coit29222

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COIT29222-Structured Programming Lecture Week 02

• Reading Textbook(4th Ed.), Chapter 2
• Textbook (6th Ed.), Chapters 4, 5
• Study Guide Book 1, Module 4
• Variables
• Branching or selection statements
• Looping statements
• Expressions, Operators, Operands, etc

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Variables

• Instead of using memory locations directly, all
  modern programming languages use names to refer
  to memory locations
• We call these things variables the values they
  refer to vary as the program runs
• Using names, instead of memory locations, makes
  programs easier to read
• Instead of

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Example Pseudocode

• write Number of marks in exam gt
• read M1
• write Students mark gt
• read M2
• set M3 to M2 / M1
• set M4 to 100 M3
• write Students percentage
• write M4

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Variables

• write Number of marks in exam gt
• read NbrMarks
• write Students mark gt
• read StudentMark
• set ProportionOfMarks to StudentMark / NbrMarks
• set Percentage to 100 ProportionOfMarks
• write Students percentage
• write Percentage
• 4 variables NbrMarks, StudentMark,
  ProportionOfMarks, Percentage

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Alternative Design

• write Number of marks in exam gt
• read NbrMarks
• write Students mark gt
• read StudentMark
• set Percentage to 100 StudentMark / NbrMarks
• write Students percentage
• write Percentage
• 3 variables NbrMarks, StudentMark,Percentage
• In C, this looks like

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Sample Program In C

• include ltiostream.hgt
• void main (void)
• int NbrMarks 0
• float StudentMark 0,
• Percentage 0
• cout ltlt "Number of marks in exam gt "
• cin gtgt NbrMarks
• cout ltlt "Students mark gt "
• cin gtgt StudentMark
• Percentage 100 StudentMark / NbrMarks
• cout ltlt " Students percentage "
• cout ltlt Percentage

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Declaring Variables

• In C, you must declare variables before you
  use them
• Also, you must declare the type of value each
  variable will hold
• The first variable declared is NbrMarks it can
  hold any integer value
• int NbrMarks 0
• An integer is any ve or ve whole number, or
  zero -3, -2, -1, 0, 1, 2, 3,

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Naming Variables

• Start with a letter - A to Z
• Can contain upper or lower case letters, or
  numerals - 0 to 9
• Good idea to limit names to 32 characters
• Must avoid C reserved words like int, float,
  void, etc
• More later

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Initialising Variables

• Our declaration of NbrMarks also gives it an
  initial value - 0
• int NbrMarks 0
• If you do not give a variable an initial value,
  it will hold whatever data was left in memory by
  the previous program
• Incorrect initialisation of variables is a very
  common logic error made by programmers
• Please read the previous point again!

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More Declarations

• Our program also declares two floating point
  variables StudentMark and Percentage
• float StudentMark 0,
• Percentage 0
• We use variables of type float to hold values
  that may not be an integer
• Examples
• An students mark 15.5 (marks)
• A distance 123.456 (meters)
• A temperature -2.3 (degrees C)

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Why Floating Point

• Some programs need to store very big numbers
  eg. 1,234,500,000,000,000,000 others need to
  store very small numbers - 0.000000000012345
• These two numbers can be represented as
• 1.2345 1018 and 1.2345 10-12
• float variables are stored in two parts
• number part 1.2345 in above cases
• power part 18 or 12 in above cases
• Decimal point floats around, depending on power
  part more details later

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More On Declarations

• We could declare these variables like this
• float StudentMark 0
• float Percentage 0
• Instead, a comma was used to separate two
  variables of the same type
• float StudentMark 0, Percentage 0
• Breaking the declaration over two lines is
  optional in C - its common practice
• float StudentMark 0,
• Percentage 0
• Notice how a comma is used here to separate two
  use of punctuation - , and

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Branching

• Our example program always performs the exact
  same actions
• get number of marks in exam
• get students mark
• calculate the students percentage
• display the students percentage
• In practice, most programs are capable of
  performing different actions, depending on the
  inputs provided by the user
• e.g., we may want to also state if the student
  passed the exam so, sometimes the program will
  need to output Pass, otherwise Fail

