Class 22: Vectors, Continued

Held Monday, October 7, 2002

Summary

Today we continue our exploration of vectors.

Today's Work

• The reading on vectors
• The lab on vectors

Assignments

• Read Naming Values with Local Bindings.

Notes on Exam 1

• I noticed a few small errors in email-eval.ss
  • The last line of email-value-helper read
    (word-value (car (car histogram))))))
    and should instead read (word-value (car (car histogram)) word-values))))
  • The line in extract-words-helper that read
    (cons (car chars) word-chars)
    should instead read
    (cons (char-downcase (car chars)) word-chars)
• Someone asked what I like to see for tests within the code. I like to see
  • (1) A display of a note that explains the test.
  • (2) A display of the Scheme code you're about to run for the test.
  • (3) The test.
  • (4) A comment showing the result
  • For example,

    (display "Testing foo on the empty list.\n")
    (display "(foo null)\n")
    (foo null)
    ; 0
    

• Are there any other questions on exam 1?

Other Notes

• Does anyone read the class news?
• Yes, Friday's class was fun. But let's strive for fewer classes that are fun in that peculiar way.
• I'm taking Daniel (youngest) to the doctor for shots this afternoon, and won't be available in my office. I will check email this evening.

Overview

• A solution: Vectors
• Common vector procedures
• Lab

An Alternative: Vectors

• The designers of Scheme decided that it would be useful to include another compound data type that handled some of the deficiencies of lists.
  • However, that meant we had to give up some of the advantages of lists.
• Vectors are compound data types that
  • Have fixed size: One you create a vector, you cannot easily add elements to the vector.
  • Permit indexed access: You can quickly access any element in a vector.
  • Support mutation: You can change any element of a vector.
• Through the rest of the term, we'll often ask ourselves whether we're better off using a vector or a list for a particular problem.
• We've seen that lists are implemented through cons cells. How are vectors implemented? There's basically just a big cons-cell-like thing that has as many boxes as there are elements in the vector (plus a notation as to the vector's length).

Common Vector Methods

• (vector val1 ... valn): a lot like list
• (make-vector size element): make a vector of a particular length containing lots of copies of the same value.
• (vector? value): determine if a value is a vector
• (vector-length vector): get the length of a vector
• (vector-ref vector pos): get an element from a vector.
• (vector-set! vector pos newval): change part of a vector. Note the use of an exclamation point to indicate that this changes its argument.
• (list->vector list): Guess
• (vector->list vector): Guess
• (vector-fill! vector value): Fill an existing vector with a particular value.
• Note the naming convention: A procedure that changes one of it's parameters ends with an exclamation point!

Lab

• Do the lab.
• Be ready to reflect.
• If there's time, we might even look at a sample vector procedure.