Algorithms IV

1
Algorithms IV

• 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 youve broken a problem down as much as
  you can and you still dont 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