Working with Data Files

• How much data are you working with?
  • very small amount, just hard code it "x = 5"
  • few pieces of data, ask user to type it in
  • more than a few pieces, you need an external file
  • external files stored on secondary storage, can be bigger than RAM capacity of the machine!
• Why use external data files?
  • handles Large amounts of data
  • data is indpendent of program, so program does not have to change if data changes and vice versa
  • it's easier to edit the data external to the program, not during the execution of the program! (can't go back to fix during a run!)
  • can use the same data as input for different programs
  • can save output files for later use
  • can use the output of one program as input for another
• Finding a File on your Disk
  • text files vs. binary files
  • we will work with text files
  • can use IDE's editor or Notepad to create a data file
  • a file must be opened and closed
  • a file is an object with methods
  • syntax: fileobject = open(filename, "r") or fileobject = open(filename,"w")
  • syntax: fileobject.close()
  • easiest place to put file so it will be found by your program is the same folder as your .py file
  • can give absolute path for filename, but don't do this for program assignments (TA may not have same file structure as you do)
  • syntax example in textbook at end of this section is missing quotes
  • what is a buffer?
  • open a file using a buffer
  • why it is so important to close a file "flush the buffer"
  • reading a text file in Python (or most languages) is done with sequential access
    • it means that the data is read starting from the beginning of the file
    • it is read in order until the last piece of data in the file is read
    • no backing up, no jumping over data
    • every piece of data must be read in - what you do with it after it is in memory is up to you
  • other types of files (binary) can be read with random access
    • means that you can tell the program where in the file to read from
    • can read forwards, backwards, jump around
    • not done in this class
• Reading from a File
  • ALL data read from a file in Python is string type!
  • a line at a time - a line is delimited by a newline character
  • for line in myfile: very powerful!
  • use split to break up the line
  • then you have a list of strings to work with
• Iterating over lines in a file
  • another way to read from a file - using a while loop

  • 		priming readline
    		while line != "":
    		     process line
    		     get another line (another readline)
    	    

  • sentinel logic!!