16: Reading and Writing Files

If you did the Study Drills from the last exercise, you should have seen all sorts of commands (methods/functions) you can give to files. Here's the list of commands I want you to remember:

• close -- Closes the file. Like File->Save.. in a text editor or word processor.
• read -- Reads the contents of the file. You can assign the result to a variable.
• readline -- Reads just one line of a text file.
• truncate -- Empties the file. Watch out if you care about the file.
• write('stuff') -- Writes "stuff" to the file.
• seek(0) -- Move the read/write location to the beginning of the file.

One way to remember what each of these does is to think of a vinyl record, cassette tape, VHS tape, DVD, or CD player. In the early days of computers data was stored on each of these kinds of media, so many of the file operations still resemble a storage system that is linear. Tape and DVD drives need to "seek" a specific spot, and then you can read or write at that spot. Today we have operating systems and storage media that blur the lines between random access memory and disk drives, but we still use the older idea of a linear tape with a read/write head that must be moved.

For now, these are the important commands you need to know. Some of them take parameters, but we do not really care about that. You only need to remember that write takes a parameter of a string you want to write to the file.

Let's use some of this to make a simple little text editor: