如何使方法 isEmpty() 为真，以便我最终可以从 InputStream 读取数据？

Question

I read Algorithm Forth Edition today and try to understand how Dijkstra's two-stack Algorithm for Expression Evaluation work, so I wrote the my implementation code to Intellij IDEA, which is almost same with the code on the book, but I found that the program always paused and waited me continue to input string, and never prints the result.

The problem seems have nothing with the algorithm itself, but is related to the method StdOut.isEmpty(). it seems always give me a false value no matter the Standard InputStream is empty or not, and let me continue to type in data when the InputStream is empty, thus I put it in "while(.StdOut,isEmpty())". so the proceed are blocked, I do not know how to let it return a true value. and let the code run correctly.

and this is the code I wrote:

import edu.princeton.cs.algs4.Stack;
import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdIn;
/**
 * Created by Stepten.Wong on 2016/8/15 0015.
 */
public class DoubleStackEE
{
public static void main(String[] args)
{
    Stack<String> ops = new Stack<String>();
    Stack<Double> vals = new Stack<Double>();
    String s;
    while(!StdIn.isEmpty())
    {
        s = StdIn.readString();
        if(s.equals("("));
        else if(s.equals("+"))      ops.push(s);
        else if(s.equals("-"))      ops.push(s);
        else if(s.equals("*"))      ops.push(s);
        else if(s.equals("/"))      ops.push(s);
        else if(s.equals("^"))      ops.push(s);
        else if(s.equals("sqrt"))   ops.push(s);
        else if (s.equals('\n')) break;
        else if(s.equals(")")) {
            double val = 0;
            String op = ops.pop();
            if(op.equals("+"))           val = vals.pop() + vals.pop();
            else if(op.equals("-"))      val = vals.pop() - vals.pop();
            else if(op.equals("*"))      val = vals.pop() * vals.pop();
            else if(op.equals("/"))      val = vals.pop() / vals.pop();
            else if(op.equals("**"))     val = Math.pow(vals.pop(),                     vals.pop());
            else if(op.equals("sqrt"))   val = Math.sqrt(vals.pop());
            vals.push(val);
        }
        else vals.push(Double.parseDouble(s));
    }
    StdOut.println(vals.pop());
}

}

---

here is the implement code of isEmpty() and readString() from their lib algs4.jar:

/**
 * Returns true if standard input is empty (except possibly for whitespace).
 * Use this method to know whether the next call to {@link #readString()}, 
 * {@link #readDouble()}, etc will succeed.
 *
 * @return <tt>true</tt> if standard input is empty (except possibly
 *         for whitespace); <tt>false</tt> otherwise
 */
public static boolean isEmpty() {
    return !scanner.hasNext();
}




 /**
 * Reads the next token  and returns the <tt>String</tt>.
 *
 * @return the next <tt>String</tt>
 * @throws NoSuchElementException if standard input is empty
 */
public static String readString() {
    return scanner.next();
}

Answer 1

The key in here is to press ctrl+z followed by pressing Enter which is the signal to the stdIn that console has reached its EOF(end of file) and now it can return true to break the looping.

Answer 2

Actually EOF in console 'CTRL + D' in my computer. If you click right in Intellij IDEA or any IDE the code part you can give a short description from the instruction usability and a link to the Princeton webpage, that give you a detailed description. If I would you recommend, if you learn and have time, it is worth reading everything in detail, you can find interesting information.