Algorithms IV: Top-Down Design

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, ltnum1gt, from the user
• 2. Get an integer, ltnum2gt, from the user
• 3. ltSumgt ltnum1gt ltnum2gt
• 4. ltaveragegt ltsumgt / 2
• 5. Display The average of ltnum1gt and
  ltnum2gt is ltaveragegt
• Steps 1, 2 5 will make use of code found in the
  run-time library.

3
Investigating Steps 1 5

• Get an integer, ltnum1gt, from the user
• Display The average ltaveragegt.
• 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

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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

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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

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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

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Pseudocode for our Main Program

• Call ltGet Customer Listgt
• Call ltSort on customer listgt by zipcode
• Call ltMake Target Listgt from customer list using
  only zipcode 21222
• Call ltPrint Envelopesgt using target list

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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.

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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.

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Another Example

• Problem Write a program that draws this picture
  of a house.

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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 ltDrawWindowgt 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
  ltDrawWindowgt.

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The Pseudocode

• Call ltDrawHouseOutlinegt giving a position
• Call ltDrawChimneygt giving position
• Call ltDrawDoorgt giving position
• Call ltDrawWindowgt giving position 1
• Call ltDrawWindowgt giving position 2
• Call ltDrawWindowgt giving position 3

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Problem Solving Techniques

• We have investigated three problem solving
  techniques
• Multiple attacks
• Working backwards
• Top-down design

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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

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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.