Handout 5

Overview

• Questions from last time
• Input-Output (continued)

Input-Output

• File. File input and output uses the same basic ideas, but is a little more complex. File input involes first creating a handle from the filename with the open() command, then reading from it, and finally closing the handle with close() (example).

  It's really easy to make a mistake creating a file handle, so it is customary to add a test to let you know if something has gone wrong. Using the or operator, we can add a command that will only be executed if the open() command fails. We use the die() command which prints a string and ends the program without execuating any more commands (example).

  Finally, we frequently want to read all the lines of a file until there are none left. There are two ways to do this. One is to use a while-loop (example), and the other is to use an array (example).

  File output is rather similar, but with several glitches. Be careful; it's easy to mess this up and accidentally overwrite important files! First, one uses the "redirection" operator > when one creates the file handle. Then one uses print with the new file handle (example). To avoid doing dangerous things, I strongly encourage you to do all your playing with file output in some new directory on your computer where there are no important files.

  There are a couple of oddments to note. First, notice that the redirection operator goes inside the quotes. Second notice that there is no comma after the handle in the print() statement.