Lecture 13: 10/10/2002CS149D Fall CS149D Elements of Computer Science Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 13: 10/10/2002

CS149D Fall Outline Problem Solving techniques An example to apply problem solving techniques First look at a C++ program

Lecture 13: 10/10/2002CS149D Fall 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: 10/10/2002CS149D Fall Strategies: Ask Questions When 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?

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

Lecture 13: 10/10/2002CS149D Fall Strategies: Divide and Conquer Break up large problems into smaller problems that are easier to handle (Top-Down approach) Hard problem Easy subproblem Hard subproblem Easy subproblem

Lecture 13: 10/10/2002CS149D Fall An Example 1/3 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* r 2 3.Print the value of circle area How do we represent more complex algorithms Pseudocode, flowcharts (will introduce flowcharts later)

Lecture 13: 10/10/2002CS149D Fall An Example 2/3 A divide and conquer block diagram of our problem Circle area Read radiusPrint 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 * radius 2 Write Circle area on the screen Stop

Lecture 13: 10/10/2002CS149D Fall An Example 3/3 Convert algorithm into a C++ program #include void main () { float pi = f; 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