Lecture 13: 10/10/2002 CS149D Fall 2002 1

CS149D Elements of Computer Science

Ayman Abdel-Hamid

Department of Computer Science

Old Dominion University

Lecture 13: 10/10/2002

Lecture 13: 10/10/2002 CS149D Fall 2002 2

Outline•Problem Solving techniques

•An example to apply problem solving techniques

•First look at a C++ program

Lecture 13: 10/10/2002 CS149D Fall 2002 3

Problem Solving Techniques

•You follow algorithms every day in your life

•We need to learn how to design algorithms not simply follow them

•Some Strategies to solve problems

Ask questions

Look for things that are familiar

Means-Ends Analysis

Divide and Conquer

Lecture 13: 10/10/2002 CS149D Fall 2002 4

Strategies: Ask QuestionsWhen you are given a problem, you ask questions (What, Why, When, and Where?)

In the context of programming

•What do I have to work with (What is my data)?

•What do the data items look like?

•How much data is there?

•How will I know when I have processed all the data?

•What should my output look like?

•How many times is the process going to be repeated?

•What special error conditions might come up?

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Strategies: Look for Familiar Things

•Never reinvent the wheel

If a solution exists USE IT

Finding the daily high and low temperatures

is really the same problem as

Finding the highest and lowest grades on a test

Both problems can be abstracted as being

Find largest and smallest values in a set of numbers

Lecture 13: 10/10/2002 CS149D Fall 2002 6

Strategies: Means-Ends Analysis

•Beginning state and End state are often given

•You need to define a set of actions that can be used to get from one to the other

•Once you have a set of actions, you need to work out the details

Translated to computer programming

Begin by writing down what the input is? (Beginning state)

What the output should be? (End state)

What actions can be performed to obtain results from input data?

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Strategies: Divide and ConquerBreak up large problems into smaller problems that are easier to handle

(Top-Down approach)

Hard problem

Easy subproblem

Easy subproblem

Hard subproblem

Easy subproblem

Easy subproblem

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An Example1/3

Compute the area of a circle

Problem statement

We need an interactive program (user will input data) that computes the area of a circle. Given the circle radius, the circle area should be displayed on the screen

Input/Output description

Input Circle radius

Output Circle area

Algorithm development (set of steps, decomposition outline)

1. Read value of circle radius (r)

2. Compute circle area as pi* r2

3. Print the value of circle area

How do we represent more complex algorithms

Pseudocode, flowcharts (will introduce flowcharts later)

Lecture 13: 10/10/2002 CS149D Fall 2002 9

An Example2/3

A divide and conquer block diagram of our problem

Circle area

Read radius Print circle areaCompute area

Pseudocode

Prompt the user for the circle radius (put a message on the screen)

Read radius

Assign Circle area the value pi * radius2

Write Circle area on the screen

Stop

Lecture 13: 10/10/2002 CS149D Fall 2002 10

An Example3/3

Convert algorithm into a C++ program

#include <iostream.h>

void main ()

{

float pi = 3.14159f;

float radius, area;

cout << "Enter the radius of the circle: ";

cin >> radius;

area = pi* radius * radius;

cout << "The area of the circle is: " << area << endl;

}

Let’s look at that program in Microsoft Visual C++ environment