Week 3

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Week 3

• Get some information in,
• Do something with it,
• Display the result.

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Learning Points

• Structure of a simple C Program
• TYPES of information
• Simple and Complex
• Variables, Constants and Literals
• Identifiers to Name memory
• Declarations
• Getting information into your program basics
• Operator basics

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(No Transcript)
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Problem Solving Techniques

• OVERCOME MENTAL BLOCK -- by rewriting the problem
  in your own words
• DRAW A FIGURE/DIAGRAM
• ASK QUESTIONS
• What information must be displayed? What is
  information do you know? what do you NOT know and
  need? what information can you assume to make
  things simpler for yourself?, What kind of
  information am I manipulating (whole numbers,
  numbers with decimal points, text? What would be
  good identifiers (names) for the information.
• Where does the information come from? Is it
  given? Can it be assumed? Do you get it from the
  user? do you calculate it? do you get it from a
  file?
• Can you identify what is input and what is to be
  output?
• What are the processes?
• Do you need to repeat steps
• What are the conditions?
• Do you need to do different things in different
  situations?
• What are the conditions

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Problem Solving Techniques

• LOOK FOR FAMILIAR THINGS -- certain situations
  arise again and again. E.g. Am I repeatedly
  getting input from the user? creating tables,
  adding up lists of numbers, reading from files,
  Searching for maximum or minimum values.
• Do you know any equations?
• Find the wall area of a room, find the stopping
  distance of a car.

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Problem Solving Techniques

• Stepwise refinement -- break up large problems
  into manageable units.
• SIMPLIFY
• Can you solve a simpler but related problem?
• For example a program to calculate wall areas of
  rooms for a decorating company.
• Solve problem assuming simple rectangular rooms
  without doors and windows.
• Solve by analogy -- it may give you a place to
  start. In a broader sense than looking for
  familiar things.
• E.g. Finding the student with the highest and
  lowest score is analogous to finding the highest
  and lowest temperature.

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Problem Solving

• USE MEANS-ENDS ANALYSIS
• Where you often know start conditions and know
  what the end goal is.
• Identify intermediate goals then think how you
  get from start to the intermediary goals choosing
  the most appropriate means for the problem.
• E.g. I am hungry and I want to be full
• What's the difference between what I have and
  what I want?  An empty stomach.  What changes the
  emptiness of my stomach?  Eating food.  But I
  don't have any food. I want to have some food. 
  What's the difference between what I have and
  what I want?  The presence of food.  What changes
  the presence of food?  Searching, shopping,
  hunting, growing food. I search for food, but
  there is nothing available. I decide to shop for
  food.  What do I need to shop?  Money. But I
  don't have any money. I want some money.  How do
  I get money? Withdraw from the bank, sell
  something, steal. I decide to withdraw from the
  bank. I want some money from the bank.  What do
  I need to get money from the bank?  I must be at
  the bank. I want to be at the bank.  What is the
  difference between where I am now and where the
  bank is?  Distance.  What changes distance? 
  Walk, cycle, drive, bus. ..... and so on

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Problem Solving Techniques

• BUILDING-BLOCK APPPROACH -- can you solve small
  pieces of the problem? And then join them up
• E.g. a large application like a word processor
  has many functions. (Text manipulation, Tables,
  drawing)
• Do not try to solve all at once.

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Result of Problem solving

• One or more pages of rough work that sketches
  your ideas of problem requirements, of user
  inputs, of outputs, of constants, of conditions
  for repetition and selection, of assumptions.
• A formal written design that include.
• inputs, Processes, Outputs, assumptions.
• Write steps to process input unambiguously using
  a semi formal notation (PDL, Flowchart or
  structure diagram?) Can you translate each step
  of the plan into C easily?
• Verification/testing procedures documented (A
  test table).

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What we are aiming for is Structured Methodical,
Systematic Approach to Programming

• Advantages
• Not one giant step, but breaks in to smaller and
  smaller chunks.
• Programmer concentrates on details
• Easy to do in teams
• Easy to keep track of development
• Creates more reliable and robust programs.
• Errors isolated
• Design written in code
• Improves reliability
• Minimizes risk of Failure
• Faster development
• Makes more possible reuse and extensibility.

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Summary

• So take any problem, apply problem solving
  approaches. To find a solution, write it down in
  English.
• Write it down more formally in Pseudo-code (PDL),
  or as a structure diagram or a flow chart.
• Start your programming environment and convert
  your pseudo-code to C
• Order the computer to convert the C to machine
  language (compile)
• Order the computer to load the machine language
  into memory and execute (Run).

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Recall from last week A Skeleton program
include ltiostreamgt using namespace std int
main ( ) return 0
type of returned value
name of function
We call main( ) a FUNCTION. Functions are used to
collect together instructions. main( ) is a
special function because it is the first thing
that is executed
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What happens?

