Learning by Example Example Program

1
Learning by Example Example Program

• / Example program computes prints users age
  in days
• Course CS 2308-xxx Author ...
  Date ...
• ... /
• include ltiostream.hgt
• int main(void)
• const int DAYS_PER_YEAR 365 // constant
  declaration
• int ageInYears, ageInDays // variable
  declarations
• cout ltlt "Enter your age in years (whole
  number) "
• cin gtgt ageInYears
• ageInDays ageInYears DAYS_PER_YEAR
• cout ltlt "Your age is about " ltlt ageInDays ltlt "
  days.\n"
• return(0)

2
Learning by Example Dissection of Example
Program

• / Example program computes prints users age
  in days
• Course CS 2308-xxx Author ...
  Date ...
• ... /
• include ltiostream.hgt
• int main(void)
• const int DAYS_PER_YEAR 365 // constant
  declaration
• int ageInYears, ageInDays // variable
  declarations
• cout ltlt "Enter your age in years (whole
  number) "
• cin gtgt ageInYears
• ageInDays ageInYears DAYS_PER_YEAR
• cout ltlt "Your age is about " ltlt ageInDays ltlt "
  days.\n"
• return(0)

3
Dissection of Example ProgramComments

• Explanatory remarks
• what entire program is about
• what each line of code (or set of same) is meant
  to do or reason behind its use
• anything useful for own future reference or for
  helping others understand your program
• Syntax 1 (C/C) / ltcommentgt /
• Syntax 2 (C Only) // ltcommentgt
• delimiters (/, / //) must appear as single
  units

4
Dissection of Example ProgramMore on Comments

• No effect on program size or execution speed
• Can be placed anywhere in program
• Syntax 1 best when across two or more lines
• As in example program
• Syntax 1 cannot nest one within another
• Dont do / A comment / Another comment / /
• Syntax 2 best when one line or same line as
  actual code
• // An example one-line comment
• int ageInYears, ageInDays // variable
  declarations

5
Dissection of Example ProgramMore on Comments
(contd)

• Almost always a good thing to have
• Most programmers dont use enough comments
• Important part of any program (esp. for this
  class)
• Remember
• Not every one is as smart as you are or think or
  see things like you do
• You may not remember undocumented stuff yourself
• Details are represented by your codes, so
  comments should concentrate on big picture, not
  details
• Should clarify any significant reason underlying
  you work (why do it why doing it the way you do)

6
Learning by Example Dissection of Example
Program (contd)

• / Example program computes prints users age
  in days
• Course CS 2308-xxx Author ...
  Date ...
• ... /
• include ltiostream.hgt
• int main(void)
• const int DAYS_PER_YEAR 365 // constant
  declaration
• int ageInYears, ageInDays // variable
  declarations
• cout ltlt "Enter your age in years (whole
  number) "
• cin gtgt ageInYears
• ageInDays ageInYears DAYS_PER_YEAR
• cout ltlt "Your age is about " ltlt ageInDays ltlt "
  days.\n"
• return(0)

7
Dissection of Example Programinclude
Preprocessor Directive

• Tells compiler to insert contents of another file
  into your source file, at where the directive is
  located
• In effect, include directive is replaced by the
  contents of the file indicated
• Syntax include lt name of file to be inserted gt
• In example program, specifically,
• include ltiostream.hgt
• tells compiler to add contents of file
  iostream.h to the source file before actual
  compilation starts

8
Dissection of Example ProgramDigression About
iostream.h

• Stands for input/output stream header file
• Comes packaged with C compiler
• Contains declarations for iostream library -
  provides interactive I/O capabilities
• Because example program uses objects cin and
  cout, declarations in iostream.h are needed so
  that compiler can recognize these objects

9
Dissection of Example ProgramDigression Header
Files

• Example iostream.h weve just discussed
• Header because they are placed at the top or
  head of C/C programs
• Other commonly used standard header files
• iomanip.h (output formatting capabilities)
• math.h (math functions)
• ctype.h (single-character processing)
• stdlib.h (commonly used functions)

10
Dissection of Example ProgramDigression Header
Files (contd)

• Inserting custom header files
• Custom header files header files written by you
• Syntax include path and name of file
• Example include myheader.h
• What difference does this make?
• Need for header files
• All programs (except trivial ones) invariably
  need one or more header files
• Explicitly include header files for portability
• Header files optional for some compilers

11
Dissection of Example ProgramDigression
Preprocessor Directives

• Numerous preprocessor directives in C/C,
  notably
• include - beaten to death already
• define - create symbolic constants (in C,
  discussed next) and macros (will not discuss in
  this course)
• Creating symbolic constants in C using define
• Symbolic (names for) constants
• Syntax define symbolic-name constant
• Example define MARKUP 1.75
• Customary use all uppercase for symbolic
  constants
• ? In C, named constant const double MARKUP
  1.75

