Class 08: Conditionals Lab

Held Wednesday, September 11, 2002

Summary

Today we continue our study of conditionals by working with them experimentally.

Due

• Homework 1: A CS151 Site.

Assignments

• Read Comments.

Notes:

• Coffee and Poptarts.
• On Friday, we're going to have our first pause for breath of the semester. We'll review what we've done to date and look at some of the key problems in more depth.
• On Friday, I'll also assign the first lab writeup and discuss the procedures for writing and turning in lab reports. It is likely that I will ask you to write up some problems from the procedures lab and some problems from the conditionals lab.
• Almost everyone turned in HW1 long before class.
  • The times some of you sent your homework were depressing.
  • "Site" and "Cite" are different words.

Overview

• The grading problem, revisited.
• Lab: Conditionals

Conditionals

• As we saw in our PBJ algorithm, many algorithms have to make choices based on the input.
• We might do different things if we have a new jar or old.
• We might do different things if our twisty-tie is twisted clockwise or counter-clockwise.
• Scheme provides a number of tools for dealing with these kinds of decisions.
• The simplest is if
• The normal form for if is
  (if test true-exp false-exp)
• The meaning is
  • Evaluate test.
  • If test is false, evaluate false-exp and use its value.
  • If test is not false, evaluate true-exp and use its value.

• There are times in which we want to decide between many different alternatives. For example, in assigning a letter grade, we give an A for 94-100, an A- for 90-93, a B+ for 87-89, and so on and so forth.
• We can write that with a lot of nested if expressions.

  (if (<= 94 numgrade 100) 'A
      (if (<= 90 numgrade 93) 'Aminus
          (if (<= 87 numgrade 89) 'Bplus
              ...)))
  

• However, it is easy to lose the logic in the midst of all that.
• Scheme provides an alternative mechanism for such cases, the cond expression.

  (cond (test1 exp1)
        (test2 exp2)
        ...
        (testn expn))
  

• The meaning is "Evaluate each test in order. For the first one that is not false, use the value of corresponding expression."
• For our grades example, we might write

  (cond
    ((<= 94 numgrade 100) 'A)
    ((<= 90 numgrade 93) 'Aminus)
    ((<= 87 numgrade 89) 'Bplus)
    ...
    ((< numgrade 60) 'F))
  

• Note that it's much easier to read this
• Note also that there are times in which none of the cases might hol, so we want a default case. You can use else or #t as the test for the default case, which must come last.

Things You Can Test

• In order for if and cond to be useful, you need to be able to test for various situations.
• Scheme provides you with a few basic tests and ways of combining tests.
• You can build your ownh tests from those.
• Scheme provides:
  • Basic type predicates (is a value a number, symbol, string, list, ....)
  • Equality comparators for all types
  • Comparators for numeric types
  • Three basic mechanisms for combining tests: and, or, and not
  • More things that we'll learn about as the semester progresses.

Conditionals Lab

• Do the lab.
• Prepare to reflect on Friday.