Class 09: Conditionals

Held Friday, September 8, 2000

Summary

Today we begin to learn how to tell Scheme to make decisions. The procedures used to make decisions are called conditionals.

Notes

• Today's reading: Conditionals
• Today's lab: Conditionals
• Reading for Monday: Recursion basics (Not yet ready; look for it tomorrow.)
• I'm working with computing services to handle the problems accessing www.cs.grinnell.edu from ResNet. For now, you might instead use www.math.grin.edu.
  • You might also ask the help desk for assistance setting your DNS server so that it includes newton.math.grin.edu (132.161.33.70).
• To turn in the homework.
  • Go to http://blackboard.grinnell.edu
  • Select this course.
  • When prompted to log in, use your normal account as the name Use a variant of your student ID number as password. Returning students use the last seven digits of student ID as password. New students use trailing non-zero digits. [Due to stupidity in the design of the system, you may be required to log in twice.]
  • Click on "Student Tools"
  • Click on "Student Drop Box"
  • Click on the "Browse" button to select a file.
  • Type in something useful, like "Homework 1 from Sam and Max"
  • Click on "Send File to Instructor"
  • Pray
• Grinnell Entrepreneurs is sponsoring a talk at 4:15 Monday, September 11th in South Lounge. The talk, entitled From a startup to millions, Grinnell style will docus on issues in starting up a company with close references to ASI Sign Systems, his own company. Mr. Latimer brings much valuable insight and perspective from personal experience. Mr. Latimer is the founder and President of ASI Sign Systems. Mr. Latimer used to work for the college and he quit in 1985 to start a business of his own in Grinnell. The company specializes in making signs. For eg: all the signs in the Fine Arts Building are made by his company. The company has now grown far beyond Grinnell and is one of the leading ones in the Midwest with offices at various locations in the midwest.

Overview

• A problem: Assigning grades
  • One solution: Using if
  • Another solution: Using cond
• Lab

A Problem

• Suppose that for fairness's sake, I'd like a program that converts numeric grades to letter grades.
• We'll look at two related grading schemes.
  • 94-100: A, 90-93: B, 87-89: B+, 84-86: B, ...
  • 10:A+ 9:A, 8:A-, 7:B+, 6:B, 5:B-, 4:C+ 3:C 2:C- 1:D 0:F
• How might you explain to someone how to turn a number into a grade?
• In either schema, can you write a procedure that turns a number into a grade, using only the Scheme you already know?

First Solution: Use if

• As you may have noted from your narrative, we are often tempted to describe the procedure in terms of the word ``if''. For example, ``if the grade is at least 94, then give the student an A, otherwise, if the grade is between 90 and 93, ...''
• In Scheme, you can write such expressions as

  (if condition success-value failure-value)
  

• For example

  (if (>= grade 94) 'A 'F)
  

• We can nest these conditions.

  (if (>= grade 94) 
      'A
      (if (<= 90 grade 93)
           'Aminus
          (if (<= 87 grade 90)
            ...
          )
      )
  )
  

Another Solution

• As you may have noticed, the code is somewhat hard to read (and awfully indented). Another alternative is to use Scheme's cond, which more closely resembles a table.
• Here's what a cond expression looks like

  (cond 
    (condition value)
    (condition value)
    (condition value)
    ...
    (else value)
  )
  

• For our problem, this seems particularly appropriate.

  (cond
    ((>= grade 94) 'A)
    ((<= 90 grade 93) 'Aminus)
    ...
  )