Planning a Program

1
Planning a Program

• HI5100 Data Structures for BioInformatics
• Lesson 3

2
OBJECTIVES

• In this lesson you will learn
• Steps that programmers use to plan a program
  before they write code.
• How to use the Input-Process-Output model to
  understand parts of programs.

3
Lesson 3 Sections

• 1.1 Why Plan?
• 1.2 Stages of Program Planning
• 1.3 Features of User-Friendly Apps
• 1.4 Logic Structures Used in Programs
• 1.5 Summary

4
What is a Computer Program?

• Step-by-step instructions that tell the computer
  what to do
• There can be no ambiguity in the statements
• Thus, strict syntax rules exist for every
  programming language
• Programs are read one character at a time
• White space (space, tab, return) separates
  words
• Words are compared to the rules of the language
  and interpreted (translated into machine language)

5
Simple Sequential Instruction Set

• Click on the Start button in the lower right hand
  corner of the screen
• Click on Enterprise Manager icon in the left
  panel to start up Enterprise Manager

6
Sometimes A More Complex Structure of
Instructions is Needed
Where is Enterprise Manager? I dont see it. What
do I do now?
7
More Complex Instruction Set Structure

• Click on the Start button in the lower right hand
  corner of the screen
• Do you see an Enterprise Manager icon in the left
  panel?
• If you answered yes, then click on the icon to
  start up Enterprise Manger
• If you answered no, then

8
Program Complexity

• Modern programs often accomplish very complex
  tasks
• Programs that accomplish complex tasks are very
  complex
• Thus, the process of creating a program often
  requires significant planning prior to writing

9
Development Stages

• Analyze
• Write specifications
• Design
• Implement/Debug/Implement
• Test/Debug/Modify/Test/Debug
• Deploy and Maintain

10
Analyze the Problem

• Analyze the problem to be solved
• Break it down into smaller tasks
• What input is required?
• What will be the output?
• Your goal is to understand the problem from the
  users perspective

11
Write Specifications

• Describe what the program will do
• At this stage, you are not planning how it will
  do things
• Specify inputs
• Specify outputs
• Specify how inputs relate to outputs

12
Design the Program

• Define subtasks and inputs and outputs for each
• Design flow of data and processing through the
  subtasks
• Develop the logic of the program
• Develop algorithms to perform tasks

13
Implement

• Translate the design into code
• Use software to compile the program
• With Python we will use the Python interpreter to
  translate the program line-by-line rather than
  create a compiled version
• Find syntax errors

14
Typos and syntax errors often occur when
translating a design into code

• Sources of Error

15
Syntax Errors

• Every programming language requires that programs
  follow a set of rules for syntax
• Compilers identify errors made in syntax
• Some IDEs identify some errors made in syntax as
  you type before the code ever gets compiled

16
Can You Spot An Error in Syntax?

• Preheat the oven to 350 F.
• Put the cake mix into a bowl.
• Add 2 slightly beaten eggs, ¼ c. vegetable oil
  and 1 ½ c. water.
• Mix for 3 minutes using medium speed of an
  electric mixer.
• For 33 minutes pour batter into greased pan and
  bake.
• Remove from oven and let cool.

17
Can You Spot an Error that is most likely a Typo?

• Preheat the oven to 350 F.
• Put the cake mix into a bowl.
• Add 2 slightly beaten eggs, ¼ c. vegetable oil
  and 1 ½ c. water.
• Mix for 30 minutes using medium speed of an
  electric mixer.
• Pour batter into greased pan and bake for 33
  minutes.
• Remove from oven and let cool.

18
Implement

• Once translated into code and compiled, the
  syntax errors and typos are gone
• The program will not execute if there are syntax
  errors and typos
• Input into the computer for testing
• The apps we create can only run on a program that
  has the Python interpreter installed
• If we compiled them down to .exe form, then they
  could run on any computer with the appropriate OS

19
Test and Debug

• Testing and debugging of complex programs should
  be integrated with implementation
• During coding, you are testing for typos and
  syntax errors
• After typos and syntax errors are gone, there
  still can be errors

20
Errors in Logic

• The program runs, but the output is not correct
  with respect to the problem the program is
  supposed to solve

21
No Syntax Errors, No Typos, No Logical Design

• Mix for 3 minutes using medium speed of an
  electric mixer.
• Preheat the oven to 350 F.
• Pour batter into greased pan and bake for 33
  minutes.
• Put the cake mix into a bowl.
• Remove from oven and let cool.
• Add 2 slightly beaten eggs, ¼ c. vegetable oil
  and 1 ½ c. water.

22
Deploy and Maintain

• When the program is thoroughly tested and revised
  based on testing, it is ready to be employed by
  users to accomplish tasks
• Often new programs are deployed to a small pilot
  group of typical users
• Pilot users provide feedback
• Suggestions for improvement
• Bugs not found during testing
• Provide all users with patches and updates

23
User-Friendly Programs

• Modern applications employ a GUI (graphical user
  interface)
• At this point, we will not begin investigating
  how to create GUIs
• If the user cannot use the program without
  extensive instruction, the program is not
  user-friendly

24
Features for User-Friendliness

• Print an introduction that tells the user what
  task the program completes
• Print any instructions the user needs
• Try to avoid creating a program that can only be
  used after reading a lengthy set of instructions

25
Three Basic Logic Structures for Programming

• A programming structure is a unit of programming
  logic
• Three basic structures
• Sequence
• Selection ? Decision
• Loop ? Iteration ? Repetition
• Computer scientists have shown that you can solve
  any problem of logic using only these three
  structures

26
Sequence

• Line by line execution
• Execute line 1, followed by line 2, followed by
  line 3,

27
Sequence

• Graphical representation
• Flowchart elements
• Terminator (oval)
• Process (rectangle)

28
Selection ? Decision

• The terms selection and decision are synonyms
  with respect to programming
• Ask a question
• Two courses of action can be taken, depending on
  the answer to the question

29
Selection ? Decision
30
Selection ? Decision

• Decision (diamond)
• In code, a basic decision structure often
  involves the key words if then - else

31
Loop ? Iteration ? Repetition

• Repeat a block of code over and over until a
  condition is met
• Code
• Decision no
• Same code
• Decision no
• Same code
• Decision yes
• Continue with something else

32
Loop ? Iteration ? Repetition

• The process is performed only if the decision is
  true
• It continues to be performed until the decision
  is false

33
Additional Logic Structures for Programming

• While computer scientists have shown that you can
  solve any problem of logic using only the three
  structures of sequence, decision, and loop,
• Most modern programming languages allow three
  more structures (for each of programming)
• Case
• Do While
• Do Until

34
Case

• There is a decision variable
• Case 1 is executed when the decision variable has
  a certain value
• Case 2 is executed when the decision variable has
  another value
• Case 3 is executed when the decision variable has
  a third value

35
Case
36
Do While and Do - Until

• With the basic loop structure, the decision is
  made before the code block is executed
• Thus the code block may not be executed at all if
  the decision is FALSE originally
• With do while and do until loops the decision
  is made after the code block is executed at least
  once

37
Do While and Do Until

• The process is performed and then the decision
  question is evaluated
• Based on the evaluation result the process is
  repeated or not

38
Summary

• You are now familiar with some basic ideas about
  how to plan a program and the logic structures
  that are often used in this process
• Each logic structure has an interpretation in the
  programming language of choice, in our case,
  Python
• We will look at Python syntax for the basic logic
  structures in the next lesson

39
End of Slides for Lesson 3

• HI5100 Data Structures for BioInformatics
• Lesson 2