1. Algorithms, Part 3 of 3 Topics Top down-design Structure charts
Reading
Sections , 3.3

2. Pseudocode and Then Some
We can write an algorithm for finding the average of two integers:
Read integer <num1>, from the user
Read integer <num2> from the user
<sum> = <num1> + <num2>
<average> = <sum> / 2
Display “The average of <num1> “and”
<num2 > “is” <average>.
Steps 1, 2 & 5 will make use of code found in the run-time library. 1 2 3 4 5

3. Investigating Steps 1 & 5 Read integer <num1>, from the user
Display “The average =” <average>.
Are these really single steps?
Each of them is really a sequence of steps, a subprogram. Two kinds of subprograms:
Functions - an answer is returned to the main program.
Procedures - no answer returned to the main program

4. Functions and Procedures
In our example, the function that gets a value from the user and the procedure that prints out the answer were already written.
Many times it is necessary for us to write our own procedures or functions.

5. Top-Down Design
If we look at a problem as a whole, it may seem impossible to solve because it is so complex. Examples:
writing a tax computation program
writing a word processor
Complex problems can be solved using top-down design, also known as stepwise refinement.
We break the problem into parts
Then break the parts into parts
Soon, each of the parts will be easy to do

6. Advantages of Top-Down Design
Breaking the problem into parts helps us to clarify what needs to be done.
At each step of refinement, the new parts become less complicated and, therefore, easier to figure out.
Parts of the solution may turn out to be reusable.
Breaking the problem into parts allows more than one person to work on the solution.

7. An Example of Top-Down Design
Scenario:
We own a home improvement company.
We do painting, roofing, and basement waterproofing.
A section of town has recently flooded (zip code 21222).
We want to send out pamphlets to our customers in that area.

8. The Top Level Get the customer list from a file.
Sort the list according to zip code.
Make a new file of only the customers with the zip code from the sorted customer list.
Print an envelope for each of these customers.
Main
Read
Sort
Select
Print

9. Pseudocode for our Main Program
Start
Call Read Customer List
Call Sort on customer list by zipcode
Call Select Target List from Customer List using only zipcode 21222
Call Print Envelopes using target list
Stop

10. Another Level?
Should any of these steps be broken down further? Possibly.
How do I know? Ask yourself whether or not you could easily write the algorithm for the step. If not, break it down again.
When you are comfortable with the breakdown, write pseudocode for each of the steps (modules) in the hierarchy.

11. Structured Programs
We will use top-down design for all of our programming projects.
This is the standard way of writing programs.
Programs produced using this method and using only the three kinds of control structures, sequential, selection and repetition, are called structured programs.
Structured programs are easier to test, modify, and are also easier for other programmers to understand.

12. Another Example
Problem: Write a program that draws this picture of a house.

13. The Top Level Draw the outline of the house Draw the chimney
Draw the door
Draw the windows
Main
Draw
Outline
Draw
Chimney
Draw
Door
Draw
Windows

14. Observation
The door has both a frame and knob. We could break this into two steps.
Main
Draw
Outline
Draw
Chimney
Draw
Door
Draw
Windows
Draw
Door Frame
Draw
Knob

15. Another Observation There are three windows to be drawn. Main Draw
Outline
Draw
Windows
Draw
Window 1
Draw
Window 2
Draw
Window 3
(x1,y1)
(x2,y2)
(x3,y3)

16. Code Reuse
But don’t the windows look the same? They just have different locations.
So, we can reuse the code that draws a window.
Simply copy the code three times and edit it to place the window in the correct location, or
Use the code three times, “sending it” the correct location each time (we will see how to do this later).
This is an example of code reuse.

17. The Pseudocode Start Call DrawHouseOutline giving a position
Call DrawChimney giving position
Call DrawDoor giving position
Call DrawWindow giving (x,y) position #1
Call DrawWindow giving (x,y) position #2
Call DrawWindow giving (x,y) position #3
Stop

18. Summary
Use the divide-and conquer strategy to split the original problem into a series of sub-problems.
Consider each sub-problem separately and split them further into smaller sub-problems, progressively refining the previous versions in a top-to-bottom manner, until no further refinement is possible. Stop the process when you have the pseudocode that can be directly translated in a C program.

19. Exam 1 All material before this page is “fair game” for the midterm.
Exam 1 will be on Wednesday October 10th
If you know that you can not make the exam, tell me today after class, so that we can schedule a possible makeup.
Only very SERIOUS conflicts will be honored.