ALGORITHMS

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ALGORITHMS

• CSC 171
• LECTURE 1

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What is Computer Science?

• What is Science?
• What is a Computer?
• What is Computation?

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Computer Science

• Is Computer Science, science?
• Is computation part of nature?
• Is information part of nature?
• How do we study computation?

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Computational Science

• A mode of scientific investigation that includes
  several phases
• Development of a mathematical model (equation) of
  some physical phenomenon or system
• Development of an algorithm to solve the model
• Implementation of the algorithm in software
• Simulation of the phenomenon using software
• Graphical visualization of the results
• Interpretation and validation of the results,
  which may lead to refinement of the original
  model and repetition of the cycle

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History

• Muhammad ibn Musa Al'Khowarizmi (ca. 825)
• Described rules to be followed for arithmetics
  using Hindu numerals.
• A written process to be followed to achieve some
  goal
• Translated into Latin in 1202 by Leonardo
  Fibonacci

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ALGORITHM

• An algorithm is a procedure for solving problems
  in terms of
• The actions to be executed (statements)
• The order in which these actions are executed
• Algorithms are independent of any specific
  computer language which may be used to for their
  implementation.
• Often expressed informally as pseudocode.

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Some Algorithms

• How do I get to the pit?
• Your classmate
• Your parents
• A robot
• Move forward x meters
• Turn right y radians

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Abstraction

• To one computer
• x y 5

• To another
• Load y in accumulator
• Load 5 in register A
• Add register 5 to accum
• Store accum in x

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Statements

• What do we need for Statements
• Syntax
• the form in which statements may be written
• Semantics
• meaning attached to syntactic constructs

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Example

• Syntax
• F (9/5) C 32
• 2 F 3)C 9 / 5(
• Semantics
• Take the Celsius temperature
• Multiply it by 1.8
• Add 32
• The result is the Fahrenheit temperature

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Example Finding square roots

1. Determine the value you want a square root of
   call it A
2. Start with 1 call it a guess
3. Divide A by the guess call the result
   second_guess
4. If guess is not close to second_guess, then set
   your guess to the average of guess and second
   guess and go to step 3.
5. All done (your last guess is pretty good)

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Square Roots

• g11
• A50
• g250/150
• g1(150)/225.5
• g250/25.51.96
• g1(25.51.96)/213.73
• g250/13.733.641

• g1(13.733.641)/2 8.68
• g2 50/8.685.76
• g1(8.685.76)/27.22
• g250/7.226.92
• g1(7.226.92)/27.07
• g2(50/7.07)7.072
• All done

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Language

• What does language do?
• Transfer information (describe)
• Action oriented languages (directions)
• Functional languages
• Area Pi Radius Radius
• Object oriented languages
• model the world around us

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Computer Languages

• To a human we might say
• Write Hello, CSC 171
• To a computer we might say
• System.out.println(Hello, CSC 171)

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History

• 1612 Napier invented and decimal points and
  logarithms.
• 1622 William Oughtred created the slide rule

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History

• 1622 William Oughtred created the slide rule

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A Computer Program

• public class myFirstProgram
• public static void main(String args)
• System.out.println(Hello, CSC171)

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Programs in General

• Programs contain 3 types of things
• Comments notes to humans
• Headers references to other code
• Code instructions to humans

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The Containment Pattern

• Starts with some begin symbol
• (
• Ends with some end symbol
• )
• General Pattern ltbegingtltcontentsgtltendgt

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Named containment

• ltheadergt ltcontainmentgt
• ltheadergt ltbegingtltcontentsgtltendgt
• Often used as a mechanism of abstraction

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The sequence pattern

• A sequence is a list of one or more instructions
  to be followed in order
• Sequence can be empty or
• Sequence can be
• ltinstructiongt ltsequencegt
• ltinstructiongt ltinstructiongt
  ltsequencegt
• Etc
• Robot example complex instructions

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Objects

• A class of things are objects with similar
  attributes
• Objects are instances of a class
• A class is a general description of an object
• A cookie cutter defines a class of cookies
• An cookie is object
• Rubber stamp

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myFirstProgram.java

• Contains a class definition, which contains
• A method named main, which contains
• An instruction to the System.out object
• The instuction is a comand to print something
• That something is a String of characters
• Hello, CSC171 is a data item