12
Dissection of Example ProgramDigression
Preprocessor Directives (contd)

• Rules
• All preprocessor directives begin with
• A preprocessor directive must be written on one
  line, except in some cases where a backslash ( \
  ) can be used to indicate continuation onto next
  line
• Only whitespace characters may appear before a
  preprocessor directive on a line
• Other uses of preprocessor directives
• Conditional execution of preprocessor directives
• will use this when separating interface from
  implementation
• Conditional compilation of program code
• will not see this in this course

13
Learning by Example Dissection of Example
Program (contd)
/ Example program computes prints users age
in days Course CS 2308-xxx Author
... Date ... ... / include
ltiostream.hgt int main(void) const int
DAYS_PER_YEAR 365 // constant declaration
int ageInYears, ageInDays // variable
declarations cout ltlt "Enter your age in years
(whole number) " cin gtgt ageInYears
ageInDays ageInYears DAYS_PER_YEAR cout
ltlt "Your age is about " ltlt ageInDays ltlt "
days.\n" return(0)
14
Dissection of Example ProgramDigression
Functions

• One of the fundamental building blocks of C/C
• A C/C program consists of one or more functions
• Example program consists almost entirely (save
  comments preprocessor directive) of a single
  main function
• Self-contained units of program code designed to
  perform specific tasks
• Subprograms (small programs) that can be called
  by other programs (calling programs) to
  accomplish particular tasks
• Essentially programs (though small), thus
  characteristics design are similar to those for
  programs (discussed earlier)
• Need some means for receiving data returning
  results

15
Dissection of Example ProgramDigression
Functions (contd)

• C/C Function Format (informally somewhat
  simplified)
• ReturningSpec CallingSpec ( ReceivingSpec )
• Function Body
• Parentheses are distinguishing feature of a
  function
• Every function must have pair of braces -
  delimit body of function
• More formal terms
• ReturningSpec ??Return Data Type
• CallingSpec ??Function Name
• ReceivingSpec ??Function Parameters (Arguments)
• To discuss basic data types in C/C naming
  rules for now

16
Dissection of Example ProgramFurther Digression
Basic C/C Data Types

• (Operating system/compiler dependent examples
  below typical for PC )
• Data Type Size (bytes) Lower Bound Upper Bound
• char 1 - -
• int 2 ?????? ??????
• unsigned int 2 ? ?????
• long int 4 ??3?? ??3???
• float 4 ??????????? ??????????
• double 8 ??????????? ????????????
• char - alphabet letters (lower upper case),
  digits, special symbols
• int, unsigned int, long int - integer (short
  int also in C)
• float - single-precision floating-point
• double - double-precision floating-point (long
  double also in C)
• ? bool - booleans (included in ANSI C
  standard)

17
Dissection of Example ProgramFurther Digression
Naming Rules

• Same rules for all C/C identifiers (functions,
  variables, etc.)
• May use only upper lower case letters, digits
  (0 to 9), and underscore ( _ ) no other
  characters (spaces included) allowed
• First character must be a letter or underscore
• Only 31 characters is recognized by most
  compilers
• Cannot be the same as a C/C keyword (reserved
  word)
• C Keywords (as listed in page 638 of textbook)
• asm class double friend new return switch union
• auto const else goto operator short template uns
  igned
• break continue enum if private signed this virtu
  al
• case default extern inline protected sizeof thro
  w void
• catch delete float int public static try volatile
  
• char do for long register struct typedef while
• ? There are additional keywords in ANSI C
  standard

18
Dissection of Example ProgramFurther Digression
Common Sense Naming Rules

• All names should be mnemonic - memory aid by
  design
• Should be carefully chosen to be as meaningful
  as self-documenting as possible
• Names that are not adequately self-documenting
  should be supplemented with comments
• All program code, including names, should in
  general be in lowercase letters - leave names
  that begin with uppercase letters or use all
  uppercase letters for special cases (all
  uppercase letters for symbolic constants, for
  example)
• Proper use of multiple-word names can improve
  readability
• Not good totalcommissionstodate
• Better total_commissions_todate or
  totalCommissionsTodate

19
Dissection of Example Program (contd)Function
Header

• Structure
• int main(void)
• Function Body
• ReturningSpec - int specifies that data type of
  result to be returned is of type int
• Defaults to int if not specified (common
  practice for main function)
• Use void if function returns nothing
• ReceivingSpec - void specifies that nothing
  will be received
• Use of void is optional (for functions that
  receive nothing)
• CallingSpec - main is name with which function
  is called

