Statements and Expressions

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Statements and Expressions
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Statements

• the statement is the basic unit for building C
  programs
• most statements are imperative (i.e., tell the
  computer to do something)
• statements end with a semicolon, not with the end
  of a line (important point!)
• Example statements
• cout ltlt "Hello" //statement that does
  output
• totPay hourlyRate numHours 10.0
  //assignment stmt

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Lines vs Statements

• the compiler ignores whitespace (i.e., spaces,
  tabs, line feeds, carriage returns)
• preprocessor directives should be on one line
• can not break strings between two lines
• C allows us to write the body of our function
  main (i.e., our program) as
• cout ltlt "\nHello World" ltlt endl return 0
• but this would be difficult to read

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Expressions

• An expression is a grouping of variables,
  constants and operators that specifies a
  computation
• An expression results in a value
• Examples amount 5
• (temp -2) rate
• An expression is not a statement
• statements end in a semicolon
• a statement tells the computer to do something
• a statement may include several expressions

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C Variables

• A variable is the name of a location in computer
  memory
• All variables need to be declared
• C is a "strongly typed" language (i.e., when a
  variable is declared, the built-in data type or
  the class of an object at which the variable will
  point must be explicitly identified)
• declaration vs definition
• declaration lets the system know you will use a
  name and specifies type
• a definition also reserves space
• int studentCount // this is a definition space
  is reserved
• Declaring a variable that will point at an object
  implicitly instantiates it (unlike Java or
  Smalltalk)
• Car firstCar

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Identifiers

• names given to program elements are referred to
  as identifiers
• these names all have similar rules for their
  construction
• use lower case and upper case letters, digits and
  underscore
• can not start with a digit
• any length, but generally ignore beyond 250
• can not use a keyword (e.g., if, while, return)

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Popular Conventions for Variable Naming

• Lots of different conventions exist -- choice
  may be influenced by a vendor or package, or be
  set by company, project or individual
• Consistent adherence to convention improves
  readability and understandability
• C code is often (usually?) written with
  inconsistent styles
• Popular C variable naming conventions
• all lower case (payrate)
• mixture of upper and lower case (PayRate)
• lots of underscores for readability (Pay_Rate)
• Java style (payRate)

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Our Convention for Variable Names

• We will apply the following conventions (similar
  to what we did for Java)
• We will use letters and numbers
• We will not use special characters including
  spaces, dots, underlines, pound signs, etc.
• The first letter will be lower case
• We will use variable names that are meaningful
  (except for occasional counters that we might
  call i, j, x, etc.)
• We will concatentate words, and capitalize each
  after the first, e.g., bankBal, thisAcctNum,
  totAmt
• If we abbreviate, we will be consistent. For
  example we will not use both bankBal and
  totalBalance as variable names.

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Our Convention for Struct and Class Names

• In creating names of structs and classes we will
  apply the same rules as for variable names,
  except the first character will be upper case
• Example
• an object's name myCar (or thisOldWreck)
• the struct or class name Car
• Another Example aPerson and Person

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Classes(A Review)

• A class refers to the model, template or master
  form for a category or type of object
• Classes might be Person, Employee, BankAccount,
  Car, TextWindow and MyProgram
• We need to differentiate these from instances of
  the classes, such as person1, anEmployee, myCar
  and person2
• Don't forget instance is another word for
  object
• You create an instance of a class (i.e., an
  object) with a constructor, which may be either
  implicitly or explicitly called
• Person thisPerson // declares thisPerson
  which now refers to a Person object has made an
  implicit call to constructor
• ptrPerson new Person() //the new operator
  requests an amount of memory from the OS, and
  returns a pointer of the appropriate type to that
  memory (note very DIFFERENT than in Java or
  Smalltalk)

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C Statements

• Some are assignment statements
• structure varName expression // l-value
  r-value
• examples balance 0.
• newAmt (myCar.getWt()) 200
• fullName lastName
• Some are not assignment statements

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C Statements

• Some statements are not assignment statements
• structure expression
• function(arguments)
• object.method(arguments)
• CarcarCount() //calling a static ftn
• examples cout ltlt "Amount " ltlt amt
• myCar.setWt(2400)
• strcpy (sameName, origName)
• duration(time1)

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When do you use an assignment statement?

