Announcements The Algorithmic Model Programming Languages Data Assignment Arithmetic

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AnnouncementsThe Algorithmic ModelProgramming
LanguagesDataAssignmentArithmetic
Lecture 2
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Announcements

• Web Page
• http//www.cc.gatech.edu/classes/AY2001/cs1311/

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Announcements

• Newsgroups
• git.cc.class.cs1311.announce
• git.cc.class.cs1311.rec
• git.cc.class.cs1311.lab
• git.cc.class.cs131x.macusers
• Reading newsgroups
• On Campus Outlook Express/Netscape/tin
• Off Campus tin

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The Algorithmic Model
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The Algorithmic Model

• What is Computer Science
• What is Programming
• Algorithms
• Defined
• Properties of Good Algorithms
• Describing Algorithms
• Examples
• Components of Algorithms

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What is Computer Science

• Not really a science in the traditional sense
• Study of algorithms and how to use them
• NOT study of computers
• Software engineering is important!
• Analysis and design of systems
• Programming
• Testing and maintenance

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What is Programming

• Programming requires two skills
• Algorithmic thinking
• Knowledge of programming language syntax
• Learning language syntax is the easy part

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How We Learn Computer Science

• We use pseudo-code to focus on learning
  algorithmic thinking.
• From there, you can go anywhere!
• Not using a computer to compile and run your
  program forces you to mentally execute your
  program and debug it!
• Too many programmers try to program by trial and
  error and twiddling.

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Algorithm Defined

• A set of logical steps to accomplish a task
• One way to solve a problem
• A recipe of action
• A way of describing behavior
• Algorithms contain
• Data
• Instructions

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Algorithms in Computing
Input Data Algorithm Output Data
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Algorithm Recipe

• Chocolate Chip Cookies DATA
  (ingredients) 2 1/4 cups flour 1 tsp salt 1
  tsp baking soda 2 eggs 3/4 cup brown sugar 1
  tsp vanilla ext. 3/4 cup grand sugar 1 cup
  soft butter 12oz. semi-sweet chocolate
  chipsINSTRUCTIONS (steps) Preheat oven to
  375. Combine flour, salt, baking soda, in
  bowl, set mixture aside. Combine sugars,
  butter, vanilla, beat until creamy. Add eggs
  and beat. Add dry mixture and mix well.
  Stir in chocolate chips Drop mixture by
  teaspoons onto ungreased cookie sheet Bake 8
  to 10 minutes

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Abstraction

• a general idea or term
• an impractical idea visionary and unrealistic
• general characteristics apart from concrete
  realities, specific objects or actual instances
• withdrawal
• absent-mindedness inattention
• a work of art stressing formal relationships

(Random House Dictionary of the English Language)
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Abstraction in Computing

• Refers to the logical grouping of concepts or
  objects
• Define/implement the general idea
• Isolate the details
• Helps readability and understandingof our
  algorithms

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Abstraction Examples

• Algorithms
• Variable names
• Procedures and functions
• Data structures
• The computer language itself!

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Abstraction in Algorithms

• The Abstraction
• Get to the College of Computing at Georgia
  Tech
• One Implementation
• begin
• get on I-85 heading towards Midtown Atlanta
• exit I-85 at the 10th street exit
• proceed along exit ramp to 10th street
• turn west on 10th street
• turn left on Atlantic Drive
• stop in front of CoC building
• end
• Other implementations of the same abstraction can
  be just as correct, better, or worse

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Why do we stress abstraction?
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Whats Wrong With This Algorithm?
(From back of shampoo bottle) Directions
Wet Hair Apply a small amount of shampoo
Lather Rinse Repeat
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Properties of Good Algorithms

• Good algorithms are
• Precise
• Unambiguous
• Complete
• Correct
• Simple
• Contain levels of abstraction

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Describing Algorithms

• Natural language (English)
• Pictures
• Pseudo-code or a specific programming language

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• Make a list of courses you want to register for,
  in order of priority
• Start with empty schedule. Number of hours 0.
• Choose highest priority class on list.
• If the chosen class is not full and its class
  time does not conflict with classes already
  scheduled, then register for the class (2 steps)
  
• 4.a. Add the class to the schedule
• 4.b. Add the class hours to the number of hours
  scheduled
• Cross that class off of your list.
• Repeat steps 3 through 5 until the number of
  hours scheduled is gt 15, or until all classes
  have been crossed out.
• Stop.

