CPSC 171 Introduction to Computer Science

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CPSC 171 Introduction to Computer Science

• Algorithm Discovery and Design

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Lab Hours, Colton 106

• Sunday Matt Young
• Monday Will Mooney
• Tuesday Cody Depew
• Wednesday Christian Ebinger
• Thursday Stefan Maurer
• Lab will be open 8-10pm starting Monday night,
  Sept. 7

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Reading Assignment

• Chapter 1 in Textbook

Homework Assignment
page 34 of Textbook Problems 5,7,8,9,13 Due Sept
4 at beginning of class
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Problem 3 Page 34

• Read problem in book
• Follow along on board

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An Algorithm is

• A well-ordered collection of
• Unambiguous and
• Effectively computable operations that, when
  executed
• Produces a result and
• Halts in a finite amount of time

Textbook definition
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Representing Algorithms

• How do we represent algorithms?
• English
• Programming Language
• We will start with pseudocode and later will look
  at various programming languages.

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Pseudocode

• Set of English Constructs designed to resemble
  statements in programming languages
• Steps presented in structured manner
• Simple, Highly readable

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Constructs We Will Use

• Sequential operations to carry out
• computation,
• input, and
• output.
• Conditional operations.
• Iterative operations.

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Sequential Operations

• computation
• Set the value of X to 3.
• Assign X a value of A B.
• X2 - C.
• Set the value of Name to the first person's name.
• input
• Get a value for X, Y, and Z.
• Input values for A1, A2, ..., Am.
• Read X, Y, and Carry.
• output
• Output the value of X.
• Print values for X, Y, and Carry.
• Print the message, "Error".

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Example Algorithm (Sequential)

• Figure 2.3
• Algorithm for Computing Average Miles per Gallon

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Practice

• Write an algorithm in pseudocode to
• Get the radius of a circle as input and output
  the circumference and area of the circle

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Conditional Operation

• If a true/false condition is true then
• first set of algorithmic operations
• Else
• second set of algorithm operations

Notice the indentation
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Example Algorithm (conditionals)

• Figure 2.5
• Second Version of the Average Miles per Gallon
  Algorithm

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Practice

• Modify your algorithm to print large circle if
  the circumference is greater than 100 and small
  circle if it is equal to or less than 100.

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Conditional Operation Picture
true-false statement
true
false
true branch
false branch
operation
operation
operation
Note either branch can be missing
operation
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Conditional Operation Picture
true-false statement
true
false
true branch
false branch
operation
operation
operation
Note either branch can be missing
operation
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Conditional Operation Picture
true-false statement
true
false
true branch
false branch
operation
operation
operation
Note either branch can be missing
operation
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Iterative Operations

• Set count to 5
• While count lt 4
• Add 1 to count
• Print count
• stop

Set count to 5 Do Add 1 to count Print
count While count lt 4 stop
Pretest loop
Posttest loop
Notice when the continuation condition is checked
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Infinite Loops

• Set count to 0
• While count gt 0
• Add 1 to count
• Print count
• stop

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Iteration Operation Picture

true-false statement
false
operation
true
operation
operation
operation
true-false statement
true
false
while loop
Do/while loop
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Example Algorithm (iteration)

• Figure 2.7
• Third Version of the Average Miles per Gallon
  Algorithm

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Practice

• Write an algorithm that gets as input a single
  data value, x, and outputs the three values x2,
  sin x, 1/x. This process should repeat until the
  input value for x is equal to 99.

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Summary of Operation Typesfor Algorithms

• Sequential operations to carry out
• computation,
• input, and
• output.
• Conditional operations.
• Iterative operations.

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There is a theorem in theoretical computer
science that proves that these operations are
sufficient to represent ANY algorithm! Algorithms
are used to do everything you see on a computer!

• Do word processing.
• Fly NASA probes to the planets.
• Run the international telephone switching system.
• Create CAT scan images.
• Process your pay checks.
• Run computer games.
• Etc.