Learning by Example Dissection of Example Program (cont

1
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)

2
Dissection of Example Program (contd) C I/O
Objects

• Defined in iostream.h header file for standard
  class called iostream
• Of particular interest in this course
• Important aspects of interactive I/O file I/O
• Interactive I/O
• Reading from or writing to system monitor
• Need for this is obvious
• File I/O
• Reading from or writing to file on disks
• Required for most homework assignments

3
Dissection of Example Program (contd)
Interactive Output Using cout Statement

• Syntax
• cout ltlt expr1 ltlt expr2 ltlt expr3 ltlt ltlt exprN
• cout - output stream object connected to system
  monitor
• ltlt - stream insertion operator
• expr1, , exprN - placeholders for valid C
  expressions
• stream - sequence of data
• What happens
• Items are inserted into output stream using
  stream insertion operator
• As items are inserted into output stream, they
  flow to system monitor
• Overall effect sequence of data displayed on
  system monitor

4
Dissection of Example Program (contd)
Interactive Output Using cout Statement (contd)

• cout ltlt 250 gives 250
• cout ltlt 7.89 gives 7.89
• cout ltlt A gives A
• cout ltlt A String gives A String
• cout ltlt 1 ltlt 2 ltlt 3 gives 123
• No automatic generation of spacing between items
• cout ltlt 1 ltlt ltlt 3 gives 1 3
• cout ltlt ageInDays gives ltvalue of ageInDaysgt
• ageInDays being a defined variable
• In example program
• cout ltlt "How old are you in years?\n"
• cout ltlt "Your age is about " ltlt ageInDays ltlt
  "\n"
• \n ? escape sequence for newline ( carriage
  return/line feed)

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Dissection of Example Program (contd)Digression
Escape Sequences

• Used in C/C to represent nonprinting characters
  (tab, newline, etc.), and special characters like
  ", ', / and \.
• Each consists of backslash (\, the escape
  character) followed by one or more characters
  (e.g., \n for newline)
• Represent single characters, though each written
  as more than one character
• When encountering \, insertion operator looks
  ahead at the next character(s) combines it/them
  with the backslash to form an escape sequence -
  \ gives signal to escape from normal way of
  interpretation

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Dissection of Example Program (contd)
Digression Escape Sequences (contd)

• Some Common Escape Sequences
• Escape Sequence Meaning
• \f form feed (move to next page)
• \n newline (move to next line)
• \t horizontal tab (move to next tab setting)
• \\ backslash ( \ )
• \ single quote ( ' )
• \ double quote ( " )

7
Dissection of Example Program (contd)
Digression \n versus endl

• Newline escape sequence (\n)
• Generates carriage return/line feed
• End-of-line manipulator (endl)
• Generates carriage return/line feed
• Flushes stream buffer
• Some systems accumulate information in buffer
  until there is enough to justify flushing
• endl manipulator forces any accumulated buffer
  information to be flushed immediately
• When to use which
• Use \n within cout statement for cursor
  control
• Use endl manipulator at end of cout statement
  to flush buffer

8
Dissection of Example Program (contd)
Digression \n versus endl (contd)

• Example
• Assuming current, resistance and voltage are
  defined variables with
• values of 0.001, 4700 and 4.7, respectively, the
  cout statement
• cout ltlt "With a current of " ltlt current ltlt "amp
  and a\n"
• ltlt "resistance of " ltlt resistance ltlt "ohm,
  the\n"
• ltlt "voltage is " ltlt voltage ltlt " volt." ltlt
  endl
• generates the output
• With a current of 0.001 amp and a
• resistance of 4700 ohm, the
• resulting voltage is 4.7 volt.
• Applying to example program
• cout ltlt "How old are you in years?" ltlt endl
• cout ltlt "Your age is about " ltlt ageInDays ltlt endl

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Dissection of Example Program (contd)
Digression Formatted Output Using Manipulators
(iomanip.h)

