Top Down Design

• Pseudocode
  • pseudocode - a mixture of English and computer language that can be used to describe an algorithm
  • Input, output
  • if (condition) then else
  • Write pseudocode compare two numbers
  • while (condition) do
  • Write code to find the largest number in a list, first given the number of numbers
  • repeat until (condition)
  • Given a set of letters, see if they are a palindrome
  • Top down design
    • Also known as functional decomposition
    • A process for solving problems
    • Really just divide and conquer
      • Start with an abstract problem
      • Break it into smaller problems
      • repeat until the problem becomes manageable
    • In CS we sometimes refer to the smaller problems as modules
      • These are usually self-contained
      • They have data inputs and outputs
      • They solve the subproblem
    • We move from the abstract to the concrete
    • To accomplish this, we can build a structure chart (p 152)
    • Example: Plan a vacation
      • Plan a vacation
        • Select Date
          • Check Schedule for work
          • Check Social Schedule
        • Select Destination
          • Decide on desired climate
          • Decide on desired activities
          • Decide on price range
        • Make Reservations
          • Decide on price
          • Decide on required options
          • Check availability
        • Pack
      • Notice, data flows throughout this process
    • A computer example: Find the area under the curve x²+3x+2, for any starting and ending point.
      • We usually work the problem out on paper first.
      • Remember Problem Solving.
      • Remember Look for something we know how to do
      • Read, write, save, loop, if.
  • Testing
    • We need to somehow verify our algorithm.
    • is it correct
    • This is throughout the problem solving method
    • For TopDown Design
      • If the modules below this are correct, is this module correct?
    • Often one writes preconditions - or things that must be true before the module is called
    • and postconditions - or things that are true after the module has been called.
    • Desk Checking is verifying the design without using a computer
    • This can be done by a "walk through" or pretend to be the computer and follow the instruction you have written