Class 22: Input and Ouptut

Held: Tuesday, February 27, 2007

Summary: Today we consider why and how to make Scheme do explicit input and output.

Related Pages:

• EBoard.
• Lab: Input and Output.
• Reading: Output in Scheme and Input in Scheme.

Notes:

• Homework 8 is now due Friday.
• Homework 9 is now due Friday.

Overview:

• Interactive programs.
• Output in Scheme.
• Input in Scheme.

Interactive Programs

• In our traditional usage of Scheme, we define procedures and then ask Scheme to evaluate one or more expressions.
• Note that this model does not follow our traditional uses of computing, in which we start a program (e.g., by typing its name in the shell) and then interact with it.
• In the simplest interactive model, a program repeatedly queries someone for information, the person enters the information, and the program
• To match this model, we need to figure out how to get input from the user and generate output for the user to read.

Program Output

• You're already seen one procedure for producing output: display.
  • (display val) displays a value, eliminating the quotation marks from around strings.
• We've also used the (newline) procedure that starts a new line of output.
• There's a third useful output procedure, write, that works a lot like display except that it also displays quotation marks around strings and displays characters with the #\.
• A fourth procedure, write-char, only works with characters.

Program Input

• What procedures are available to read input?
  • Primarily read, which reads one Scheme value, skipping whitespace as appropriate.
  • Also read-char, which reads a single character for input.
• read is an interesting procedure. In many cases, it will continue to read for more than one line.

Lab

• You will, of course, experiment with these issues in the lab.
• Be prepared to reflect.