Stacks

[Stephanie Dears]

I have a program that is supposed to reverse a string of words. The code is below. I get an error code for "word = (String)stack.pop();" It says inconvertible types.

What does that mean?

public class ReverseSentence{
 
  private static BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
 
  public static void main (String [] args) throws IOException{
 
    final int STACK_SIZE = 200;
       String word;
       ArrayStack stack = new ArrayStack(STACK_SIZE);
 
    System.out.println("Enter a sequence of strings" + " one per line, an empty string" +  
       " when done:");
    word = stdin.readLine();
      while (!word.equals("")) {
        stack.push(word);
        word = stdin.readLine();
      }
    System.out.println("The words in reverse order are:");
      while (!stack.isEmpty()) {
        word = (String)stack.pop(); 
    System.out.println(word);
      }
  }
}

[ November 26, 2005: Message edited by: Jim Yingst ]

[fred rosenberger]

i don't remeber the exact precedence, but it's possible that java is casting your stack to a string, and then calling the pop() method - although it doesn't really get that far.

try something like

word = (String)(stack.pop())

[Stan James]

I changed your ArrayStack to Stack (ArrayStack must be something you made?) and compiled with 1.5 and did not see any errors. Can you copy the exact message in?

[Marilyn de Queiroz]

import java.io.*;
 
public class ReverseSentence{
 
    private static BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
 
    public static void main (String [] args) throws IOException{
 
        final int STACK_SIZE = 200;
        String word;
        ArrayStack stack = new ArrayStack(STACK_SIZE);
 
        System.out.println("Enter a sequence of strings" + " one per line, an empty string" +
            " when done:");
        word = stdin.readLine();
        while (!word.equals("")) {
            stack.push(word);
            word = stdin.readLine();
        }
        System.out.println("The words in reverse order are:");
        while (!stack.isEmpty()) {
            word = (String)stack.pop();
            System.out.println(word);
        }
    }
} 
  
import java.util.Stack;
  
class ArrayStack extends Stack {
    ArrayStack(int stackSize)
    {
    }
}

And I compiled and ran with jdk1.4 where ArrayStack extends Stack with no problem. Can you copy the error from your output rather than just trying to summarize it?
[ November 25, 2005: Message edited by: Marilyn de Queiroz ]

[Stephanie Dears]

Inconvertible types is the exact message.

[Jim Yingst]

Exactly what is this "ArrayStack" you're using? Stan and Marilyn tried assuming it's some sort of java.util.Stack, and their code works. If yours is not working, I suspect this "ArrayStack" is something other than a java.util.Stack. In particular, I think that the pop() method returns something other than a String. Which means it can't be cast to a String. What does pop() return?

[Stan James]

This one? http://struts.apache.org/struts-doc-1.0.2/api/org/apache/struts/util/ArrayStack.html

pop returns Object, ought to be fine, no?

[Stephanie Dears]

This is the ArrayStack file:

package ch04.stacks;
 
public class ArrayStack implements StackInterface 
{
  private Object[] stack;           // Array that holds stack elements
  private int topIndex = -1;        // index of top element in stack
 
  // Constructors
  public ArrayStack() 
  {
    stack = new Object[100];
  }
 
  public ArrayStack(int maxSize) 
  {
    stack = new Object[maxSize];
  }
 
  public void push(Object item)
  // Adds an element to the top of this stack
  {      
    if (!isFull())
    {
      topIndex++;
      stack[topIndex] = item;
    }
    else
      throw new StackOverflowException("Push attempted on a full stack.");
  }
 
  public void pop()
  // Removes an element from the top of this stack
  {                  
    if (!isEmpty())
    {
      stack[topIndex] = null;
      topIndex--;
    }
    else
      throw new StackUnderflowException("Pop attempted on an empty stack.");
  }
 
  public Object top()
  // Returns the element on top of this stack
  {                 
    Object topOfStack = null;
    if (!isEmpty())
      topOfStack = stack[topIndex];
    else
      throw new StackUnderflowException("Top attempted on an empty stack.");
    return topOfStack;
  }
 
  public boolean isEmpty()
  // Checks if this stack is empty
  {              
    if (topIndex == -1) 
      return true;
    else
      return false;
  }
 
  public boolean isFull()
  // Checks if this stack is full
  {              
    if (topIndex == (stack.length - 1)) 
      return true;
    else
      return false;
  }
}

[ November 26, 2005: Message edited by: Jim Yingst ]

[Ernest Friedman-Hill]

There you go. Your pop() returns void. You can't cast void to a String (or to anything else!)

