1. Tracing through the execution of a program

2. Variations on Loops/Repetition (while loop)
   - counting up (how much accomplished)
   - counting down (how much left to do)
   - keeping track of current index

Tracing through the execution of a program

The following slides show the step-by-step execution of the SmileyApp:

Row of Smileys execution
Loop Variations

2.1. Version 1 (summary)




The meaning we assign to a variable tells us how to use it.

In the first version of drawSmileyRow we decided that:
count represents number of smileys drawn so far

This meaning of count tells us how to initialize, update,
and use it in the loop condition:

- init:    int count = 0;        * no faces drawn so far
- update:  count = count + 1;    * one more face drawn
- loop:    while ( count < n )   * keep drawing as long as
                                   less than n faces drawn




void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = 0;    // how many drawn so far

     while ( count < n )    // as long as not all have been drawn
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count + 1;    // another one drawn
      }
}







2.2. Version 2



    
We could have assigned a different meaning to count:
count represents number of smileys left to draw

The new meaning of count changes how we initialize, update,
and use it in the loop condition:

- init:   int count = n;         * all faces left to draw
- update: count = count - 1;     * one less left to draw
- loop:   while ( count > 0 )    * keep drawing as long as
                                   smileys left to draw





void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = n;    // how many left to draw

     while ( count > 0 )    // as long as smileys left to draw
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count - 1;    // one less left to draw
      }
}





2.3. Version 3



    
In the previous versions we kept track of:

* how much work was accomplished (Version 1)
* how much work was left to do (Version 2)

Yet another way to think about the problem is to keep
track of the index of the smiley we are about to draw,
i.e. the 1st, the 2nd, the 3rd, etc. 

count represents the index of the smiley to be drawn

The new meaning of count changes how we initialize, update,
and use it in the loop condition:

- init:    int count = 1;        * about to draw the 1st smiley
- update:  count = count + 1;    * change to index of next smiley
- loop:    while ( count <= n )  * keep drawing as long as the index
				   is for one of the smileys;
                                   if we are asked to draw n=4 smileys,
				   the indices are 1,2,3,4




void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = 1;    // index of smiley to draw

     while ( count <= n )    // as long as valid index
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count + 1;    // move to index of next smiley
      }
}





Here are the three complete versions to compare:




count means "how many drawn so far"
count means "how many left to draw"
count means "index of smiley to draw"





void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = 0;    // how many drawn so far

     while ( count < n )    // as long as not all have been drawn
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count + 1;    // another one drawn
      }
}






void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = n;    // how many left to draw

     while ( count > 0 )    // as long as smileys left to draw
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count - 1;    // one less left to draw
      }
}






void drawSmileyRow( double x, double y, double radius, int n )
{
     double curX = x; 
     double curY = y; 

     int count = 1;    // index of smiley to draw

     while ( count <= n )    // as long as valid index
     {
           drawSmiley( curX, curY, radius );

           curX = curX + 2*radius;        

           count = count + 1;    // move to index of next smiley
      }
}