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Sample Dialogs

• Number of marks in exam gt 80
• Students mark gt 60
• Students percentage 75 (Pass)
• or
• Number of marks in exam gt 80
• Students mark gt 20
• Students percentage 25 (Fail)

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Branching Statements

• To enable programs to perform different actions
  (depending on user input) all programming
  languages include one or more branching or
  selection statements
• The branching statement selects between one of
  more possible paths through the program
• All programming languages include some form of
  if-then-else statement

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Pseudocode Example

• write Number of marks in exam gt
• read NbrMarks
• write Students mark gt
• read StudentMark
• set Percentage to 100 StudentMark / NbrMarks
• write Students percentage
• write Percentage
• if Percentage lt 50 then
• write (Fail)
• else
• write (Pass)

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Example of if-then-else

• write Number of marks in exam gt
• read NbrMarks
• write Students mark gt
• read StudentMark
• set Percentage to 100 StudentMark / NbrMarks
• write Students percentage
• write Percentage
• if Percentage lt 50 then
• write (Fail)
  NbrMarks ?
• else
  StudentMark ?
• write (Pass)
  Percentage ?
• in C, this looks like

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if-else Statements

• include ltiostream.hgt
• void main (void)
• float StudentMark 0
• cout ltlt "Students mark gt "
• cin gtgt StudentMark
• if (StudentMark gt 50)
• cout ltlt "Pass!"
• else
• cout ltlt "Fail!"

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if-else Statements

• Notice that comparison between StudentMark and 50
  is enclosed in parentheses
• if (StudentMark gt 50)
• Also, braces mark start and end of the if-branch
  (shown below), and the else-branch
• cout ltlt "Pass!"
• Indentation makes code easier to read its not
  required by the compiler

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if-else-if Statements

• if-else-if statements can be coded as
• if (StudentMark gt 75)
• cout ltlt Distinction!"
• else if (StudentMark gt 50)
• cout ltlt Pass!"
• else
• cout ltlt "Fail!"

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Activity Code in C

• write Select conversion - (1) C to F, (2) F to C
  gt
• read ConversionType
• write Input temperature gt
• read Temperature
• write Converts to
• if ConversionType 1 then
• write 32 (Temperature 1.8)
• write degrees Fahrenheit
• else
• write (Temperature 32) / 1.8
• write degrees Centigrade

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A Start

• include ltiostream.hgt
• void main (void)
• int ConversionType 0
• float Temperature 0
• cout ltlt "Select conversion - (1) C to F, (2) F
  to C gt "
• cin gtgt ConversionType

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A Solution

• include ltiostream.hgt
• void main (void)
• int ConversionType 0
• float Temperature 0
• cout ltlt "Select conversion - (1) C to F, (2) F
  to C gt "
• cin gtgt ConversionType
• cout ltlt "Input temperature gt "
• cin gtgt Temperature
• if (ConversionType 1)
• cout ltlt 32 (Temperature 1.8)
• cout ltlt " degrees Fahrenheit"
• else
• cout ltlt (Temperature - 32) / 1.8
• cout ltlt " degrees Centigrade"

note C uses instead of for testing
equality
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Looping

• Our program to calculate the exam percentage for
  a single student performs the following 5 tasks
• get number of marks in exam
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• To calculate the exam percentage for more than
  one student, a program must perform some of these
  tasks more than once

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Looping

• To calculate the percentage for 2 students, a
  program must perform 9 tasks
• get number of marks in exam
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail

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Looping

• To calculate the percentage for 3 students, a
  program must perform 13 tasks
• get number of marks in exam
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail

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Looping

• We could have
• -One program to handle 2 students,
• -Another program to handle 3 students, etc
• It would be better to have a single program to
  handle any number of students
• We would like to design a program to repeat some
  actions more than once
• We want our program to
• get the number of marks in exam from user
• - repeat as required -
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail

