Algorithms III

1. Algorithms III Top-Down Design

2. Pseudocode and Then Some • We can write an algorithm for finding the average of two integers: Get an integer, num1, from the user Get an integer, num2, from the user sum = num1 + num2 average = sum / 2 Print “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 Print “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. More Problem Solving • 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 21222 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