Chapter 1: An Introduction to Computer Science

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Chapter 1 An Introduction to Computer Science

• Invitation to Computer Science,
• C Version, Third Edition

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Objectives

• In this chapter, you will learn about
• The definition of algorithm
• The definition of computer science
• Overiew of the course

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What is an Algorithm?

• 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.

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Example of Algorithm

• An algorithm is a list that looks like
• STEP 1 Do something
• STEP 2 Do something
• STEP 3 Do something
• . .
• . .
• . .
• STEP N Stop, you are finished

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The Formal Definition of an Algorithm

• Algorithm
• A well-ordered collection of unambiguous and
  effectively computable operations that, when
  executed, produces a result and halts in a finite
  amount of time
• Example Make Pasta
• Boil two quarts of water
• Add ½ lb of pasta in the water
• Strain the water
• Add the sauce

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The Formal Definition of an Algorithm (continued)

• Unambiguous operation
• An operation that can be understood and carried
  out directly by the computing agent without
  needing to be further simplified or explained
• Ex.
• Boil some water (ambiguous)
• Boil 2 quarts of water (unambiguous)
• An operation that is unambiguous is called a
  primitive operation (or just a primitive)
• Q. - What are the primitives of a computer?
• A. -

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The Formal Definition of an Algorithm (continued)

• Effectively computable
• Computational process exists that allows
  computing agent to complete that operation
  successfully
• Note that a given collection of operations may be
  an algorithm with respect to one computing agent,
  but not with respect to another computing agent!!
• Example of not effectively computable operations
• Write all the real numbers between 0 and 1.

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The Formal Definition of an Algorithm (continued)

• The result of the algorithm must be produced
  after the execution of a finite number of
  operations
• i.e. Can the user of the algorithm observe a
  result produced by the algorithm?
• A result can be a sign, a sound, an alarm, a
  number, a message of error, ect

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The Formal Definition of an Algorithm (continued)

• It halts in a finite amount of time.
• Infinite loop
• The algorithm has no provisions to terminate
• A common error in the designing of algorithms
• Do not confuse, "not finite" with "very, very
  large".
• Q. - Is this loop finite or very, very large?
• Step 1. Write the number 1 on the board.
• Step 2. Add 1 to the number you just wrote and
  write the result on the board
• Step 3. Repeat Step 2.
• Step 4. Stop.

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The Importance of Algorithmic Problem Solving

• Algorithmic solutions can be
• Encoded into some appropriate language
• Given to a computing agent to execute
• The computing agent
• Would mechanically follow these instructions and
  successfully complete the task specified
• Would not have to understand
• Creative processes that went into discovery of
  solution
• Principles and concepts that underlie the problem

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Which of those is an algorithm?

• Write all the possible numbers on a paper.
• Fill 1 cup of sugar. Mix it to 3 cups of milk.
• Think of a number. Add 20 to it.
• Either wet your hair or lather your hair. Then
  rinse your hair.
• If your hair are dirty, wet your hair lather
  your hair rinse your hair else go back to study.

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Common misconceptions about Computer Science

• Misconception 1
• Computer science is the study of computers
• Misconception 2
• Computer science is the study of how to write
  computer programs
• Misconception 3
• Computer science is the study of the uses and
  applications of computers and software

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The Definition of Computer Science

• Computer science is the study of algorithms
  including
• 1. Their formal and mathematical properties
• 2. Their hardware realizations
• 3. Their linguistic realizations
• 4. Their applications

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1. Their formal and mathematical properties

• It is not enough to design an algorithm to solve
  a problem.
• We must worry about some additional properties of
  an algorithm
• How efficient is it?
• What kinds of resources must be used to execute
  it?
• How does it compare to other algorithms that
  solve the same problem?

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2. Their hardware realizations

• Computing entities are required to execute
  algorithms
• Computing entities are not necessarily machines!
• However our interest will lie with algorithms
  that execute on computing entities called
  "computers".
• How are computer constructed?
• The emphasis of the course will be on the logical
  construction of a computer, not the physical
  construction.

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3. Their linguistic realizations

• Which language do we use to code the algorithms?
• We will start with one linguistic realization,
  called pseudocode and later will look at possible
  different realizations in various programming
  languages.
• In particular we will do some introductory
  programming in C.

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4. Their applications

• What are some of the many important and popular
  applications of computers in current use
  including
• numerical problem solving
• information retrieval
• graphics
• networking
• artificial intelligence
• telecommunications
• modeling and simulation

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Overview of the course

• 1. The algorithmic foundations of computer
  science. (Ch 1-3)
• 2. The hardware world. (Ch 4-5)
• 3. The virtual machine. (Ch 6-7)
• 4. The software world. (Ch 8-11)
• 5. Applications. (Ch 12-14)
• 6. Social issues. (Ch 15)
• Note these correspond to the levels of the
  pyramid on the cover of your text.
• Each level addresses one aspect of the definition
  of computer science
• Computer science/Algorithms

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• Figure 1.9
• Organization of the Text into a Six-Layer
  Hierarchy

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Summary

• Computer science is the study of algorithms
• 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
• If we can specify an algorithm to solve a
  problem, then we can automate its solution
• Computers developed from mechanical calculating
  devices to modern electronic marvels of
  miniaturization

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BE SURE TO CHECK THE CLASS WEBSITE DAILY FOR YOUR

• Reading assignments
• Homework assignments
• Other materials related to this course

http//www.personal.edu/aguercioa/Spring06/CS1005
1-600.html