s) {
original = s;
}
public String reverse () {
return new StringBuilder(original).reverse().toString();
}
public boolean isPalindrome()
return reverse().equals(original);
}
}

I don't see the variable racer anywhere, but since you're using reverse inside a method, I'd recommend making it
Most likely, racer was never defined
Either that or the method was called w/o quotes
isPalindrome(racer)//note the lack of quotes
change reverse() to this
private() String reverse () {
String reverse= "";
int x = 1;
int length = original.length();
while (length - x >= 0) {
reverse = reverse + original.substring(length -x);
x++;
}
return reverse;

How to Convert a char to a local variable of Type Letter Called Guessed.?

Switch back to the Phrase class.  Add a new method guessLetter that returns a boolean 
and takes a char as an argument.  This will be used to see if a player guesses a letter 
correctly.  This method should:
● Convert the char to a local variable of type Letter called guessed
● Return true if guessed is in letters, otherwise return false
public class Phrase {
private String phrase;
public Phrase(String phrase) {
phrase = phrase.toUpperCase();
for(char c : phrase.toCharArray()) {
letters.add(new Letter(c));
}
}
public String getPhrase() {
return phrase;
}
// public String phrase;
ArrayList<Letter> letters = new ArrayList<Letter>();
public ArrayList<Letter> getLetters() {
return letters;
}
public boolean guessLetter(char c) {
char c = new char(Letter);
c = guessed;
return false;
}
}
Thanks. I can't figure this one out.
package edu.htc.java1.phrasegame.model;
public class Letter {
private char letter;
private boolean isHidden;
public int getLetter() {
return letter;
}
public boolean isHidden() {
return isHidden;
}
public void unhide() {
isHidden = false;
}
public Letter(char letter) {
this.letter = letter;
if (String.valueOf(letter).matches("[A-Z]")) {
isHidden = true;
}
}
}

char c = new char(Letter); should probably be Letter guessedLetter = new Letter(c);
I'm not sure what c = guessed; is supposed to be doing.
Then you'll need to see if guessedLetter is in letters. How you go about that depends on the implementation of Letter.
EDIT: Now that I see the Letter implementation, the correct solution is to override equals()in Letter, probably comparing letter. You can get away without implementing hashCode() in this exercise, but you really should override it too. Then your guessLetter() method can be:
public boolean guessLetter(char c) {
return letters.contains(new Letter(c));
}
Or to meet the constraints in the problem:
public boolean guessLetter(char c) {
Letter guessed = new Letter(c);
return letters.contains(guessed);
}

My current assumption is this
public boolean guessLetter(char c)
{
// convert received character to letter
Letter letter = new Letter(c);
// loop through your list of letters
for(Letter l : letters)
{
// if list of letters contains same letter as the one you received then return true
if(l.getLetter() == letter.getLetter())
return true;
}
// we did not find the letter, so we return false
return false;
}
Please do make sure that you read this code and understand it. No one is going to cry if you fail the class because you chose to copy paste the answer.