Lecture Notes 82503

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Lecture Notes 8/25/03

• Program Design

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Algorithms

• What is an algorithm
• Most popular analogy is a recipe.
• Like a recipe, you follow a well-defined series
  of steps to achieve desired results.
• Example
• How do you start your car?

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Algorithms Cont

• Without knowing it, most people have an algorithm
  for starting their car.
• Unlock door
• Open door
• Sit in drivers seat
• Insert key into ignition
• Check mascara in rear-view mirror
• Turn key and hold until car is started.

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Steps in Program Development

• 1) Define the problem
• Basically any program can be looked at like a
  black box.

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Steps in Program Development...

• Outline the solution
• Here we delve into the black box. Once we know
  what we need to do, its time to start designing
  a solution.
• The outline may include
• Major processing steps involved
• Major subtasks
• Major Control structures (e.g. Repetition loops)
• Major variables and record structures
• The mainline logic

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Steps in Program Development

• Develop the outline into an algorithm
• The algorithm should represent the precise steps
  to be taken and the order in which they should be
  carried out.
• For this class, algorithmpsuedocode
• We will be covering psuedocode over the next 2-3
  lectures Fun fun fun!!!!

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Steps in Program Development

• Test the algorithm for correctness.
• Run through the psuedocode and make sure it makes
  sense.
• Example
• Start your car
• Unlock door
• Open door
• Sit in drivers seat
• Put Car in reverse
• Place brick on accelerator
• Turn key and dive out of drivers side window

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Steps in Program Development

• Code the algorithm into a specific programming
  language.
• C
• C
• Java
• Perl
• Fortran
• Cobol
• Etc

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Steps in Program Development

• Run the program on the computer
• Here we can detect the two main types of errors.
• Syntax errors (program wont run)
• Logic errors (program runs but doesnt work
  properly)
• This step can be quite fun.
• Its analogous to building a model rocket. You
  put it all together, but you really dont know
  what will happen until you light the fuse.
• If it flies, you get to enjoy watching it.
• If it doesnt, you get to try to figure out why
  (hopefully without blowing your fingers off).

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Steps in Program Development

• Document and maintain the program
• The two main types of documentation are
• External (algorithm, test data results, etc)
• Internal (commenting your code)

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Stuctured Programming

• Top Down Development
• Instead of jumping right in, programmers have
  found that using a more systematic approach is
  much more powerful.
• We look at major tasks and break those tasks into
  subtasks,
• Then we break the subtasks into further subtasks.
• We repeat until the subtasks are manageable.

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Structured Programming

• Modular Design
• Subtasks are often repeated. So it is useful to
  modularize them so that we dont have to reinvent
  the wheel every time we need to do the same
  subtask.
• Example. Filling out paper-work for a new job.
• Honestly, how many times do you need to write
  your name and address, and then sign a piece of
  paper!?

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Structured Programming

• Structure theorem
• Any computer program can be written using the
  following three basic control structures
• Sequence
• Selection (If-Then-Else)
• Repetition (DoWhile)

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Psuedocode

• This book establishes a standard for psuedocode
  which we will follow in this class.
• Note There really is no standard for psuedocode
  that everyone knows or follows

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Pseudocode

• Five standards this book follows
• Statements are written in simple english
• Each instruction is written on a separate line.
• Each set of instructions is written from top to
  bottom, with only one entry and one exit.
• Groups of statements may be formed into modules,
  and that group given a name.

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Program Data

• Variables, constants, and literals
• Variables
• Names given to represent data. That data can be
  manipulated and changed.
• Constants
• Names given to represent data. That data CANNOT
  be manipulated or changed.
• Literals
• Basically, a constant whose name is the written
  representation of its value.

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Elementary Data Items

• This is what we will deal with for most of the
  semester.
• One value is represented as one of the
  aforementioned types of program data.
• E.g. The integer 10 may be represented one of
  three ways
• Ten (variable)
• Ten (constant)
• 10

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Data Structures

• The only data structures we will cover in this
  class are
• File
• Array
• String
• In CS 202, record data structures are covered.
  These are things like structs and classes.

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Summary

• Now that were all utterly confused -)
• This lecture is simply an introduction to some
  ideas.
• At this point, you are not really expected to
  know anything about programming. I just want to
  get you thinking about how we really go about
  problem solving.
• Read Chapters 1 2 in Simple Program Design.