20
Dissection of Example Program (contd)Digression
main Function Specials

• All C/C programs must have a function called
  main
• For a C/C program w/ only 1 function, that
  function must be called main - this is the case
  with example program
• If there is no function called main - linker
  will signal error
• Execution of all C/C programs starts with main
• In most programs, the main function plays a
  driver role - calls other functions to carry
  out programs real work
• For this reason, it is sometimes referred to as
  the driver function or simply the driver

21
Learning by Example Dissection of Example
Program (contd)
/ Example program computes prints users age
in days Course CS 2308-xxx Author
... Date ... ... / include
ltiostream.hgt int main(void) const int
DAYS_PER_YEAR 365 // constant declaration
int ageInYears, ageInDays // variable
declarations cout ltlt "Enter your age in years
(whole number) " cin gtgt ageInYears
ageInDays ageInYears DAYS_PER_YEAR cout
ltlt "Your age is about " ltlt ageInDays ltlt "
days.\n" return(0)
22
Dissection of Example Program (contd)Function
Body

• Structure
• C/C Statements
• Purpose performs all necessary operations to
  accomplish the specific task for which function
  is designed
• Includes processing of data received (if any)
• Can directly return at most one value back to
  calling program (multiple values can be returned,
  but indirectly, more on this to come)
• May call other functions

Compound Statement (Block)
23
Dissection of Example Program (contd)Function
Body (contd)

• Statements
• Fundamental unit of C/C programming
• Many C/C statements must be ended with
  (crucial part of syntax but very easy to forget
  - syntax error)
• Preprocessor directives - not statements (dont
  end with )
• Declaration Statements
• Naming and defining variables (more on this to
  come)
• Must appear before executable statements in C
  (not in C)
• Executable Statements
• Cause computer to perform some operations
• Constitute majority of statements
• Much of what you have to learn in this class

24
Learning by Example Dissection of Example
Program (contd)
/ Example program computes prints users age
in days Course CS 2308-xxx Author
... Date ... ... / include
ltiostream.hgt int main(void) const int
DAYS_PER_YEAR 365 // constant declaration
int ageInYears, ageInDays // variable
declarations cout ltlt "Enter your age in years
(whole number) " cin gtgt ageInYears
ageInDays ageInYears DAYS_PER_YEAR cout
ltlt "Your age is about " ltlt ageInDays ltlt "
days.\n" return(0)
25
Dissection of Example Program (contd)Declaration
of Named Constants

• Declaration statement
• const int DAYS_PER_YEAR 365
• declares DAYS_PER_YEAR to be an integer
  constant whose value is 365
• Constants
• Cannot be assigned another value (even if its
  the same value)

26
Dissection of Example Program (contd)Declaration
of Variables

• Declaration statement
• int ageInYears, ageInDays
• declares that variables ageInYears
  ageInDays are intended to hold data of type int
• Variables
• Most fundamental part of any language
• Symbolic names - refer to locations in computer
  memory
• When value is assigned to a variable, value is
  stored in memory location referred to by the
  variable
• Must be defined before they can be used
• C only must be defined before first executable
  statement

27
Dissection of Example Program (contd)Digression
Declarations and Definitions

• Subtle distinction as applied to variables
• Declaration introduces a variable into program
  and specifies its data type
• If declaration also sets aside memory for
  variable (up to compiler to decide), it is called
  definition
• Most declarations are also definitions
• Cases where declarations are not definitions most
  commonly occur when multiple programs are
  involved (may encounter later in this class)

28
Learning by Example Dissection of Example
Program (contd)

• / Example program computes prints users age
  in days
• Course CS 2308-xxx Author ...
  Date ...
• ... /
• include ltiostream.hgt
• int main(void)
• const int DAYS_PER_YEAR 365 // constant
  declaration
• int ageInYears, ageInDays // variable
  declarations
• cout ltlt "Enter your age in years (whole
  number) "
• cin gtgt ageInYears
• ageInDays ageInYears DAYS_PER_YEAR
• cout ltlt "Your age is about " ltlt ageInDays ltlt "
  days.\n"
• return(0)

29
Dissection of Example Program (contd)Digression
Operators in C/C

• Arithmetic Operators in C/C
• Operation C/C Operator Algebraic E.g. E.g. in
  C/C
• Addition x 7 x 7
• Subtraction - x - y x - y
• Multiplication xy or x ??y or x ??y x
  y
• Division / x / y
• Modulus x mod y x y
• Operands must represent numeric values
  (characters okay, why?)
• Modulus - integer operands, non-zero second
  operand
• Division - non-zero second operand
• No exponentiation operator in C/C (use math
  function pow)