This stack implements a "two-phase pop:" you call top() to get the top element, then pop() to discard it and bring the next one to the top.

[Stephanie Dears]

That's what I thought. So how do I get my sentence string to print in reverse?

[Jim Yingst]

Um... by folloing EFH's last suggestion? I'm not sure what "that's what I thought" means here. Did you already knw this? Does the code compile now? I just did what EFH said and the code worked fine - input of

one
two
three

became

three
two
one

Isn't that what you want it to do?

[Jim Yingst]

Also, I added [code] tags to your previous posts. Please use these in the future whenever you post code, to preserve the original indentation. That makes it more readable for everyone. Thanks.

[Stephanie Dears]

Right I figured that out...the top then the pop. I just can't figure out the correct pop syntax. This is what I've tried.

System.out.println("The words in reverse order are:");
while (!stack.isEmpty()) {
word = (String) stack.top();
System.out.println(word);
stack.pop();

and

System.out.println("The words in reverse order are:");
while (!stack.isEmpty()) {
word = (String) stack.top();
stack.pop();
System.out.println(word);

Both ways, It spits out the original sentence. I've tried other things too, but then I either get an error about inconvertible types or when I hit return, it spits out the sentence over and over and over again.

Please help.

[Jim Yingst]

Hey, look! More code! If only there were some way to get that code to display with proper indentation. Hmmm....

For the correct way to use top() & pop(), I don't think it matters. Either one above is fine. As it happens I used the first one in my code.

When you say "sentence", are you putting all these words on one line? (Like a normal sentence?) Did you notice the instructions say "Enter a sequence of strings one per line..."?
[ November 26, 2005: Message edited by: Jim Yingst ]

[Stephanie Dears]

Sorry, I didn't read that part of your post until after I posted. I'll try to remember next time.

Yes, I noticed. The instructions are to input a sentence, like "My dog has fleas" and it's supposed to return "fleas has dog My." Obviously there's something wrong with the other stuff. I just tried it one word at a time and it reversed, but it needs to be in a sentence.

More help please.

[ November 26, 2005: Message edited by: Stephanie Dears ]
[ November 26, 2005: Message edited by: Stephanie Dears ]

[Jim Yingst]

OK, so if the program is reversing lines correctly, then the top() and pop() are working correctly. There's no way you can reverse words within a line just by modifying the top() and pop(). The problem is that earlier, you read lines with readLine(), and put the whole line onto the stack at once. I think after you read a line, you will need to split it up somehow into separate words, then push each word onto the stack, separately. There are several classes you could use for this: StringTokenizer, the split() method in String or the Scanner class in JDK 5. To use the last two you would need to know a bit about regular expressions; if you don't know abou these yet you should probably look at StringTokenizer first.

[Stephanie Dears]

Ok, when I type in my sentence "Josie o mine", I get "Josie o minemine o Josie"

Here's my code:

  
import ch04.stacks.* ;
import java.io.* ;
import java.util.*;
 
public class ReverseSentence{
 
  public static void main (String [] args) throws IOException{
 
    final int STACK_SIZE = 200;
    String phrase;
    ArrayStack stack = new ArrayStack(STACK_SIZE);
 
    Scanner stdin = new Scanner(System.in); 
    phrase = "" ;
 
      while (!phrase.equals("")) {
        stack.push(phrase);
        phrase = stdin.nextLine();
      }
 
    System.out.println("Enter a sentence:");
    phrase = stdin.nextLine();
 
    StringBuffer s = new StringBuffer (phrase);
    s.reverse();
    StringTokenizer stok = new StringTokenizer(s.toString());
  
      while (stok.hasMoreTokens()){
        phrase += new StringBuffer (stok.nextToken()).reverse() + " ";
      } //end while
      phrase.trim();
 
      System.out.println("The words in reverse order are:");
      while (!stack.isEmpty()) {
        phrase = (String) stack.top();
 
        stack.pop(); 
      }
 
      System.out.println(phrase);
   }
}

[Jim Yingst]

Continued here.