Chapter 1 Analysis and Design

Page last updated 08/23/99

Goals of the chapter

Understand an example of program development
Understand the characteristics of a good algorithm
Understand how algorithmic patterns can help design programs
Provide the deliverables for two phases of program development: analysis and design

Exercises, Analysis and Design Projects

1.1 Program Development

One program development strategy has these three steps:

Step	Process	Deliverables
Analysis	Understand the problem	A document that lists the useful data objects
Design	Organize the solution	An algorithm that outlines a solution
Implementation	Get the solution running	An executable program

Program development is the progression from analysis to design to implementation

1.2 Analysis

Synonyms of analysis

inquiry, examination, study

Activities of analysis

Read and understand the problem statement.
Name the pieces of information necessary to solve the problem.

these names of data are part of the solution ("variables").

Active Learning Example

Compute the maximum airplane takeoff weight.

Name a data object that stores the answer: _________________________
Name the data object(s) that must be supplied to compute that answer: _________________________

There is more to these data than just the names

The data things are called objects and have these three important characteristics:

a name
state more commonly known as value(s)
a set of operations to manipulate the state (the values); the operations may be called methods or functions

How are these objects used?

Output: Information that the computer must display after the processing has occurred.
Input: Information that the user must supply to solve the problem.
Objects may also be used for temporary storage or calculation

Example of Analysis

Find the length of a line segment.

The line is represented by Cartesian coordinates of its end points, e.g. (1,1) to (5,4)

The variables or objects to hold this data are: _______________________________

The formula for the length of a line segment is = 5

A variable to hold this calculation: ________________________

What are input variables and output variables? ____________________________________

More Examples

Consider the following problems. What variables would be necessary in the solution? Choose useful names and give a sample value for each. Would that variable be used as input or output?

Problem Description	Object Name / Sample Data	Input or Output?
Compute the average of three test scores
Compute the roots of a quadratic equation: ax² + bx + c
Compute a loan payment

Summary of Analysis

Activities performed during analysis

Read and understand the problem (ask questions, dialog with users)
Decide what object(s) represent the answer—the output
Decide what object(s) the user must enter to get the answer—the input
Create a document that summarizes the analysis. This document is used for the next phase of program development—design.

Analysis Example

Using the grade assessment scale (weighted average) below, compute the weighted average for any combination of two tests and a final.

Test 1 is 25%
Test 2 is 25%
Final is 50%

Objects	Sample Problem 1	Sample Problem 2	Input or Output

1.3 Design

Synonyms of design: model, think, plan, devise, pattern, propose, outline

We'll use these design tools:

algorithms
algorithmic patterns
algorithm walkthroughs

Algorithms

An algorithm is a set of activities that solves a problem.

An algorithm must:

list activities that must be performed
list the activities in the proper order.

Algorithm Example

Bake a Cake

A recipe (a.k.a. an algorithm)
- Preheat Oven
- Grease Pan
- Mix ingredients
- Place ingredients into pan
- place pan in oven
- remove pan after 35 minutes
Switch some activities around
What's missing?

1.3.1 Algorithmic Patterns

Pattern: Anything shaped or designed to serve as a model or guide in making something else.
Algorithmic Pattern: A pattern that occurs frequently during program development.
The Input/Process/Output (IPO) Pattern is used during the case study of Chapter 1.

IPO Algorithmic Pattern

Pattern: Input/Process/Output (IPO)

Problem situation: The program requires input to generate the desired info.

Outline of IPO:

obtain input data from user
process input data
output the results

Patterns ala Alexander

"Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice."

From A Pattern Language, Christopher Alexander, Oxford University Press

1.3.2 Example of Algorithm Design

The deliverable from this design phase will be an algorithm.

The IPO patterns provides a guide to design this more specific algorithm (that is a bit sketchy:

IPO Model One Specific IPO Case Study

I)nput Obtain test1, test2, and finalExam
P)rocess Compute the course grade
O)utput Display the course grade

Refinement

Usually a number of steps must be refined into substeps.

For example, Compute the course grade might now be refined with the C++ mathematical addition + and multiplication * symbols and names for the objects:

courseGrade = test1 * 0.25 + test2 * 0.25 + finalExam * 0.50 ;

1.3.3 Algorithm Walkthrough

Suggestion: Use an algorithm walkthrough to review the algorithm

Simulate what the computer would do if given the instructions.

If input occurs, copy values by object names
if processing occurs, change an object's value
if output occurs, write that output

An example walkthrough

Input) Retrieve some example values from the user and store them into the objects as shown:

test1 is 80
test2 is 90
finalExam is 100

Processing) Retrieve input data and compute courseGrade as follows

courseGrade=(0.25*test1)+(0.25*test2)+(0.50*finalExam)

(0.25*80.0) + (0.25*90.0)+(0.50*100.0)

(20.0 + 22.5 + 50.0)

courseGrade = 92.5

Output) Show course grade to the user:

Note: Retrieve the data stored in courseGrade to show: 92.5%