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Output to a file

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• Opening a file
  • Often, you need to write state information to a file for analysis.

  • To open a file for writing, use:

    open FileName ACCESS

    • FileName is the name of the file to open.

    • ACCESS is the access mode:
      • r: Open the file for reading. The file must already exist.
      • r+: Open the file for reading and writing. The file must already exist.
      • w: Open the file for writing. File will be erased first.
      • w+: Open the file for writing. File will NOT be erased first.
      • a: Open the file for appending. The file must already exist.
      • a+: Open the file for appending. The file need not already exist.
    • The open command returns a "token" (value) that Tcl file access command use to access the file.

  • Example:

    set infile [open "input.data" r] set outfile [open "output.data" w]

    The variables infile and outfile will be used in further file access commands (such as reading from or writing to a file)

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• gets: reading a line from a file
  • The gets command reads a line from an input file:

  • Syntax of gets:

    gets $FileID nextLine

    • FileID can be one of:
      • a File identifier returned by open
      • stdin

    • gets reads a line of input from FileID and store it into the variable nextLine

      (It also discards the terminating newline character)

    • gets returns the number of characters in the string, and -1 if EOF.

  • Example 1:

    gets stdin nextLine

    Type in something at the "empty prompt" !

    Try typing in "Control-D" !!

  • Example 2:

    set infile [open "input.data" r] gets $infile nextLine

    Create a data file named "input.data" !

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• puts: writing a line to a file
  • The puts command writes a line to an output file:

  • Syntax of puts:

    puts $FileID $nextLine

    • FileID can be one of:
      • a File identifier returned by open
      • stdout
      • stderr

    • puts write a line in the variable nextLine to the output file FileID

      (It also add a terminating newline character)

    • gets returns the number of characters in the string, and -1 if EOF.

  • Example 1:

    set nextLine Hello puts stdout $nextLine

  • Example 2:

    set outfile [open "output.data" w] set nextLine Hello puts $outfile $nextLine

    Create a data file named "input.data" !

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• Closing the file
  • Don't forget to close a file at the end of the program:

    flush $fileID // flush any output that has been buffered close $fileID // close the file

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• Further reading
  • File access 101: click here