Interactive IO

1
Interactive I/O

• Input from keyboard
• Must prompt user
• User friendly
• echo checking

2
Noninteractive processing

• Batch processing
• Store values in a file
• File input and Output
• File - section of secondary storage with a name
• C can write and read from files just as to
  keyboard and screen
• standard input file - keyboard
• standard output file - terminal or printer

3
Advantages to noninteractive

• can edit data, prepare ahead of time
• Can rerun file without reentering data
• Can examine output at leisure
• Can display output to screen or print
• Can use out as input into another program
• means of communicating between programs

4
files

• file - named secondary memory area that holds a
  collection of information
• usually stored on an auxiliary storage device
• examples
• external file - a file that is used to
  communicate with people or programs. It is stored
  externally to the program.
• internal file - a file that is created but not
  saved
• (scratch file)

5
text files

• composed of characters only
• Differs from .doc file
• Size of file
• Can be created in notepad, Visual c.net

6
File organization

• naming rules(dos) - 1 to 8 character name,
  optionally followed by a period and up to a 3
  character extension
• extension tells the type of file
•  .cpp c program file
• .doc Word file
• .exe executable file
• .OBJ - Compiled programs - object files
• .TXT - Text files(ASCII form, printable)
• .DAT - data files
• file is composed of tracks, sectors
• FAT table

7
commands
8
files

• program files - contains program instructions
• data files - contains data, collection of records
• record - one entity
• i.e. customer file - each record holds data for
  each customer
•  fields - data element
•  customer file has name field, account field,...
• record key or key field

9
File Organization

• Sequential - records stored one after another
• Random - use key to directly access record
• Indexed - combines sequential and random
•  ISAM - Indexed sequential
•  each record has a unique key
•  input records arranged in key sequence
•  one area for index - gives location of record

10
Writing Data to a File

• 1. Open the file as an output file.
• 2. Write the elements.
• 3. Close the file.
• Data will be output from the program and written
  on the disk. This is used to create a data file. 

11
Opening for output causes

• The directory is searched for "filename". If it
  doesn't exist, it is created. If it does exist,
  it will be lost. The file is set to write mode.
• A buffer is created in memory. WRITE causes data
  to go into the buffer. When buffer is filled,
  then gets written to disk.
• A file pointer is set to the beginning of file.
  This is updated with each write.

12
Reading the Data

• Open the file for input.
• Read the data (and print).
• Close the file. 
• Data will be input into the program from the
  disk. Reads data previously stored on the disk.

13
Opening for input causes

• The directory is searched for "filename". The
  file is set to read mode.
• A buffer is created in memory.
• A file pointer is set to the beginning of file.

14
Using Files in c

• 1. include ltfstream.hgt
• 2.declare file streamsifstream inFile //input
  only (reading)ofstream outFile //output only
  (writing)
• 3. open each file for reading or writing
  filevar.open(filename)
• 4.specify names of file streams in input and
  output statement

15
Declaring Files

• ifstream input file stream
• ofstream output file streamgtgt, get, ignore
  also valid for ifstreamltlt endl,setw,
  setprecision also valid for ostream
• declareifstream inMPGofstream outMPG

16
Open Files

• inMPG.open("inmpg.dat")outMPG.open("outmpg.dat")
  1. associates a stream variable with file2.
  output file
• check if it exists, overwrite, if not create
  sets marker to beginning of file
• good idea to open all files at the beginning

17
input and output-

• use var name in input and output statements in
  place of cin and coutinMPG gtgt amt1 gtgt amt2 gtgt
  amt3 gtgt amt4 gtgt startMiles
• gtgt endMiles
• outMPG ltlt "the mileage per gallon is " ltlt mpg ltlt
  endl

18
Input Failure

• invalid data -gt fail state
• after fail state, any further i/o operations have
  no effect
• opening file that doesn't exist -gt fail state
• Important to test for this!!!!!!inMPG.open("inmpg
  .dat")
• if(!inMPG)
• cout ltlt Cant open inmpg.dat
• return 1