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Flowcharts
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Components of Algorithms

• Any computing algorithm will have AT MOST five
  kinds of components
• Data structures to hold data
• Instructions change data values
• Conditional expressions to make decisions
• Control structures to act on decisions
• Modules to make the algorithm manageable by
  abstraction, i.e., grouping related components

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Questions?
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View of Programming Languages
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Overview of Programming Languages

• Allows programmers to describe desired behaviors
  (algorithms)
• The audiences are
• Humans
• Computers (only understand binary)
• Consists of
• Built-in
• User-defined

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Program Language
Language
Built-in
User-defined
Data
Operators Assignment Arithmetic Input/Output Relat
ional Boolean
Data
Operators
Atomic Number Character Boolean Pointer
Complex String
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Relations between Problems, Algorithms, and
Programs
Problem
Algorithm
. . . .
Algorithm
. . . .
Program
Program
Program
Program
. . . .
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Questions?
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Data
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Data Structures

• Data Structures are containers for data.
• The simplest of them are called atomic because
  they hold only a single value and cannot be
  subdivided into lower-level components
• Other complex data structures may hold multiple
  pieces of data, and are constructed from atomic
  types.

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Built-in Data Types

• Four built-in atomic types
• - Character (char)
• - Number (num)
• - Boolean (boolean)
• - Pointer (ptr)
• One built-in complex type
• - String (string)

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Memory Lane
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Abstracting the Memory

• Memory consists of 1s and 0s
• We want to abstract out the details
• Create a space in memory for a specific type of
  information
• Access this location in memory using a meaningful
  name

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Three Steps in Using Memory

• Declare
• Initialize (no default value)
• Manipulate

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Declaring Variables

• Variables are data structures whose value may be
  changed by the algorithm.
• Each variable must be declared to have an
  identifier (i.e., a name) and be of some data
  type.
• For example to declare a variable named
  counter that can hold a numeric value
• counter isoftype Num

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Initialization

• Each variable must first be assigned a value of
  the appropriate type after being declared.
• For example to assign the value 0 to the
  variable counter
• counter lt- 0
• Variables do not contain information until they
  are initialized (I.e. no default value).

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Numbers

• Declared as
• my_num isoftype Num
• my_num can now store one (integer or real) number
  of any magnitude.
• Examples
• my_num lt- 1 // this is OK
• my_num lt- -65 // so is this
• my_num lt- 14.5 // and so is this
• my_num lt- a // an ERROR

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Characters

• Declared as
• your_grade isoftype Char
• your_grade can now store any one alphanumeric
  character.
• Single-quotes used to distinguish Char values.
• Examples
• your_grade lt- A // this is OK
• your_grade lt- A- // an ERROR
• your_grade lt- 9 // this is OK
• your_grade lt- 9 // an ERROR

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Booleans

• Declared as
• this_test isoftype Boolean
• this_test can now hold one boolean (TRUE or
  FALSE) value.
• Examples
• this_test lt- TRUE // this is OK
• this_test lt- FALSE // so is this
• this_test lt- FALSE // an ERROR
• this_test lt- TRUE // an ERROR

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String

• Strings can holds a collection of characters.
• Declared as
• this_string isoftype String
• this_string can now hold a string of any number
  of alphanumeric characters.
• Examples
• this_string lt- hello world // OK
• this_string lt- hello world // ERROR
• this_string lt- 45 // OK
• this_string lt- 45 // ERROR

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Distinguishing Data Types

• Woman, without her, man is nothing
• Woman, without her man, is nothing
• my_char lt- a // single quotes
• my_string lt- a // double quotes

