Held: Wednesday, 6 November 2013
Back to Outline 35 - Trees.
On to Outline 37 - Randomized (Unpredictable) Drawing.
Summary
We consider vectors, an alternative to lists for
storing collections of data.
Related Pages
Overview
- Problems with lists.
- A solution: Vectors.
- Behind the scenes: How Scheme implements vectors.
- Important vector procedures.
Administrivia
- Today's writeup: Exercise 6
- New assignment: Mostly building color trees.
- Upcoming extra credit opportunities:
- Any one Grinnell prize event this week.
- Thursday extras next week: Reports from internships (I think)
- Grinnell Town Hall Meeting, Nov 13 noon or 7:30, I believe
- CS Table, Friday: TBD (it's been a bad week)
Data Types
- At the start of the semester, we decided that "basic values and
operations on those values" are key to writing algorithms.
- We tend to use the word "type" to express these two concepts.
- We've seen a variety of characteristics of types in 151.
- Some types are defined in terms of a list of possible values or a simple construction method: Character, Boolean, RGB color, etc.
- Some types that can potentially be defined recursively: Drawings-as-value,
Maybe integers, Lists.
- Some types are designed to collect other kinds of values: Lists. (Okay, maybe that's it for now.)
- We're about to learn a few more. Vectors today. Pair structures (particularly trees) on Monday.
List Deficiencies
- Now that we've worked with lists for a while, we've identified
some things that make lists inappropriate for some situations.
- List are expensive to use; to find the length of a list
or to access an element late in the list, you need to cdr down the
list.
- Lists are fixed; you can't easily change an element of a
list.
- At the same time, there are some advantages to lists:
- Lists are dynamic; it is easy to grow and shrink a list.
- Lists are inherently recursive; the type is defined
recursively.
- Lists are simple; you can build almost every list operation
through just a few basic operations (
car, cdr,
null, and null?).
An Alternative: Vectors
- Vectors provide an alternative to lists.
- They have two primary advantages:
- Vectors are indexed: You can quickly access elements by number.
- Vectors are mutable: You can change the elements of a vector.
- In order to obtain these benefits, vectors lack some key features of lists. In particular,
- Vectors are static: Once you've created a vector, you cannot change its length.
- Some key vector procedures:
(vector val1 ... valn): Create a vector(make-vector length val): Make a vector of specified length, with duplicates of val as the contents.(vector-ref vector position): Extract a value from a vector.(vector-set! vector position newvalue): Change an element of a vector.(vector-length vector)
Implementing Vectors
- To be done live in class: Memory layout and more.
Lab