• You use an assignment statement when you need to
  capture or retain the value or object created by
  evaluating a C expression
• no capture myCar.getWt()
• capture currentWeight myCar.getWt()
• Other cases can be more complicated
• may depend on what statements or functions are
  available
• a function can change values of variables used as
  parameters
• sometimes either is possible
• Sometimes the choice can be complicated. Rely
  on
• analysis
• experience

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When Do You Need to Preserve a Value or Object?

• If you will need to refer to it later, such as
  for the following purposes
• to use in a calculation
• if it is an object, to send it a message
• to print it out
• to send it to another routine (i.e., to use as an
  argument for a function)

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C Expressions Using Objects

• A variable is a reference to a location in memory
  where a data type, a struct or an object (i.e.,
  an instance of a class) is stored
• Objects can receive messages
• a variable myCar.getWeight()
• class CreditCardnextID()
• a data type no! No! NO!!!!
• What is the role of the object, the member
  function (e.g., method) and the argument or
  arguments?

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What can an expression return?

• What is returned or created as a result of
  executing each of the following C expressions
• myCar.getWeight()
• inputWindow.prompt("Enter Name ")
• 4 (15.8 - 12.3/7.)
• myCar.setWeight(3000)
• inputStreamObject.readDouble()
• myFile.locatePerson("Jones")

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Not All Expressions Return a Result

• Which expressions do not return a result
• xxx myCar.getWeight()
• xxx inputWindow.prompt("Enter Name ")
• xxx 4 (15.8 - 12.3/7.)
• xxx myCar.setWeight(3000)
• xxx inputStreamObject.readDouble()
• xxx myFile.locatePerson("Jones")
• Do you use an assignment statement where the
  expression does not return a result?

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Declaring variable types to match the result of
an expression

• What type of variable (e.g., int, string, Person,
  etc.) would we need for each assignment
  statement?
• xxx myCar.getWeight()
• xxx inputWindow.prompt("Enter Name ")
• xxx 4 (15.8 - 12.3/7.)
• xxx myCar.setWeight(3000)
• xxx inputStreamObject.readDouble()
• xxx myFile.locatePerson("Jones")

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Arguments

• An argument can be a
• Literal data type (e.g., 5, -43.7, 'z')
• Variable containing a data type (custBal)
• Literal objects and arrays (e.g., "Sally Wynn")
• Variable name containing an object (thisCar)
• An expression (e.g., toupper(middleInitial),
  (custBal 15.6)/2.3)

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Learning More C

• To learn more C
• Create a separate, small program with minimal
  code to test a new feature
• Use printed and on-line sources (see syllabus)
• text
• reference books
• C on-line help system
• on-line tutorials, etc. (see syllabus)
• check "Resources" discussion list

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Looking at Values During Execution

• When your program is not producing correct
  results, you need to examine what your program is
  actually doing
• You can
• Trace the logic by hand (always a good first
  step)
• Look at the values of variables and the internal
  content of objects during execution. You can
• print out values
• use Integrated Development Environment's debug
  features

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MS Visual C 6.0 Debug Features

• We will learn how to
• Set breakpoints
• Do step by step execution
• Watch values
• Trace values at termination

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Some Key Points (Slide 1)

• A variable is a name of a location in memory
  where a data type, struct or object may be stored
• C is "strongly typed" which means that you have
  to declare what each variable can store
• Follow the rules and our agreed-upon conventions
  for creating C variable names, structs and
  class names
• C works with data types, structs and objects,
  and they are different
• When you evaluate a C expression, the result
  can be a data type, a struct, an object, or
  nothing at all

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Some Key Points (Slide 2)

• You need to know what is kind of value/object is
  produced by the evaluation of a C expression
  (so you can do something with it, or assign it to
  a variable)
• C comes with an extensive library, with many
  specialized libraries available too
• C expressions and statements are easy to write,
  after some practice
• Remember that is the assignment operator and
  not an algebraic equal sign