Ch. 10 Input/Output

1
Ch. 10 Input/Output

• Oregon State University
• Timothy Budd

2
Introduction

• Two competing I/O systems in C.
• Standard I/O Library, stdio
• Stream I/O Library, iostream
• Never use both Library in the same program.

3
The stdio Library

• Inherited from the C.
• Widely known and available.
• Based on stable and well-exercised libraries.
• Not as extendable or adaptable as the newer
  Stream I/O Library.
• For developing new programs, the Stream I/O
  Library is preferred.

4
Examples of stdio

• include ltcstdiogt
• int c getchar() // read a single input
  character putchar('x') // print a single
  character
• char text "please enter your
  name"puts(text) // print the text
  stringchar buffer120gets(buffer) // read a
  line of input
• FILE fp fopen("mydata.dat", "r")if (fp
  NULL)puts("file cannot be opened")
• fputc('z', fp) // write character to fpint c
  fgetc(fp) // read character from fpchar msg
  "unrecoverable program error"fputs(msg,
  stderr) // write message to standard error output

5
Formatted Output

• The printf facility works only for formatted
  primitive values, such as integers and floats.
• Not easily extendable to user-defined data types.
• Always verify that formatting commands match the
  argument types.

6
Examples of Formatted Output

• d integer decimal value
• o integer printed as octal
• x integer printed as hexadecimal
• c integer printed as character
• u unsigned integer decimal value
• f floating point value
• g floating point value
• s null terminated string value
• percent sign

7

• int i 3
• int j 7
• double d i / (double) j
• printf("the value of d over d is g", i, j,
  d)
• char fileName ...
• if (fopen(fileName, "r") null)
• fprintf(stderr,"Cannot open file s\n",
  fileName)
• char buffer180
• sprintf(buffer, "the value is d\n", sum)
• double d 23.5
• printf("the value is d\n", d) // error -- float
  printed as int
• scanf("d f", i, f) // read an int, then a
  float

8
The Stream I/O Facility

• Whenever possible use the Stream I/O Library
  rather than the Standard I/O Library.
• Use the ability to overload function names in
  C.
• Better possibilities for extendability as well as
  improved error detection.

9
Figure 10.2 Various Overloading Versions of the
ltlt Operator

• ostream operator ltlt (ostream out, const int
  value)
• // print signed integer values on a
  stream unsigned int usvalue if (value lt 0)
  // print leading minus sign out ltlt
  '-' usvalue - value else usvalue
  value // print non-negative number out ltlt
  usvalue return out

10

• inline char digitCharacter(unsigned int value)
• // convert non-negative integer digit into
  printable digit // assume value is less than
  nine return value '0'
• ostream operator ltlt (ostream out, const
  unsigned int value)
• // print unsigned integer values on a stream
• if (value lt 10) out ltlt digitCharacter(value)
  else out ltlt (value / 10) // recursive
  call out ltlt digitCharacter(value 10)
• // print single char return out

11
Examples of Stream I/O

• include ltiostreamgt
• cout ltlt "n " ltlt n ltlt " m " ltlt m ltlt " average " ltlt
  (nm)/2.0 ltlt '\n'
• Easy to provide formatting capabilities for a new
  data type.
• ostream operator ltlt (ostream out, const
  rational value) // print representation of
  rational number on an output stream out ltlt
  value.numerator() ltlt '/' ltlt value.denominator()
  return out
• rational frac(3,4)
• cout ltlt "fraction of " ltlt 3 ltlt " and " ltlt 4 ltlt "
  is
• ltlt frac ltlt endl

12
Another Examples

• A manipulator is used to change features of the
  I/O system.
• ostream endl (ostream out) // write the end
  of line characterout ltlt '\n'// then flush the
  bufferout.fflush()// then return the
  bufferreturn out
• ostream operator ltlt (ostream out, ostream
  (fun)(ostream )) // simply execute
  functionreturn fun (out)

13
Stream Input

• Stream input always ignores white space.
• cin gtgt intval
• while (cin gtgt intval) // process
  intval ...// reach this point on end of
  input...
• Visualize gtgt and ltlt as arrows
• Input operator, gtgt x points data into x
• Output operator, ltlt x copies data out of x

14
String Streams

• A string stream writes to or reads from a string.
• Must include the sstream header file.
• ostringstream creates an output stream.
• Values are buffered in an internal string and can
  be accessed with the member function str.
• Useful for formatting complex output not only
  perform catenation, but also automatically
  convert the right argument from numerous
  primitive data types into character values.
• Can be used as the string catenation operator
  commonly used in Java

15

• include ltsstreamgt
• int n ...
• int m ...
• ostringstream formatter
• formatter ltlt "the average of " ltlt n ltlt " and "
• ltlt m ltlt " is " ltlt ((n m)/2.0)
• string s formatter.str()
• string text "Isn't this a wonderful feature"
• istringstream breaker(text)
• string word
• while (breaker gtgt word)cout ltlt word ltlt endl //
  print each word on a separate line

16
File Streams

• A file stream is a stream that reads from or
  writes to a file.
• Must include the header file
• include ltfstreamgt
• fstream header file includes the iostream header,
  so not necessary to include both.
• The class ifstream and ofstream are used to creat
  streams that are attached to input and output
  files.

17

• A conversion operator changes a file stream into
  a boolean value, whereby the value indicates the
  success or failure of the file opening operation.
• char fileName "outfile.dat"
• ofstream ofd(fileName) // create file for
  output
• if (! ofd) cerr ltlt " cannot open file " ltlt
  fileName
• else ofd ltlt "first line in file" ...
• File operations in C throw far fewer exceptions
  than do in Java.

18
An Example Program

• include ltiostreamgt
• include ltstringgt
• int main() int wordCount 0double
  totalWordLen 0double totalSenCount
  0string wordwhile (cin gtgt word) int
  wordLen word.length() wordCount if
  (wordwordLen-1 '.') totalWordLen
  (wordLen-1) totalSenCount else
  totalWordLen wordLen

19

• cout ltlt "total number of words " ltlt wordCount ltlt
  endlif (wordCount gt 0)cout ltlt "average word
  length "
• ltlt totalWordLen / wordCount ltlt endlcout ltlt
  "total number of sentences "
• ltlt totalSenCount ltlt endl
• if (totalSenCount gt 0) cout ltlt "average
  sentence length "
• ltlt wordCount / totalSenCount ltlt
  endlreturn 0