Class 35: Program Input and Output

Held Tuesday, November 5, 2002

Summary

Today we investigate mechanisms for reading input in Scheme. We also revisit Scheme's primary mechanisms for generating output.

Assignments

• Read Files.

Notes

• I'm working to get all the procedures for tomorrow's reading documented. Please wait a few hours before following the links.
• Are there questions on Homework 7?
  • Do you have to document your procedures? Yes!
  • All of them? Yes!
  • What width (or height) should you use when the images have different widths (or heights)? The smaller width (height).
  • Can you show me a sample merge function? Certainly

    ;;; Procedure:
    ;;;   merge-colors-1
    ;;; Parameters:
    ;;;   color1, a color
    ;;;   color2, a color
    ;;; Purpose:
    ;;;   To merge color1 and color2 into a new color by using the red
    ;;;   component of the first color, the green component of the second
    ;;;   color, and the average blue component of the two colors.
    ;;; Produces:
    ;;;   newcolor, the merged color
    ;;; Preconditions:
    ;;;   Each color is a list of three integers in the range [0..255]
    ;;;   The three integers represent red, green, and blue components of
    ;;;     the color.
    ;;; Postconditions:
    ;;;   newcolor is a list of three integers in the range [0..255]
    ;;;
    (define merge-colors-1
      (lambda (color1 color2)
        (let ((red1 (car color1))
              (red2 (car color2))
              (green1 (cadr color1))
              (green2 (cadr color2))
              (blue1 (caddr color1))
              (blue2 (caddr color2)))
          (list red1 green2 (trunc (/ (+ blue1 blue2) 2))))))
    

• I'm not sure that I can both write the exam and grade homework 6 by tomorrow. Which would you prefer that I do first?
• Don't forget to vote today.

Overview

• Interacting with programs
• Scheme's output procedures
• Scheme's input procedures
• Lab

Program Input

• 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 presents some output.
• We already know how to produce output: write, display, and newline.
  • Review: What does each do?
• 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.