1. Algorithms IV: Top-Down Design
Topics
more pseudocode
problem solving strategies
Home-Exercise: The Box
Home-Exercise: Drawing a Rectangle
Reading
None

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

3. Investigating Steps 1 & 5 Get an 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. Problem Solving Strategies
If we look at the problem as a whole; it may seem impossible, because it is so complex.
Complex problems can be solved using a Top-Down approach aka Stepwise refinement
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 down 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.
What if you’ve broken a problem down as much as you can and you still don’t know?
Missing information
More detail is needed

7. A Practical Example Scenario: We own a home improvement company
We do painting, roofing and basement waterproofing
A section of town has recently had flooding
We want to send out waterproofing pamphlets to our customers in that area

8. Our Algorithm Get the customer list from a file
Sort the list according to zipcode
Make a new file of only the customers with the zipcode from the sorted customer list
Print an envelope for each customer in the new file

9. Single steps ? None of our steps are single steps
Employ Top-Down Design
Start with the large idea and break into large parts
Each part will end up being a function or a procedure

10. Pseudocode for our Main Program
Call <Get Customer List>
Call <Sort on customer list> by zipcode
Call <Make Target List> from customer list using only zipcode 21222
Call <Print Envelopes> using target list

11. The Next Step
Write algorithms for each of the functions or procedures.
These may include steps that are really several steps, too.
Give names to these new functions and procedures.
Write algorithms for them.
Continue this process until done.

12. Structured Programs
Top-down design will be used for all of our programming projects
This is the standard way of writing programs.
Programs produced using this method and using only the 3 kinds of structures, sequential, selection and repetition are called structured programs.
Structured programs are easier to modify later and also easier for other programmers to understand.

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

14. The Algorithm Draw the outline of the house Draw the chimney
Draw the door and doorknob
Draw the windows

15. Similarities All three windows are identical.
They are simply in different positions.
We can write a function called <DrawWindow> that will draw one window at a position we specify and call it 3 times with the three different positions.
Another benefit of Top-down design is reuseability. We are reusing the code for <DrawWindow>.

16. The Pseudocode Call <DrawHouseOutline> giving a position
Call <DrawChimney> giving position
Call <DrawDoor> giving position
Call <DrawWindow> giving position #1
Call <DrawWindow> giving position #2
Call <DrawWindow> giving position #3

17. Problem Solving Techniques
We have investigated three problem solving techniques:
Multiple attacks
Working backwards
Top-down design

18. Writing Algorithms from Scratch
Do the following 2 projects as exercises at home:
Given a problem statement, write the corresponding generic algorithm for the solution.
Use the following procedure:
Determine the algorithm inputs and outputs
Pseudocode a rough algorithm
Complete the pseuodocode

19. The Box
Problem: Write an interactive program to compute and display the volume and surface area of a box. The program must also display the box dimensions. Error checking should be done to be sure that all box dimensions are greater than zero.

20. Drawing a Rectangle
Problem: Write an interactive program that will draw a solid rectangle of asterisks (*). The program must also display the dimensions of the rectangle. Error checking must be done to be sure that the dimensions are greater than zero.