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Looping

• One design is
• Ask user for number of students at start
• get number of marks in exam
• get number of students
• - repeat once for each student -
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• Another design is

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Looping

• Keep asking if the user has more students
• get number of marks in exam
• - repeat while user wants more students -
• get students mark
• calculate the students percentage
• display the students percentage
• display Pass or Fail
• ask user if they have more students

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Looping Statements

• To enable programs to perform different numbers
  of actions (depending on data inputs) all
  programming languages include one or more looping
  or repetition statements
• All programming languages include some form of
  while statement...
• Other common looping statements are do-while and
  for

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Pseudocode Example

• write Number of marks in exam gt
• read NbrMarks
• write Number of students gt
• read NbrStudents
• set NbrProcessed to 0
• while NbrProcessed lt NbrStudents
• write Students mark gt
• read StudentMark
• set Percentage to 100 StudentMark /
  NbrMarks
• write Students percentage
• write Percentage
• set NbrProcessed to NbrProcessed 1

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Example of while
write Number of marks in exam gt
NbrMarks 80 read NbrMarks
NbrStudents 2 write Number of
students gt NbrProcessed 2 read
NbrStudents
StudentMark 40 set NbrProcessed to 0
Percentage 50 while NbrProcessed
lt NbrStudents write Students mark gt
read StudentMark set Percentage to 100
StudentMark / NbrMarks write Students
percentage write Percentage set
NbrProcessed to NbrProcessed 1
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Activity
Show the output produced by the following
pseudocode. set TotalCost to 20 set ItemCost to
5 set NumberOfItems to 0 while TotalCost gt 0
set TotalCost to TotalCost - ItemCost set
NumberOfItems to NumberOfItems 1 write Number
of items purchased write NumberOfItems
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while Statement

• include ltiostream.hgt
• void main (void)
• int NbrTimesTold 0,
• NbrTimesToTell 0
• cout ltlt "How many times must I tell you? gt "
• cin gtgt NbrTimesToTell
• while (NbrTimesTold lt NbrTimesToTell)
• cout ltlt No new taxes!"
• cout ltlt endl
• NbrTimesTold NbrTimesTold 1

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Activity Feedback

• The program displays No new taxes! the number
  of times requested by the user
• cout ltlt endl moves to start of next line

How many times must I tell you? gt 3 No
new taxes! No new taxes! No new taxes!
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while Statement

• Things to notice
• Comparison enclosed in parentheses
• while (NbrTimesTold lt NbrTimesToTell)
• cout ltlt endl
• cout ltlt " No new taxes!"
• NbrTimesTold NbrTimesTold 1
• Use of braces and indentation

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One More Detail About C

• All compilers are fussy about punctuation
• C compilers are also fussy about case
• The following code has 3 compilation errors
• Include ltiostream.hgt
• void Main (void)
• Cout ltlt "Hello World!"

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C is Case-Sensitive

• Include ltiostream.hgt
• void Main (void)
• Cout ltlt "Hello World!"
• This program has three errors
• it should be include , not Include
• it should be main , not Main
• it should be cout , not Cout
• C compilers are case-sensitive

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Data Processing

• We said that computers are data processing
  devices they convert input data into output
  data
• Where does all the data processing occur in a
  program?
• Recall our sample program...

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Sample Program

• write Number of marks
• read NbrMarks
• write Students mark
• read StudentMark
• set ProportionOfMarks to StudentMark / NbrMarks
• set PercentageOfMarks to ProportionOfMarks 100
• write Students percentage
• write PercentageOfMarks

• In this program, all the data processing occurs
  in assignment statements

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Anatomy of an Assignment

• Lets analyse the assignment statement
• Here, the two assignment statements are
• set ProportionOfMarks to StudentMark / NbrMarks
  
• set PercentageOfMarks to ProportionOfMarks
  100
• That is
• set lt variable gt to lt value of interest gt
• Here, the values of interest are
• StudentMark / NbrMarks
• ProportionOfMarks 100