• The operating system (XP) Loads the program
• Think of it as XP calling the main function.
• Instructions in main are carried out one at a
  time.
• Notice the int in front of main this line is
  like a contract to the operating system
• I will send you a piece of information in the
  form of an integer.
• Hence the NEED for a return instruction. return 0
  sends a zero to windows XP

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Discussion of structure

• Your program must have a function called main
• Note that Main, MAIN, mAiN will cause an error
• The round brackets ( and )after main tells
  the computer that main is a function rather than
  a variable.
• The curly brackets and mark the beginning
  and end of instructions that form the body of
  main.
• The semi-colon after return 0 is an
  instruction terminator or separator.

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The Hello World Program
accesses standard toolbox, i.e. we are using
the display (cout) tool, the send to tool and
the endl tool
include ltiostreamgt using namespace std int
main ( ) cout ltlt Hello World ltlt endl
return 0
Instructions that tell computer that I want to
use iostream toolbox and access the standard
(std) tools for input and output
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Discussion of Hello World Program

• There are now two instructions in main
• we added a display information instruction using
  a tool cout
• cout represents the display screen. It hides all
  the complicated business of displaying things
  from you. All you need to know is how to use
  cout.
• cout receives information using the ltlt send to
  operator (insertion operator).
• So we literally send the string Hello world to
  the display!
• endl (last letter is lowercase L not number 1!)
  is also sent to the display, this is interpreted
  by cout as a new-line command.

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Discussion of Hello World

• cout is an OBJECT that represents our screen.
• Screens are hardware.
• Sending information to screens varies greatly
  from machine to machine.
• So cout became standard. Customised couts were
  created for all the major types of computer in
  the world.
• So that you can write one piece of code that
  should compile on all the main types of computer
  in the world.

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Discussion of Hello World

• Because cout NOT BUILT IN we need to tell the
  computer that we want to make use of the
  standard facilities for input and output
• HENCE the lines
• include ltiostreamgt
• using namespace standard

This is one of the original reasons for C
popularity its relative ease of porting from one
type of computer to another
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Some Definitions

• Statements
• A statement is what programmers often call an
  instruction.
• Your code consists of many instructions/statements
  
• Statements end with a semi-colon
• blocks
• Any section of code which is surrounded by curly
  brackets is a block

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Example A Block of 4 statements

• cout ltlt"A fraction "ltlt5.0/8.0 ltltendl
• cout ltlt"Big "ltlt 7000.07000.0ltltendl
• cout ltlt8 5 ltlt" is the sum of 8 5\n"
• cout ltlt Hello world

Block denoted by Curly braces
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TYPES of information computers use

• Simple (needs little or no effort to use)
• To hold whole numbers (integers)
• To hold numbers with fractions (float and double)
• To hold individual characters (char)
• Complex (needs a little more work to use)
• Strings
• cin and cout

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Simple Information
int

• Integer (Whole numbers)
• For counting things
• To represent values which only have whole numbers
• Pounds, Pence?
• Grades, Scores (Chelsea 0 Penge United 10)
• Categories (days of week codes as numbers 1-7)

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Simple Information
float
double

• Floating points numbers
• For representing numbers that may contain
  fractions
• Averages, measurements, money, pi

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Simple information
char

• Single characters
• Can represents your initials
• Can represent single key responses to questions
• y for yes and n for no
• Not much else??

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What about Strings
include ltstringgt
string

• Some languages this is a simple piece of
  information
• In C it is not. A string is complex in that it
  is made up of lots of chars.
• In C we use the standard string tools

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Recap!

• Your programs will usually entail
• Reserving and labelling some space to store
  information
• Getting information into the space you reserved
• This information will either be whole numbers,
  floating point numbers, single characters or
  strings.
• More complex information we will cover at a later
  date.
• Doing something (operate on) with the information
  (int, float, double, char or string)
• Displaying results

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Constants, Variables and Literals
Fundamental building blocks of programs

• Remember at school
• Area of a circle is pr2
• Circumference of a circle is 2 pr
• p is a constant representing the number 3.142
• r is a variable representing the radius of a
  circle
• 2 literally represents itself, it is a literal

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Key programming concept

• You can categorise the information you are using
  as either
• constant
• variable
• literal
• Programs use, variables, constants and literals
  to store information needed to solve problems

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Fundamental task isGetting information in to the
computer

• We enter values of literals directly into code.
• We instruct the computer to reserve space for a
  constant and we enter the value of the constant
  directly into code
• We instruct the computer to reserve space for a
  variable. We have a choice on how we enter values
  of variables.
• We can enter values directly in code
  (assignments)
• Values can be set interactively with a user.
• Values can be set interactively with a file on
  disk.

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Using literals

• char, string, integer, and double values are
  referred to by value not by a name.
• We type these directly in to code

include ltiostreamgt using namespace std int
main() cout ltltThe sum of one plus three is
ltlt 1 3 ltlt endl return 0
string literal
2 integer literals
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char literals
include ltiostreamgt using namespace std int
main() cout ltltFirst letter of the alphabet is
ltlt A ltlt endl return 0
char literals in SINGLE QUOTES
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floating point literals

• floating point number can contain fractions.
• floating point literals are double

include ltiostreamgt using namespace std int
main() cout ltltone point one plus three point
two is ltlt 1.1 3.2 ltlt endl return 0
2 double literals
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Discussion

• You can enter information directly into code
  using literals
• This is obviously very limiting if a program
  wanted to reuse a value we need to keep typing in
  its value every time.
• Also if a program makes use of a value many times
  it is hard work to change all the CORRECT
  references in a long program, especially is the
  number, say 10 refers to a count in one part of
  the program and the number 10 means a grade in
  another. How can we differentiate between them?
• It can also be very confusing for another person
  to understand what the numbers mean.

