CS149D Elements of Computer Science

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

• Problem Solving techniques
• An example to apply problem solving techniques
• First look at a C program

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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

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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?

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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

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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 Conquer
Break 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)
• Read value of circle radius (r)
• Compute circle area as pi r2
• Print the value of circle area
• How do we represent more complex algorithms
• Pseudocode, flowcharts (will introduce flowcharts
  later)

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An Example2/3
A divide and conquer block diagram of our problem
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
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An Example3/3
Convert algorithm into a C program include
ltiostream.hgt void main () float pi
3.14159f float radius, area cout ltlt "Enter
the radius of the circle " cin gtgt
radius area pi radius radius cout ltlt
"The area of the circle is " ltlt area ltlt endl
Lets look at that program in Microsoft Visual
C environment