ENERGY 211 CME 211

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ENERGY 211 / CME 211

• Lecture 3
• September 26, 2008

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What's in a C Program?

• A program consists primarily of the declarations
  and definitions of its entities
• Functions for performing operations
• Variables for storing data
• A declaration includes a name and type
• The definition of an entity includes its
  declaration, but more importantly, it describes
  its code and data

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Variables and Functions

• A variable is a named memory location
• Its type indicates the nature of the data stored
  in this memory location
• A function consists of
• The declaration of its name and type
• Zero or more statements
• A function's type consists of the types of its
  input arguments, and the type of its return value

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Statements

• Statements describe the operations performed by a
  function
• Types of statements
• Variable declarations or definitions
• Expressions
• Flow-control statements
• Compound statements (enclosed in )
• Non-compound statements must be terminated with a
  semicolon

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A Simple Program

• include ltiostreamgt // std objects defined in
  iostream
• int main()
• int i // i is an integer
• int largest(0) // largest is initially zero
• do // following statements are to be
  repeated
• stdcout ltlt "Enter a positive number "
• stdcin gtgt i // read integer, store in
  i
• if (i lt 1)
• break // terminate repetition
• if (i gt largest)
• largest i // update largest value
• while (true) // repeat forever
• stdcout ltlt "Largest is " ltlt largest ltlt
  stdendl
• // print largest value and exit

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Variable Definitions

• The statements
• int i // i is an integer
• int largest(0) // largest is initially zero
• declare that i and largest are variables of type
  int (for integer) and define both (i.e. allocate
  storage)
• The (0) initializes the value to zero
• Initialization is not required
• Unlike MATLAB, the type of a variable must be
  declared before it is used

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Scope

• A variable defined within a compound
• statement is unknown outside of it
• A variable declared outside of any
• function is at file scope, and known to
• any function within the file
• A variable declared with the extern
• qualifier is declared, not defined (unless
• initialized), can be used in several files
• A variable can only be defined once
• within a given scope

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What is a Type?

• A type is a specification for entities that have
  specified behavior
• Categories of types
• Value two instances with the same data are
  interchangeable
• Object each instance has its own identity
• For now, we only discuss value types
• Instances of value types can exist as pure
  values, called literals

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Examples of Literals

• Character 'a', '\n' (newline), '\223' (ASCII
  code for ô, in octal)
• String "Hello"
• Boolean true or false
• Integer
• Decimal 0, 2, -3
• Octal 040 ( 32 in decimal)
• Hexadecimal 0xA5 ( 165 in decimal)
• Floating-point -1.0, 1e-4, .14E5

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Fundamental Types

• Fundamental types are value types on which all
  other types are built
• All fundamental types in C are either integral,
  or floating-point
• Integral types
• char (ASCII characters),
• bool (Boolean values, true or false),
• integer types

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Signed Integer Types

• One bit used to indicate sign
• short
• at least 2 bytes (16 bits)
• Supports values from 32,767 to 32,767
• int
• "natural" integer type for given hardware
• usually 4 bytes (32 bits)
• long
• at least 4 bytes
• Supports values from 2,147,483,647 to
  2,147,483,647

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Unsigned Integer Types

• unsigned short
• At least 2 bytes
• Supports values from 0 to 65,535
• unsigned int
• Usually 4 bytes
• unsigned long
• At least 4 bytes
• Supports values from 0 to 4,294,967,295

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Names in C

• The names given to variables and functions are
  known as identifiers
• Identifiers contain letters, digits, underscores
• Must begin with letter or underscore
• Any length is allowed
• Identifiers are case-sensitive!
• Identifiers in simple program main, i, largest,
  std, cout, cin
• Cant use keywords! (see p. 47 in text)

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Beware of Bad Users

• The preceding program expects the user to input
  integers
• The gtgt operator, followed by an integer variable,
  assumes an integer is followed by a space or
  newline character (so it knows when to stop)
• gtgt knows to skip over "white space"
• What if the user types something else?
• In this case, stdcin has the bool value false

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Handling Bad Input

• int main()
• try // attempt to run the following code
• int i int biggest(0)
• do
• stdcout ltlt "Enter a positive
  number "
• stdcin gtgt i
• // bad input, report failure
• if (stdcin false)
• throw stdexception()
• if (i lt 1) break
• if (i gt biggest) biggest i
• while (true)
• stdcout ltlt "Largest is " ltlt biggest ltlt
  '\n'
• // end of try block
• catch (...)
• // any exception sends control to this
  point
• stdcerr ltlt "Exception thrown"
• // end of catch block

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Graceful Failure

• int i
• int biggest(0)
• do
• stdcout ltlt "Enter a positive number "
• stdcin gtgt i
• if (stdcin false)
• // any code in this try block, after the
  throw,
• // will not be executed, so we should
  print out
• // the largest value now
• stdcout ltlt "Largest is " ltlt biggest ltlt
  '\n'
• throw stdexception()
• if ( i lt 1 ) break
• if ( i gt biggest ) biggest i
• while ( true )
• stdcout ltlt "Largest is " ltlt biggest ltlt '\n'

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Characters

• A variable of type char is stored in a single
  byte (8 bits)
• The data stored in this byte is the ASCII code of
  the corresponding character
• The value zero denotes a null character, which
  marks the end of a string
• Literals for special characters, such as
  non-printing characters, are specified using
  escape sequences (like \n for newline) or the
  ASCII code in octal

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Reading a Line of Text

• include ltexceptiongt
• include ltiostreamgt
• int main()
• try
• char c int count(0) // initially count
  is zero
• stdcout ltlt "Type a line." ltlt stdendl
• do // repeat the following
• c stdcin.get() // read
  character
• if ( stdcin false ) throw
  stdexception()
• if ( c '\n' ) break // quit on
  newline
• count // update count of
  characters entered
• stdcout ltlt c // echo character
• while ( true )
• stdcout ltlt "You typed " ltlt count ltlt "
  characters\n"
• catch (...)
• stdcerr ltlt "Exception thrown"

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Floating-point Types

• float
• usually 4 bytes
• At least 6 significant digits of precision
• Not a good choice unless space is limited
• double
• usually 8 bytes
• At least 10 digits of precision
• Should almost always be used
• long double
• Usually 10 bytes
• Not supported by many C compilers

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First Floating-Point Program

• include ltexceptiongt
• include ltiostreamgt
• include ltcmathgt // needed to use math functions
• int main()
• try // initially total 0.0, value is
  uninitialized
• double total(0.0) double value
• do
• stdcout ltlt "Type in a number lt
  9999. "
• stdcin gtgt value
• if ( stdcin false ) throw
  stdexception()
• if ( value gt 9999.0 ) break
• total value value // sum of
  squares
• while ( true )
• // print length of vector of values,
  which is the
• // square root of the sum of squares
• stdcout ltlt "Length is " ltlt sqrt(total)
  ltlt '\n'
• catch (...) stdcerr ltlt "Exception
  thrown\n"

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More About Literals

• Integer literals can have suffixes
• L or l to be interpreted as a long
• U or u to be interpreted as unsigned
• Floating-point literals followed by f or F are of
  type float rather than double, or by l or L for
  extended precision
• String or character literals preceded by L are
  stored as wide-character strings (2-4 bytes each)

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Next Time

• Probably finishing these slides because you asked
  too many questions P
• Ok, its really because I had too many slides
• Arrays