30
Dissection of Example Program (contd)Digression
Operators in C/C (contd)

• Relational Operators in C/C
• Standard
• Algebraic C/C
• Relational Relational C/C Meaning of
• Operator Operator Example C/C Example
• ? gt x gt y x greater than y
• ?? lt x lt y x less than y
• ?? gt x gt y x greater or equal to y
• ? lt x lt y x less than or equal to y
• No spaces between the two symbols in gt and lt
• The two symbols in gt and lt cannot be reversed

31
Dissection of Example Program (contd)Digression
Operators in C/C (contd)

• Equality Logical Operators in C/C
• Standard
• Algebraic C/C
• Equality/Logical Equality/Logical
  C/C Meaning of
• Operator Operator Example C/C E.g.
• ? x y x equal y
• ?? ! x ! y x not equal y
• AND? x y x AND y
• OR x y x OR y
• NOT ! !x NOT x
• No spaces between the two symbols in , !,
  and

32
Dissection of Example Program (contd)Digression
Operators in C/C (contd)

• More Operators in C/C
• Name Operator Example Effect
• unary minus - -x reverse sign of x
• preincrement x increment x by 1 then use
  new
• value of x in expression
• postincrement x use current value of x in
• expression then increment x by 1
• predecrement -- --x decrement x by 1 then use
  new
• value of x in expression
• postdecrement -- x-- use current value of x
  in
• expression then decrement x by 1
• cast (type) (int) x create temporary integer
  copy of x
• No spaces between the two symbols in and --

33
Dissection of Example Program (contd)Digression
Operators in C/C (contd)

• Still More Operators in C/C (Still Many More
  Not Listed)
• Name Operator Example Effect
• assignment x y assign (store) value of y
  in x
• x y x x y
• - x - y x x - y
• x y x x y
• / x / y x x / y
• x y x x y
• address of x address of variable x
• indirection x variable whose address is
  stored in x
• conditional ? (x lt y) ? x y if x is less
  than y then x else y
• (see Appendix D of text, pages 641-642, for
  complete list of C/C operators)

arithmetic assignment
34
Dissection of Example Program (contd)
Digression Operators in C/C (contd)

• Operator Precedence Associativity
• Precedence order-of-evaluation hierarchy
  according to which C/C operators are grouped
• Operations with higher precedence carried out
  first
• Example , / and belong to one group, and
  and - belong to another
• Former group has higher precedence than latter
  group
• Mult., div. mod. operations carried out
  before add. sub.
• Natural order of evaluation can be altered
  through use of parentheses
• Associativity order in which consecutive
  operations of the same precedence are carried
  out
• left-to-right or right-to-left
• See Appendix D of text (pages 641-642) for
  complete listing

35
Dissection of Example Program (contd)Digression
Expressions in C/C

• Expression combination of 1 or more operands
  0 or more operators that usually evaluates to
  a value
• single entity - constant, variable, function
  reference, etc.
• combination of such entities - connected by
  operators
• Almost all of C/C executable statements use
  expressions
• Examples 12.3 A
• years 10 a b c
• x y c a b
• x y counter
• To repeat, a C/C expression usually evaluates
  to a value
• In expression x 5, both variable x and
  expression x 5 have value 5
• Since is an operator, this makes expressions
  like x y z 0 valid

36
Dissection of Example Program (contd)Digression
Expressions in C/C (contd)

• Some Notes on Arithmetic Expressions
• Integer expression (expression containing only
  integer operands) results in integer value
• Floating-point expression (expression containing
  only floating-point operands) results in
  double-precision value
• Mixed-mode expression (expression containing both
  integer and floating-point variables) results in
  double-precision value
• Division of two integers always produces integer
  result - any fractional part is dropped
  (truncated)
• Example value of 17 / 5 is 3
• Terminating expression with results in
  statement

37
Dissection of Example Program (contd)Assignment
Statements

• Tell computer to assign (store) values into
  variables
• Syntax variable any valid C/C expression
  
• Always have (assignment operator) a variable
  name immediately to the left of the operator
• Value of expression on the right of first
  evaluated then stored in variable whose name
  appears on the left of
• Expression on the right of can involve the
  same variable that appears on the left of
  (e.g., x x 5)
• x 10 y z is not a valid statement, why?
• Since expressions like x y z 0 are valid
  in C/C, statements like x y z 0 are
  valid in C/C

38
Learning by Example Dissection of Example
Program (Next Time)

• / Example program computes prints users age
  in days
• Course CS 2308-xxx Author ...
  Date ...
• ... /
• include ltiostream.hgt
• int main(void)
• const int DAYS_PER_YEAR 365 // constant
  declaration
• int ageInYears, ageInDays // variable
  declarations
• cout ltlt "Enter your age in years (whole
  number) "
• cin gtgt ageInYears
• ageInDays ageInYears DAYS_PER_YEAR
• cout ltlt "Your age is about " ltlt ageInDays ltlt "
  days.\n"
• return(0)