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Pointers

• Declared asthis_num_ptr isoftype ptr toa Num
• this_num_ptr can now know where a Num variable
  lives in memory.

this_num_ptr
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Don't worry about pointers for now.
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Questions?
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Assignment
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The Assignment Operator

• The assignment operator stores a particular value
  in a variable.
• Assignment lt-
• Format
• variable lt- value/variable

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Type Matching

• The type of variable on the left side must match
  the type of value/variable on the right side
• my_num lt- 42
• my_num lt- your_num

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Assignment Examples

• Examples
• my_age lt- 43
• your_age lt- my_age // duplicates?
• your_grade lt- A
• this_test lt- TRUE
• hi_there lt- Hello World
• TRUE and FALSE are each single boolean values.

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More Examples

• a lt- b c
• Get the value of b from memory
• Get the value of c from memory
• Add the two values together
• Store the result in memory location a

a lt- a 1 Get the value of a from memory Add
the number 1 to it. Store the result in memory
location a
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Pointer Assignment

• Given
• this_ptr isoftype ptr toa Num
• that_ptr isoftype ptr toa Num
• Then
• this_ptr lt- new(Num)
• that_ptr lt- new(Num)
• Gives each pointer variable a new number variable
  to which it refers

that_ptr
this_ptr
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Pointer Assignment

• Given above, then
• this_ptr lt- 4
• that_ptr lt- 8
• Assigns values to the variable to which the
  pointer refers.

this_ptr
4
that_ptr
8
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Pointer Assignment

• Given
• other_ptr lt- this_ptr
• this_ptr lt- that_ptr
• Assigns/reassigns pointers variables to reference
  other variables

this_ptr
4
that_ptr
8
that_ptr
this_ptr
8
4
other_ptr
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Questions?
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Arithmetic
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Arithmetic Operators

• Arithmetic operators allow us to express
  mathematical operations
• Addition
• Subtraction -
• Multiplication
• Division /
• Integer Division DIV
• Modulo MOD

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Order of Operations

• Follow standard arithmetic order of operations
• Multiplication Division
• Addition Subtraction
• Left to Right

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Parentheses

• Use parentheses to control order of execution.
• Suppose we want
• Will x lt- a b / c - d work?

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So we want

• x lt- (a b) / (c - d)

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DIV and MOD

• DIV keep whole number quotient, and throw away
  the remainder
• MOD keep the remainder, and throw away the whole
  number quotient
• Examples
• 8 DIV 3 2 // 3 goes into 8 two times
• 8 DIV 9 0 // 9 doesnt go into 8
• 7 MOD 3 1 // remainder of 1
• 7 MOD 9 7 // remainder of 7

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MOD DIV
7 -3 0
1 2 0 1
2 0

• 7.3333 DIV 1
• -6.3 DIV 2
• 0 MOD 3
• 1 MOD 3
• 2 MOD 3
• 3 MOD 3
• 4 MOD 3
• 5 MOD 3
• 6 MOD 3

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Questions?
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Input and Output
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Input and Output

• I/O Operators allow us to communicate with the
  outside world, the world beyond the algorithm
  itself.
• Were not concerned with formatting.

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Print

• Displays output items to the user
• Syntax
• print(item_1, item_2, ..., item_n)
• Examples
• print(Please enter your info.)
• print(num_one, my_char, is_Student)

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Read

• Obtains input items from the user
• Syntax
• read(item_1, item_2, ..., item_n)
• Examples
• read(menu_choice)
• read(is_Student, name, age)
• No automatic prompting!

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Input and Output Examples

• algorithm IO_Example
• num_one, num_two, average isoftype Num
• // obtain two numbers
• print(Please enter two numbers)
• read (num_one, num_two)
• // output a literal text message and the
• // value of the sum
• print (Sum , num_one num_two)
• // output a string literal and average
• average lt- (num_one num_two) / 2
• print (Average , average)
• endalgorithm

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Questions?
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