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SolutionVariables and Constants

• These are NAMED areas of memory.
• The programmer instructs the computer to reserve
  space for the kind of information he/she wants
  and at the same time tell the computer the name
  that will be used to refer to that bit of
  information.
• This kind of instruction is called a DECLARATION

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DECLARATION

• We can declare
• Integers
• floating point numbers
• chars
• strings

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Constants and Variable Declarations

• When a variable is declared the computer marks
  the bit of memory reserves so that it allows its
  contents to change (vary) at any time.
• When a constant is declared the computer
  effectively locks the memory reserved and
  prevents the contents being updated.

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Creating Constants and Variables
include ltiostreamgt using namespace std int
main() const double Pi 3.142 double
radius double Area, Circumference Area
Piradiusradius Circumference
2Piradius return 0
Declarations are instructions to reserve
memory space big enough to store our
information. It also creates an identifier (a
name) for us to refer to this memory space
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Rules for Creating Constants
const lttypegt ltidentifiergt ltvaluegt
lttypegt choose one of int char float double string
ltvaluegt You provide the constant value e.g. for
Pi value was 3.142
ltidentifiergt You create a name. It must obey the
rules for identifiers (see rules in a minute)
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Examples
const int maxnumber 10 const double Pi
3.142 const char AGrade A const string
MyName Chris
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Rules for Creating Variables
lttypegt ltidentifiergt or lttypegt ltidentifiergt
ltvaluegt or lttypegt ltidentifiergt, ltidentifiergt,

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Examples
int number double Result char response string
UserName
int n1, n2, n3, n4
Declare a double variable and set its initial
value
double x 5.6
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Rules for Creating Identifiers

• An identifier must start with a letter or
  underscore, and be followed by zero or more
  letters
• (A-Z, a-z), digits (0-9), or underscores
• VALID
• age_of_dog taxRateY2K
• PrintHeading ageOfHorse
• NOT VALID (Why?)
• age 2000TaxRate Age-Of-Cat
• Age of Cat

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Meaningful Identifiers
Age-Of-Cat Age of Cat
ILLEGAL!!
Use underscore to link words or run words
together and capitalise the start of each word
LEGAL!!
Age_Of_Cat AgeOfCat
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More About Identifiers

• C is case sensitive so
• NUMBER Number number
• are all legitimate but DIFFERENT identifiers
• BE CONSISTENT in your code!
• It is NOT good practice to have long identifiers
• Why?

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Long Identifier Names

• Some C compilers recognize only the first 32
  characters of an identifier as significant
• then these identifiers are considered the same
• age_Of_This_Old_Rhinoceros_At_My_Zoo
• age_Of_This_Old_Rhinoceros_At_My_Safari
• Also it is very annoying to keep typing in!

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Meaningful Identifiers

• It is common sense to try to create identifiers
  that are
• Meaningful- they describe the information they
  refer to
• E.g. Height, Count, BloodPressure, CarSpeed
• Terse- They are only as long as necessary to
  convey meaning.

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Reserved Words

• Identifiers CANNOT be a reserved word.
• Reserved words are built in words that have
  special meanings in the language. They must be
  used only for their specified purpose. Using them
  for any other purpose will result in a error.
• e.g. do if switch while else return

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Reserved Words

• C is case sensitive. ALL reserved words are
  lower case

WHILE While while Are all different identifiers
only the last is a reserved word
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Operators

• All the data types we have seen (int, float,
  double, char and string) have a set of operators
  that can be applied to them
• E.g. Numerical data uses the familiar - / for
  add subtract and divide.
• The asterisk is used for multiplication
• These work in the usual manner

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string operators

• arithmetic is not meaningful when applied to
  strings
• You do not multiply first names!
• strings operators do other things
• E.g. The operator applied to strings joins them
  (concatenates) together
• We will see other operations we want to do with
  strings like searching a string for a substring
  or comparing strings.

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Expressions

• A VALID arrangement of variables, constants,
  literals and operators

Expressions are evaluated and compute to a VALUE
of a given type E.g. Expression 9 4 computes
to 13
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A special OPERATOR
The assignment operator
Causes much confusion! IT DOES NOT WORK LIKE
EQUALS IN MATHS
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variable expression
Interpretation of Takes the value of or
becomes

• number 4
• result 10 number
• number number 1

expression simply consists of the literal int 4.
number takes the value of 4
expression evaluates to 40. result takes the
value of 40
expression evaluates to 5 number takes the value
of 5
KEY POINT expression is evaluated BEFORE it is
applied
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Using assignments to get information into the
computer

• int myage
• string myname
• double mysalary
• myage 21
• myname Chris
• mysalary 1000.00

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Interactive input formally covered later

• using cin and cout
• using data files

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Bye Bye