Class 07: Pause for breath

Held Tuesday, September 5, 2000

Summary

Today we pause for breath and consider how to solve some supposedly simple problems in Scheme.

Notes

• Handouts:
  • Homework 1
  • Guidelines for laboratory writeups
• Lab:
  • User-Defined Procedures
• Readings:
  • Defining Procedures (for today)
  • Comments in Scheme (for tomorrow)
• I was sorry to see so few of you at yesterday's brown-bag lunch. It was actually quite interesting; there were videos of old GUI applications that seem more advanced than most things available today.
• Quiz: a. Take out a piece of paper b. Write your name at the top c. On the paper, write Scheme code for extracting the A from (do (you (deserve an) (A))) d. Raise your hand when you're done

Overview

• Quiz
  • Some morals
• About and and or

Quiz Discussion

• The problem: Extract the A from (do (you (deserve an) (A)))

First Solution

• For our first solution, we used only car and cdr.

Second Solution

• For our second solution, we used only list-ref and reverse.

Morals

• Some problems are stupid/silly.
  • Surprisingly enough, some of these silly problems really do occur in practice.
• Getting the details right is hard.
• Everybody has trouble with this stuff.
• Solve the problem a step at a time. Keep making small incremental updates that get you in the right direction.
• Work with other people.

Combining Booleans

• At times, we want to combine boolean expressions. For example, ``Between X and Y, inclusive'' may be more precisely represented as ``At least as large as X and no larger than Y''.
• Similarly, ``Abnormal grade'' might be represented as ``smaller than 60 or larger than 100''.
• In Scheme, you can combine boolean expression with the and and or operators, which work similarly to the way they work in English.
• The and of two expressions is true if both expressions are true.
• The and of two expressions is false if either expression is false.
• The or of two expressions is true if either expression is true.
• The or of two expressions is false if both expressions are false.