Class 50: Records

Held Wednesday, May 2, 2001

Summary

Today we consider how to build records in Scheme. Records group data and permit access to those data by name.

Notes:

• For tomorrow, read Object-Oriented Programming (pdf).
• Are there questions on HW6?
  • Script-Fu is not quite Scheme. You can find documentation on SIOD, the version of Scheme that Script-Fu is based on, at http://people.delphi.com/gjc/siod.html.
  • I would not recommend that you use both GIMP and CGI for this assignment. If I can spare the time, I will try to find a way for you to combine them, but that may take awhile.

Overview

Grouping Data

• As you've seen in a number of examples, it is often convient to group data together into what I typically call an entry.
• You currently know two basic ways to group data: lists and records.
• Lists provide dynamic-sized groups of data but have costly access.
• Arrays provide fixed-sized groups of data with quick access to elements by number.

Records

• However, as we've seen in our work, it is often more convenient to refer to the parts of a group of data by name.
• For example, given a record for a student, we might ask for the student's first name, last name, or grade.
• Computer scientists use record as the name for a fixed-size group of data with quick access to elements by name.
• Unfortunately, Scheme lacks a built-in record type.

Implementing Records

• Fortunately, we can implement records on our own.
• The easiest thing to do is to build a vector and treat the elements of the vector as the fields of the record.
  • We write procedures to access each part of the vector.
• However, we'd like to be able to identify whether or not each vector is supposed to represent one of our desired objects.
• One clever trick is to build a special value that others find it hard to duplicate and to put it at the front of the vector.
• Consider

  (define produce-type-mark
    (let ((type-mark (list 'compound)))
      (lambda () type-mark)))
  

• The let creates a new symbol table with one entry whose name is type-mark and whose value is (compound).
• The lambda gets that value.
• Any other list of the symbol compound will be equal? to the type mark, but not eq?.
• Hence, we can use that value.
• Is this ideal? No. Next class, we'll see another way to build things like records.

Lab

• Do the lab on records.
• We'll reflect a little bit at the start of class tomorrow.