• setw(width)
• Display next value in a field with specified
  width (integer)
• Default is 0, automatically grows to accommodate
• setprecision(precision)
• Display values with specified precision (integer)
• Usual default is 6
• setiosflags( flag1 flag2 ... flagN )
• Set formatting flag(s) like
• iosshowpoint - display decimal point
  trailing zeroes
• iosfixed - display real values in
  fixed-point form
• iosscientific - display real values in
  floating-point form
• iosleft - display values left-justified
  within field
• iosright - display values right-justified
  within field

10
Dissection of Example Program (contd)
Digression Formatted Output Using Manipulators
(contd)

• Example
• Given that x y are defined variables with
  values of 1.234 5, the output statement
• cout ltlt setiosflags(iosshowpoint iosfixed)
• ltlt setprecision(3) ltlt "\n("
• ltlt setw(10) ltlt x ltlt ")\n("
• ltlt setw(10) ltlt y ltlt ")" ltlt endl
• generates the output
• ( 1.234)
• ( 5.000)

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Dissection of Example Program (contd)Digression
Formatted Output Using Member Functions

• Example (Using Stream Object Member Functions)
• Given that x y are defined variables with
  values of 1.234 5, the output statements
• cout.setf(iosfixed)
• cout.setf(iosshowpoint)
• cout.precision(3)
• cout.width(10)
• cout ltlt "\n(" ltlt x ltlt ")\n(" ltlt y ltlt ")" ltlt
  endl
• generates the output
• ( 1.234)
• ( 5.000)

12
Dissection of Example Program (contd)
Interactive Input Using cin Statement

• Syntax cin gtgt var1 gtgt var2 gtgt var3 gtgt gtgt
  varN
• cin - input stream object connected (by
  default) to system keyboard
• gtgt - stream extraction operator
• var1, , varN - variables defined in program
• Notes
• gtgt operator extracts data items to be read from
  input buffer
• Items extracted are assigned, in respective
  order, to listed variables
• Variables must be defined prior to appearing in
  cin statement
• Any white space (spaces, tabs or newlines) can be
  used to separate data items (gtgt operator ignores
  all white-space characters)
• Program execution is suspended until all
  extraction operations in an input statement are
  complete
• Good practice to precede cin statements with
  appropriate prompts

13
Dissection of Example Program (contd)
Interactive Input Using cin Statement (contd)

• Examples
• Single data item
• cin gtgt x
• Multiple data items
• cin gtgt x gtgt y gtgt z
• In example program (prompt preceding cin
  statement)
• cout ltlt "How old are you in years?\n"
• cin gtgt ageInYears
• cout is flushed automatically whenever cin is
  used

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Dissection of Example Program (contd)
Interactive Input Using cin Statement (contd)

• Consider following program
• include ltiostream.hgt
• int main()
• int score1 float score2
• cout ltlt "Enter value for score1 " ltlt endl
• cin gtgt score1
• cout ltlt "Enter value for score2 " ltlt endl
• cin gtgt score2
• return(0)
• What happens when, in response to the prompts,
• 98.5 followed by 78 are entered ?
• 98 followed by 78 are entered ?

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Dissection of Example Program (contd)
Interactive Input Using cin Statement (contd)

• Reading Character/String Data
• When reading character data using cin, keep mind
  that
• Only one character is read each time
• White spaces (spaces, tabs, newlines, etc.) are
  ignored by cin when using the gtgt operator
• check out get( ) function (page 279 of
  textbook)
• syntax cin.get( char_variable )
• Numeric values can be read as characters - each
  digit is read as separate character
• Difficulties will also arise when one tries to
  use cin with the gtgt operator to read string
  data
• Check out functions like getline( ) (page 397 of
  textbook)

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Dissection of Example Program (contd)Digression
File Processing in C

• Need for File Processing
• Have seen how data/results can be interactively
  input/output (from/to standard input/output
  device) - cin cout
• Often times data/results must be written to or
  read from secondary storage devices (disks) for
  sharing between users/programs
• Future assignments of this class, for example,
• Will need to use my data to test your programs
• Will need to write results to file to allow
  viewing/printing
• Need to know how to process (text) files in C
• Declaring, initializing closing file stream
  (fstream) objects
• Reading from files using fstream objects
• Writing to files using fstream objects

