Week 3

1
Week 3

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

2
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

3
Shortest C Program.A Skeleton program
type of returned value
name of function
int main ( ) return 0 //Write and
run this Chris
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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

5
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.

6
The Hello World Program
accesses standard toolbox, i.e. we are using
the display (cout) tool
include ltiostreamgt using namespace std int
main ( ) cout ltlt Hello World ltlt endl
return 0 //Write and run this Chris
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

17
Recap!

• Your programs will usually entail
• Reserving 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

18
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

• 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

• Key ideas
• 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!

37
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
40
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

• 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

46
Interactive input formally covered later

• using cin and cout
• using data files

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Bye Bye