I/O in C

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I/O in C

• October 7, 2008.
• Junaed Sattar

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Stream I/O

• a stream is a flow of bytes/characters/ints or
  any type of data
• input streams to the program
• output streams from the program
• note I use plural
• one program can have multiple I/O streams
  associated
• and vice-versa

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Input/Output

• Console based, no GUI
• standard streams
• cin standard input
• cout standard output
• cerr standard error

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Extraction/Insertion

• cout ltlt Hello world!
• cout ltlt The value of i is ltlt i ltlt endl
• //endl puts a new line
• cout ltlt Please enter your name
• string name
• cin gtgt name

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What are cin and cout?

• Stream classes
• Classes have methods, as we know
• so does cin and cout
• some common to all I/O stream classes in C
• File I/O, binary/text mode I/O, console I/O

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One example

• cin inputs ints, chars, null-terminated strings,
  string objects
• but terminates when encounters space (ascii
  character 32)?
• workaround?
• use the get method

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Snippet

• char tData100
• // This is a method in C istream classes
  for
• // inputting text
• //including spaces
• cin.get( tData, 99 )
• // or
• cin.get(tData,99,'\n')

• Inputting i am oh so cool
• cin.get gets the entire line
• just cin will get I
• space termination

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Or,

• Use the getline function
• getline( cin, name )

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File I/O

• Reading from or writing to files on disk
• ifstream and ofstream classes
• dedicated for input and output respectively
• or, use fstream

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Example Files Program(filesdemo)

• ofstream myofile
• myofile.open( sample.txt )
• myofile ltlt This is a sample line I'm writing\n
• myofile.close()
• ...
• ifstream myifile
• myifile.open( sample.txt )
• string oneLine
• getline( myifile, oneLine )
• cout ltlt oneLine
• myifile.close()

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Read/Write to files (files1/2)?

• Similar to how we use cin and cout
• remember, these are I/O streams too
• myfile is a file stream object, then
• to write an int
• int i 10myfile ltlt i
• to read an int
• int imyfile gtgt i

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Binary files

• As opposed to text files, they are unformatted as
  ascii.
• text files stores everything as ascii text
  strings
• even numbers
• binary files do not
• Example consider outout of the program in the
  previous slide

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Difference?

• Example program
• Accepts student ID (I input 1010)
• Accepts name (I input Junaed)
• Accepts CGPA (I input 4.5)
• Save into two files, as text and binary

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Storage
TEXT FILE
BINARY FILE
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Binary files

• Files by default are text
• Different methods to write and read
• requires casting (we'll see casting soon)?
• different data format
• If time permits, we'll revisit

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Failures?

• If open fails?
• Check before use
• if( !myifile ) cerr ltlt Cannot open file!
  exit(1)
• End of file?
• while( myifile.fail() ) //do your operations
  here

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Random vs Sequential

• Random access files
• nonsequential,
• as a result faster access times,
• content must be suitable for random access
• for example. not on network streams!
• or console input

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File heads

• Access positions
• one each for read and write
• hence two methods
• seekg (as in get) for reading
• seekp (as in put) for writing
• ifstreams have seekg
• ofstreams have seekp

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seeking

• seekg( position, mode) //(same for seekp)?
• position is a long integer signed offset
• mode can be
• iosbeg from the beginning
• iosend from the end
• ioscur from current position

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telling

• tellg and tellp
• returns as long integer, the position of the get
  and put positions, respectively

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example seeks

• file.seekg( 20L, iosbeg )
• file.seekp( -100L, ioscur )
• long pPosition file.tellp()
• long gPosition file.tellg()