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More with `while`

What does the following method return?

/** Announce a countdown, starting at 10.
 * @return A countdown starting at 10.  
 *   That is, "10...9...8... [etc] 2...1...Liftoff!"
 */
public String countdown() 
{
  String chantSoFar = "";  /* A local variable, to accumulate the answer. */
  int t = 10;         /* The next number to count down. */

  while (t >= 0) 
  {
    chantSoFar = chantSoFar + (t + "...");
    t = t-1;
    System.out.println( "t is now " + t );
    System.out.println( "chantSoFar is now " + chantSoFar );
  }

  return (chantSoFar + "Liftoff!");
}

In order to walk through this, we need to keep track of what chant and t are, in the environment.

Fix the bug about including 0.
Change the function so that it starts the countdown from a specified point, not just 10.
style tip: Don't just make the parameter named t and then assign to it (but mention that approach). Keeping the parameter untouched will be more important in some future problems.

Another example of a while loop:
We saw earler, that if we have a source of characters (like the keyboard), we can make a java.util.Scanner who can take those individual characters and turn them into numbers (ints or doubles) or strings for us, with the method nextInt(), nextDouble(), next() (which returns the next word, as a string). There are other useful Scanner methods: nextLine() which returns the entire next line (a String), as well as a boolean method hasNext(), which determines whether the source of characters has run dry.

We'll talk about the following program, which repeatedly: reads a line from a web page, and increments a variable “linesSoFar”. That is, the method counts the lines in a web page! Everything in gray is something you don't need to know right now.

  /** Open up a URL, and read the input.
   * As a side-effect (for demonstration only), print each line to the console.
   * @return The number of lines in a given URL.
   */
  public static int countLines() throws java.net.MalformedURLException, java.io.IOException
  {
    String sourcePage = "http://www.radford.edu/itec120/2009spring/";

    // Open sourcePage for input: (Create a URL object based on sourcePage, open that URL as a stream-of-characters,
    //                             and create a Scanner who will group that stream's characters into Strings etc.)
    java.util.Scanner src = new java.util.Scanner( new java.net.URL( sourcePage ).openStream() );

    int linesSoFar = 0;      // Keep track of how many lines we've seen.

    while ( src.hasNext() ) 
    {
      src.nextLine();   // Have the scanner consume the next line.  We ignore its return value.
      linesSoFar = linesSoFar + 1;
    }

    return linesSoFar;
  }

Task: First, modify this method so that it prints out each line to the screen. Then, make the modification useful: have the method return one big String containing the entire contents of the file.

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©2009, Ian Barland, Radford University
Last modified 2009.Mar.30 (Mon) Please mail any suggestions
(incl. typos, broken links)
to iba�rlandrad�ford.edu

lect10a More with `while`

lect10a
More with `while`