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Declaring File Stream (fstream) Objects
• For C program to read values from file, an
  ifstream (input file stream) object must first
  be constructed (declared) to serve as connection
  from file to program
• Syntax ifstream InputStreamName
• Example ifstream inputStream1
• Similarly, for C program to write values to
  file, an ofstream (output file stream) object
  must first be constructed (declared)
• Syntax ofstream OutputStreamName
• Example ofstream outputStream1
• ifstream ofstream classes are declared in
  fstream library
• fstream.h header file must be included to use
  library
• After fstream (ifstream or ofstream) object is
  constructed, various operations can then be
  applied to it (will see)

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Dissection of Example Program (contd)
Digression File Processing in C (contd)

• Initializing File Stream Objects
• fstream objects are uninitialized when declared
  (constructed)
• They serve as potential connections between
  program files
• Actual connections are made using open( )
  member function
• Syntax fstream_name.open ( file_name , mode )
• fstream_name is name of fstream object to be
  initialized
• file_name is name (character string) of file
• mode is optional argument indicating how file is
  to be opened
• Example inputStream1.open("in.dat", iosin)
• inputStream1 is ifstream object previously
  declared
• in.dat is name of file - as character string
• iosin specifies how file is to be opened (more
  on this next)

19
Dissection of Example Program (contd)
Digression File Processing in C (contd)

• fstream Modes (not an exhaustive list)
• Mode Description
• iosin Open file for input, nondestructively,
  with read position
• at files beginning. Default mode for
  ifstream objects.
• iosout Open file for output, destructively.
  Default mode for
• ofstream objects.
• iosapp Open file for output,
  nondestructively, with write position
• at files end (I.e., for appending)
• iosate Open existing file with read position
  (if ifstream object)
• or write position (if ofstream object) at end
  of file
• iosbinary Open file in binary mode

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Initializing fstream Objects at Declaration
• C allows initializing an fsteam object when it
  is declared
• ifstream inputStream1("in.dat", iosin)
• is equivalent to
• ifstream inputStream1
• inputStream1.open("in.dat", iosin)
• This is similar to initializing a variable at
  declaration such as
• int total 0
• For this reason, open( ) member function is
  rarely used in practice

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Testing Success of File Opening
• File opening operation may fail for some reasons
• If so, any subsequent read or write attempts to
  that file will also fail
• Thus, success in opening file should always be
  tested
• Sample code segment
• if ( fstream_name.fail() )
• cerr ltlt "Error opening " ltlt file_name ltlt endl
• exit(1)
• fstream_name is name of fstream object for which
  connection to file named file_name has been
  attempted

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Closing fstream Objects
• Syntax fstream_name.close( )
• Two things accomplished
• Executing program data file (that have been
  connected through fstream object named
  fstream_name) are disconnected
• fstream_name becomes undefined
• Example
• inputStream1.close()
• Good programming practice to close (disconnect)
  fstream objects explicitly when they are no
  longer needed
• Important when program uses many files -
  operating system usually has limits on of files
  program may have open simultaneously

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Reading from/Writing to fstream Objects
• Reading using input operator (gtgt)
• Consistent with extraction operator used in
  interactive input (cin)
• Example
• inputStream1 gtgt Reading
• Writing using output operator (ltlt)
• Consistent with insertion operator used in
  interactive output (cout)
• Example
• outputStream1 ltlt "Average " ltlt Avg ltlt ".\n"

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Dissection of Example Program (contd)Digression
File Processing in C (contd)

• Two Other Items of Interest
• eof( )
• inputStream1.get(reading)
• while ( !inputStream1.eof() )
• // some desired processing statements here
• inputStream1.get(reading)
• getline( )
• Reads characters from fstream object, storing
  them in string object, until newline character is
  read
• Newline character not stored in string object

25
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)

26
Learning by Example Dissection of Example
Program

• The return Statement
• Allows executing function to directly return
  value to calling function
• Only one value can be returned using the
  statement
• Syntax
• return expression
• Examples
• return (0)
• return (x y z)
• return ( sin(alpha) cos(beta) )