Class 24: Revisiting Lists

Held: Wednesday, 7 October 2009

Summary: We explore further details of Scheme's lists. Lists can be a key mechanism for structuring data. We also consider how lists can be used to represent drawings.

Related Pages:

• EBoard.
• Lab: Exploring Lists.
• Reading: Building Data Structures with Heterogeneous Lists.
• Due: Assignment 5: A Miscellany of Image Models.

Notes:

• A few problems on the homework were a bit more difficult than we anticipated. I apologize. (However, I think you're probably now a bit better with building anonymous procedures, and I think that's a good thing.)
• Friday's quiz will include conditionals, image-compute, and the new list operations, like cons, car, and cdr.
• Assignment 6 is now ready, and is due Wednesday of next week.
• Looking ahead: No large assignment will be due the week after break.
• Looking further ahead: GIMP (the other one) presents Rent (Nov. 13-15).

Overview:

• Lists, Revisited
• New List Operations
• Drawings as Values

Lists, Revisited

• Original notion of lists:
  • Collection of values
  • Homogeneous: All the values in a list are the same type
  • Used primarily with map and for-each
• Extending that notion
  • Can also be heterogeneous: Values in the list may have multiple types
  • Can examine individual elements with car, cdr, and list-ref.
  • Can be extended (creating new lists) with cons.
• Why is the new view important?
  • It will soon give us the opportunity to manipulate lists in new and interesting ways.

Implementing Drawings

• We can (and do) use lists to implement drawings.
• The reading considers a simplified representation, in which the width and height of a drawing are the same.
• But the general process is the same
  • Group the values in a list
  • When we need to render or manipulate a drawing, use the list operations to extract parts.