Class 05: Symbols and Lists

Held: Monday, January 29, 2007

Summary: Today we consider two of Scheme's most important types of data: symbolic values and lists.

Related Pages:

• EBoard.
• Lab: Symbols and Lists.
• Reading: Symbols in Scheme and Lists in Scheme.

Notes:

• Are there questions on HW3?

Overview:

• Symbolic values.
• Lists.
• Lab.

Symbols

• A key aspect of Scheme.
• Symbols are seful when you want to represent something atomically (as an idea that can't be pulled apart).
• The easiest way to build a symbol is with the quote operation.

  > 'a
  a
  > a
  reference to undefined identifier: a
  > (quote a)
  a
  

• Note that symbols and variables are different

  > (define a 12)
  > a
  12
  > 'a
  a
  

• As you might be able to tell, the quote means Take it literally; don't attempt to interpret or evaluate it.
  • Note that quote can be used for other types of values, although I will usually discourage you from doing so.

Lists

• The core of Scheme.
• Lists are ordered collections of information.
• Dynamic: Lists can "grow" and "shrink"
  • Or at least you can build larger or smaller lists from other lists.
• Lists look a lot like procedure application (intentionally).
• The simplest list: the empty list is null or ().
• You extend lists at the front (or, more precisely, build new lists) with

  (cons thing-for-the-front rest-of-the-list)
  

• For example,

  > (cons 'a null)
  (a)
  > (cons 'b (cons 'a null))
  (b a)
  

• You can also create lists with list.

  > (list 'a 'b 'c)
  (a b c)
  

• You extract parts of lists with three operations (we'll reflect on the naming later in the semester):
  • car: get the initial element of the list.
  • cdr: get all but the initial element of the list.
  • list-ref: get the ith element of the list.
• One disadvantage: Scheme lists look a lot like procedure applications, so it's sometimes hard for beginning programmers to tell what's going on.
  • You can always tell by context, but it takes some time to get used to the context.
• We'll look at some behind-the-scenes issues in a few weeks.

Lab

• Do the lists lab.
• We'll try to use the last five minutes of class to reflect.
• You may have to complete the remainder of the lab on your own.