Streams

1
Streams
Chapter 8
2
C Input/Output Streams

• The basic data type for I/O in C is the stream
• We've used the standard input/output streams
• istream cin built-in input stream variable
• ostream cout built-in output stream variable
• The I/O functions of C can be also be used in a
  C program
• From library ltstdiogt scanf (input), printf
  (output)
• In CS-201, use streams for input and output of
  data

3
iostream Library Header Files

• iostream library
• ltiostreamgt cin, cout, cerr, and clog objects
• ltiomanipgt parameterized stream manipulators
• ltfstreamgt information important to
  user-controlled file processing operations

4
Output the Insertion Operator

• The standard I/O streams are defined in the
  iostream library. Accessing the objects and
  functions from this library requires the
  following pre-processor directive include
  ltiostreamgt
• Then output is achieved with a statement like
  cout ltlt " X " ltlt X
• The ltlt (insertion operator) is
• Binary, left associative
• Requires an output stream (like cout) for its
  left argument
• Supports many data type for its right argument
• Returns an output stream object
• Thus Insertions to an output stream can be
  "chained" together

5
Tracing Output Execution

• Assume X 25, A 10, B -3
• cout ltlt " X " ltlt X //"X " is output
• cout ltlt X //"25" is output
• cout //no effect
• cout ltlt (A B) ltlt " " ltlt (A B) ltlt endl
• cout ltlt 7 ltlt " " ltlt (A B) ltlt endl
• cout ltlt 7 ltlt " " ltlt 13 ltlt endl
• cout ltlt " " ltlt 13 ltlt endl
• cout ltlt 13 ltlt endl
• cout ltlt endl
• cout

6
Input the Extraction Operator

• As with the standard output stream, cout, to use
  the standard input stream (cin) a program must
  use the pre-compiler directive include
  ltiostreamgt
• Then input is achieved with a statement like
  cin gtgt X where X is the name of the variable
  assigned the value read from the keyboard

7
Input the Extraction Operator

• gtgt (stream-extraction) used to perform stream
  input
• Normally ignores whitespaces (spaces, tabs,
  newlines)
• Returns zero (false) when EOF is encountered
• otherwise returns a reference to the object from
  which it was invoked (i.e. cin)
• This enables cascaded input. cin gtgt x gtgt y

8
iostream Stream Objects

• istream input streams
• cin gtgt someVariable
• cin knows what type of data is to be assigned to
  someVariable (based on the type of someVariable)
• ostream output streams
• cout ltlt someVariable
• cout knows the type of data to output
• cerr ltlt someString
• Unbuffered. Prints someString immediately
• clog ltlt someString
• Buffered. Prints someString only when output
  buffer is full or flushed

9
Priority of Stream Operators

• gtgt and ltlt have relatively high precedence
• conditional assignment expressions must be in
  parenthesescout ltlt "1 2 " ltlt (x 5)
• NOTcout ltlt "1 2 " ltlt x 5

10
Character output with put

• put member function
• Outputs one character to specified stream
• cout.put('A')
• May be called with an ASCII-valued expression
• cout.put(65)
• outputs A

11
get Member Functions

• cin.get() inputs a character from stream (even
  white spaces) and returns it
• cin.get(c) inputs a character from stream and
  stores it in c
• Reference parameter

12
Type-Safe I/O

• ltlt and gtgt operators
• Designed to accept data of different types
• When unexpected data encountered, error flags set
• Program stays in control

13
Extraction Operator and Whitespace

• In programming, common characters that do not
  produce a visible image on a page or in a file
  are referred to as whitespace
• The most common whitespace characters are
• newline (\n)
• tab (\t)
• space
• By default, the extraction operator in C will
  ignore leading whitespace characters
• That is, the extraction operator will remove
  leading whitespace characters from the input
  stream and discard them
• A mechanism exists for overiding this behavior

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Details of an Extraction
Assume the input stream cin contains 12 17.3
-19

• Suppose that X is declared as an int, and the
  following statement is executed cin gtgt X
• The type of X determines how the extraction is
  performed
• First, any leading whitespace characters are
  discarded
• Since an integer value is being read, the
  extraction will stop when a character that
  couldn't be part of an integer is found
• The digits '1' and '2' are extracted, and the
  next character is whitespace, so the extraction
  stops and X gets the value 12
• The whitespace after the '2' is left in the input
  stream

15
ignore() Member Function

• There is also a way to remove and discard
  characters from an input stream cin.ignore(5,
  '')
• read and discard characters from the input stream
  until
• 5 characters have been discardedor
• A '' character has been read and discarded

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Stream Manipulators Member Functions

• stream manipulator/member function capabilities
• setting field widths
• setting precisions
• setting the format of bool output
• setting the fill character in fields
• flushing streams
• inserting a newline in the output stream and
  flushing
• inserting a null character in the output stream
• skipping whitespace in the input stream

17
Some examples of cout member functions

• precision member function
• sets number of digits to the right of decimal
  point
• cout.precision(2)
• cout.precision() returns current precision
  setting, unchanged
• setf member function
• Used to set one of many formats
• Often used in place of manipulators
• cout.setf(ios_baseboolalpha)