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					           Algorithms IV



           Top-Down
            Design



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        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
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                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:
           o Functions - an answer is returned to the
             main program.
           o Procedures - no answer returned to the
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           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.



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                 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
           o Break the problem into parts
           o Then break the parts into parts
           o 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 you’ve broken a problem down as
           much as you can and you still don’t know?
            o Missing information
            o More detail is needed
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                   A Practical Example

          Scenario:
           o We own a home improvement company
           o We do painting, roofing and basement
             waterproofing
           o A section of town has recently had flooding
           o 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 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




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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




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                    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.
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                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



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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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posted:9/18/2011
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