C Stream IO Including Files

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C Stream I/OIncluding Files

• R. Johnson

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Stream I/O

• cin and cout are streams
• Stream I/O is made up of series of characters
• Work of I/O is performed by ltlt and gtgt operators
  cin, cout are source, destination
• gtgt and ltlt translate data types to/from chars
• Cin, cout act as middle man between our program
  and the hardware operating system!
• Objects predefined (declared) globally in
  ltiostreamgt

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File Stream I/O

• Streams also used for file I/O (to disk).
• include ltfstreamgt
• Must explicitly declare ourselves.
• File stream is NOT the file on disk!!!!!
• Formatting, actual I/O exactly like cin/cout.
• BUT must open and close stream for actual
  file before after.
• Text files always read/written from beginning
  (sequential).
• Current position advances thru file with each I/O
  operation.

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File Stream Output

• include ltfstreamgt
• Declare ofstream prt
• Open creates/overwrites existing
  prt.open(data.txt)
  prt.open(filename.c_str())
• Append? prt.open(filename.c_str(),iosapp)
• Formatting prt.setf( prt.precison(
• Output prt ltlt var1 ltlt var2ltlt endl
• Close prt.close()

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File Stream InputMUST know layout of input file!

• Declare ifstream read
• Open read.open(filename.c_str())
• Exist? if (read.fail())
  cout ltlt File not found!
  exit(1)
• Input read gtgt var1 gtgt var2 treats
  spaces, newline just like cingtgt
• Close read.close()

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Repeated OutputProduce columnar output with
loops (while,for)

• Declare open stream
• Headers lines? Formatting?
• Start of loop getting input or generating data
• Get input
• Use the data calculations?
• Output a line to the file.
• End of loop
• Close the stream.

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Repeated InputReading columnar input with a
loop, works with unknown of lines!

• Declare open stream
• Read any non-repeating header lines
• While not at end-of-file yet while(!read.eof())
• Get input data current line
• Use the data calculations, output?
• End of loop
• Close the stream.
• PROBLEM when using gtgt, can stumble on presence
  absence of final newline char!!!!

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Repeated Input-IMPROVED-

• Declare open stream
• Read any non-repeating header lines
• While an input succeeds while(readgtgtxgtgty)
• Use the data calculations, output?
• End of loop
• Close the stream.
• FIX when using gtgt, will NOT stumble on
  presence/absence of final newline char
• WHY? Value of stream read looks like True after
  a successful read, like False on attempt to read
  past end of file!

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Problem with string input form keyboard and file

• cin gtgt and read gtgt see blank spaces as
  separating real values.
• What about entire line of input as one string,
  including spaces?
• getline(cin, stringVar) for ltstringgt
• Reads all chars up to \n (newline char)
• Discards closing \n without putting it in string
  variable!
• NOTE cin.getline() is NOT the same!!

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Problem with getline()

• Using getline() after cingtgt can give empty
  string.
• Why? cingtgt sees \n as separator, but leaves it
  on the stream!
• A following getline() ends before ever getting ot
  next real line of input.
• FIX cingtgt x cin.ignore(80,\n)
  getline(cin, header1)cin gtgt y
  //Note NOT a problem after!

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Summary getline() and .ignore()

• cingtgtname user types Jim Dye
• gtgt operator only puts Jim into string name
  because it sees any spaces as separating the
  inputs.
• getline(cin, name) reads in all characters
  until \n is encountered.
• Usually need to use cin.ignore(80,\n) between
  using cin gtgt and getline() in that order.
• Use cin.getline(name) ONLY if name is declared
  as a c-string instead of standard string!