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Expressions

• We use the term expression to mean
• the description of a value of interest
• we describe the value that we wish to assign to a
  data object in an expression
• so
• StudentMark / NbrMarks
• ProportionOfMarks 100
• are two expressions

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Data Processing

• So, where does the data processing happen?
• answer some of it happens in
• assignment statements
• It can also happen in output statements

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Alternative Design

• write Number of marks
• read NbrMarks
• write Students mark
• read StudentMark
• set ProportionOfMarks to StudentMark / NbrMarks
• write Percentage
• write ProportionOfMarks 100

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Anatomy of an Output

• The anatomy of our assignment statement is
• set lt variable gt to lt expression gt
• the anatomy of our output statement is
• write lt expression gt
• so, where does all the data processing happen?

Expressions !
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Expressions

• Clearly, expressions are important - thats
  where the data processing happens
• Lets take a closer look at expressions
• Previously, we said that data was numbers and
  text -for now, we just deal with expressions to
  process numbers
• The anatomy of an expression is one weve seen
  before...

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Expressions as a Black Box

• We can think of an expression as a black box
• Expressions have one or more input values and
  produce one output value - the input-process-outpu
  t model again
• Example StudentMark / NbrMarks
• input process
  output
• StudentMark
  ?
• NbrMarks
  (a single value -
• 
  depends on inputs)

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Operators

• we use the term operator to mean
• a symbol, or name, used to represent an
  operation that can be performed on data
• in the two expressions
• StudentMark / NbrMarks
• ProportionOfMarks 100
• the operators are
• / for division
• for multiplication
• and - are used for addition and subtraction
• , -, , / all work in C as you would expect

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Operands

• We use the term operand to mean
• an input to an expression
• In the two expressions
• StudentMark / NbrMarks
• ProportionOfMarks 100
• The operands are
• StudentMark and NbrMarks
• ProportionOfMarks and 100

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Binary Operators

• In the following examples
• StudentMark / NbrMarks
• ProportionOfMarks 100
• NbrMarks - StudentMark
• StudentMark 10
• Each operator is used with two operands
• So / , , - and are binary operators
  they can all be used with two operands

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Unary Operators

• The and - operators are also unary
  operators (they can be used with just one
  operand)
• Examples
• -273.15 as in set AbsoluteZero to
  -273.15
• 100 as in set BoilingPointOfWater
  to 100
• expression
• - 273.15
• operand
• operator

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Numeric Expressions

• expressions that evaluate to a number are called
  numeric expressions
• numeric expression come in all shapes and sizes
• a number by itself a literal
• set NbrTimesTold to 0
• the name of a variable
• write Percentage
• expressions that use operators
• set NbrTimesTold to NbrTimesTold 1

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Power of Expressions

• The arithmetic operators , -, and / give us
  a powerful language to process numbers
• The power comes from the ability to nest little
  expressions inside bigger expressions
• Instead of
• set ProportionOfMarks to StudentMark / NbrMarks
• write ProportionOfMarks 100
• We can write
• write StudentMark / NbrMarks 100
• question which operator is applied first here?
  and, does it matter?

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Nested Expressions

• which operator is applied first here?
• Is the division first?
• StudentMark / NbrMarks 100
• divide StudentMark by NbrMarks, then multiply by
  100
• Or is the multiplication first?
• StudentMark / NbrMarks 100
• multiply NbrMarks by 100, then divide StudentMark
  by result of multiplication
• Activity does it matter?

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Activity Feedback

• Using StudentMark 50, NbrMarks 100
• Division first
• (StudentMark / NbrMarks) 100
• (50 / 100) 100
• 50
• Multiplication first
• StudentMark / (NbrMarks 100)
• 50 / (100 100)
• 0.005
• Will a C program do it in the correct order?

