Sections 0101, 0102, 0103 and Honors, Fall 1995

Tuesday October 24, 1995

Assigned Reading: 7.2 - 7.4

Handouts (available on-line): none

Topics Covered:

• Discussed the notion of interfaces.

• We explored the functions in the graphics library. First, we wrote a simple program that drew an "X" on the screen. See output.

• We wrote a program that drew many concentric circles on the screen. See output.

• We can also use DrawArc to draw parts of circles. We wrote a program that drew arcs starting a 0 degrees with increasing sweep. See output. We also tried a program that always drew 90 degree arcs, but at different starting point. This required us to use the sin and cos functions from the math library. See output.

• This graphics package makes it particularly easy to draw rectangles with rounded corners. See program and output.

• We can also write a program that draws more complicated objects. In this case, we drew a pumpkin. We cleaned up the code and wrote a program that did not use so many magic numbers.

• We can also turn the main program of the previous version into a function. Then, we can write a program that calls this function several times to draw many pumpkins.

• We can modify the program to draw pumpkins of different sizes. We can also modify the program to draw pumpkins of different widths.

• Finally, we commented on this graphics package. We pointed out that it is device independent and could be used to generate output for the laser printer. (Viewing this file requires a postscript previewer, such as ghostscript on UNIX.) Another issue in interface design is stability. If an interface keeps changing, then programs written for previous versions of the library will not work. For example, the DrawArc function for a previous version of the graphics libary assumed that the pen will be at the center of the arc rather than the arc itself. So, programs written for this previous version will not work with the current version of the graphics library. The old pumpkin drawing programs produces strange output when they are compiled with the new library.