School of Engineering and Applied Science
Department of Computer Science
CSci 53 -- Introduction to Software Development
http://www.seas.gwu.edu/~csci53/spring05
Prof. Michael B. Feldman
mfeldman@gwu.edu

Lab Exercise #5 -- Loop the loops
Friday, March 4, 2005

The purpose of this lab is to go beyond Sect. 3.6-3.8, and explore the behavior of the for-loop and while-loop constructs of Java. First, let's look at programs53/ForLoop.java.
 
   1. //--------------------------------------------------------------
   2. //| Very simple example illustrating for-loop construct
   3. //| Author: M. B. Feldman, The George Washington University
   4. //| Last Modified: October 2002
   5. //--------------------------------------------------------------
   6. public class ForLoop
   7. {
   8.   public static void main (String[] args)
   9.   {
  10.
  11.     for (int counter = 1;     // INITIALIZATION
  12.          counter <= 10;       // TERMINATION
  13.          counter++)           // MODIFICATION
  14.     {
  15.       System.out.print (counter + " ");
  16.     }
  17.   }
  18. }

Copy and compile this program. What is the output?
 


 

Interchange the 1 in line 11 with the 10 in line 12. Compile and run. What is the output?


 

Change ++ to -- in line 13. Compile and run; describe the result.
 


 

Change -- back to ++; now add a semicolon after the ) of line 13. Compile; describe the result.
 


 

Remove the spurious semicolon. Now modify the program so it counts backwards from 10 to 1. Write down the modified lines:


 
 

 

Now modify the program so it displays only the odd numbers. HINT: put the display statement in a conditional (if) statement inside the loop body. Write down this conditional statement here:


 
 

 

Now add statements so that the user is prompted for the starting and ending values. You'll need variables for these -- call them start and end. Run the program several times; experiment with different starting and ending values. Try at least one pair such that the starting value

Now let's look at another example, programs53/NestedFor.java.

   1. //--------------------------------------------------------------
   2. //| Very simple example illustrating nested for-loop construct
   3. //| Author: M. B. Feldman, The George Washington University
   4. //| Last Modified: October 2002
   5. //--------------------------------------------------------------
   6. public class NestedFor
   7. {
   8.   public static void main (String[] args)
   9.   {
  10.
  11.     for (int outer = 1; outer <= 5; outer++)
  12.     {
  13.       System.out.print (outer + " | ");
  14.       for (int inner = 1; inner <= 10; inner++)
  15.       {
  16.         System.out.print (inner + " ");
  17.       }
  18.       System.out.println("");
  19.     }
  20.   }
  21. }

Copy, compile, and run it. What is the output?


 
 
 

 

In line 11, change 5 to 7, and in line 14, change 1 to 6. Now what is the output?


 
 
 

 

Now in line 14, change 6 to outer. Now what is the output?


 
 
 

 

Modify the program so it displays a triangular shape with the point at the top, like
1 | 1
2 | 1 2
3 | 1 2 3
4 | 1 2 3 4
etc. Write down the modified lines here.


 
 
 

 

Finally, let's look at a while-loop program, programs53/WhileLoop.java. This program is logically equivalent to the original ForLoop.java.

    1. //--------------------------------------------------------------
   2. //| Very simple example illustrating while-loop construct
   3. //| Author: M. B. Feldman, The George Washington University
   4. //| Last Modified: October 2002
   5. //--------------------------------------------------------------
   6. public class WhileLoop
   7. {
   8.   public static void main (String[] args)
   9.   {
  10.
  11.     int counter;
  12.
  13.     counter = 1;               // INITIALIZATION
  14.     while (counter <= 10)      // TERMINATION
  15.     {
  16.       System.out.print (counter + " ");
  17.       counter++;               // MODIFICATION
  18.     }
  19.   }
  20. }

Copy and compile this one. Write down the output:


 

Now in line 14, change <= to <. Compile and run; what's the output now?


 

Comment out line 17; compile and run it. What's the result?


 

Finally, remove the comment marks from line 17, but add a semicolon after the ) in line 14. Now what happens? Why?


 
 

 
(end of lab)