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Order of Use

• There are rules to decide the order in which
  operators in an expression are applied
• Unary operators before binary operators
• Multiplication () and division (/) before
  addition () and subtraction (-)
• Otherwise, left to right
• Evaluate the following
• 4 -2 3
• 2 12 / 4 3
• Will the following be evaluated correctly?
• StudentMark / NbrMarks 100

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Activity Feedback

• Evaluate
• 4 -2 3
• unary operator first (- applies to 2)
• multiplication before addition
• (4 -2) 3
• -8 3
• -5

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Activity Feedback

• Evaluate the following
• 2 12 / 4 3
• Multiplication and division before addition
• Left to right otherwise so division before
  multiplication here
• 2 (12 / 4) 3
• 2 3 3
• Multiplication before addition
• 2 (3 3)
• 2 9
• 11

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Activity Feedback

• Will the following be evaluated correctly?
• StudentMark / NbrMarks 100
• Yes it will since the division occurs before
  the multiplication, this is the same as
• (StudentMark / NbrMarks) 100

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Order of Use

• Avoid errors by using parentheses
• (4 -2) 3
• 2 ( ( 12 / 4 ) 3 )
• Sometimes you can rewrite an expression to make
  it easier to read instead of
• StudentMark / NbrMarks 100
• You can write
• 100 StudentMark / NbrMarks
• Is this easier to understand? if so, why?

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Activity Feedback

• The expression
• 100 StudentMark / NbrMarks
• may seem easier to read than
• StudentMark / NbrMarks 100
• Possibly because, in the first expression above,
  the order in which operators are applied doesnt
  matter
• left for student to check
• Always keep you code as simple as possible
• The following program is designed to convert
  temperatures between Fahrenheit and Centigrade
• It has a logic error fix it

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• include ltiostream.hgt
• void main (void)
• int ConversionType 0
• float Temperature 0
• cout ltlt "Select conversion - (1) C to F, (2) F
  to C gt "
• cin gtgt ConversionType
• cout ltlt "Input temperature gt "
• cin gtgt Temperature
• if (ConversionType 1)
• cout ltlt 32 Temperature 1.8
• cout ltlt " degrees Fahrenheit"
• else
• cout ltlt Temperature - 32 / 1.8
• cout ltlt " degrees Centigrade"

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Feedback

• include ltiostream.hgt
• void main (void)
• int ConversionType 0
• float Temperature 0
• cout ltlt "Select conversion - (1) C to F, (2) F
  to C gt "
• cin gtgt ConversionType
• cout ltlt "Input temperature gt "
• cin gtgt Temperature
• if (ConversionType 1)
• cout ltlt 32 Temperature 1.8
• cout ltlt " degrees Fahrenheit"
• else
• cout ltlt Temperature - 32 / 1.8
• cout ltlt " degrees Centigrade"

problem here division occurs before subtraction
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Feedback

• include ltiostream.hgt
• void main (void)
• int ConversionType 0
• float Temperature 0
• cout ltlt "Select conversion - (1) C to F, (2) F
  to C gt "
• cin gtgt ConversionType
• cout ltlt "Input temperature gt "
• cin gtgt Temperature
• if (ConversionType 1)
• cout ltlt 32 (Temperature 1.8)
• cout ltlt " degrees Fahrenheit"
• else
• cout ltlt (Temperature 32) / 1.8
• cout ltlt " degrees Centigrade"

clarification parentheses make intention clear
a solution enclose subtraction in parentheses
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C Syntax Summary

• input cin gtgt ltvariablegt
• output cout ltlt ltexpressiongt
• assignment ltvariablegt ltexpressiongt
• A selection statement
• if ( lttestgt )
• ltif-block statementsgt
• else
• ltelse-block statementsgt
• A repetition statement
• while ( lttestgt )
• ltwhile-block statementsgt

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Summary

• Five base programming statements are input,
  output, assignment, selection, repetition
• These statements provide a platform for
  programming in all popular languages
• Data processing happens in expressions
• Different types of expressions literals,
  variables names, ones that use operators
• Arithmetic operators are , -, , /
• Rules control order of application
• Parentheses are used to impose ordering