CSC151.02 2003F, Class 44: Binary Search and Such

Admin:
* No reading for Thursday
* Think about sorting mechanisms
* Questions on the homework?

Overview:
* Background on binary search
  * What purpose do the parameters to binary-search serve?
  * Explain the parameters to the kernel.
* Lab
* Reflection

(define binary-search
  (lambda (key vec get-key less-than?)
    ; Search a portion of the vector from lower-bound to upper-bound
    (let search-portion ((lower-bound 0)
                         (upper-bound (- (vector-length vec) 1)))
      (if (<= lower-bound upper-bound)
          (let* ((midpoint (quotient (+ lower-bound upper-bound) 2))
                 (middle-element (vector-ref vec midpoint))
                 (middle-key (get-key middle-element)))
            (cond ((less-than? key middle-key)
                   (search-portion lower-bound (- midpoint 1)))
                  ((less-than? middle-key key)
                   (search-portion (+ midpoint 1) upper-bound))
                  (else midpoint)))
          -1))))
key, vec, get-key, less-than?
What is key? The value we're looking for.
What is vec?  A vector of "keyed lists"
  (key value value value)
  ("Rebelsky" "Sam" "4410")
  ("Stone" "John David" "1234")
What is get-key? Something that given a data item returns the key of that data item
* If we're sorted by last name and our entries look like the above, it's car
* If we're sorted by phone number and our entries look like the above, it's caddr
* If we're sorted by last name and first name and we want to look for the combination of the two, it's probably soemthing like 
  (lambda (entry) (string-append (car entry) ", " (cadr entry)))
* If we have a vector of numbers (that is, each entry is a number) and we want to simply see if a number is there, get-key will be ... (lambda (x) x)

Lab

* You can compare two strings with
  string-ci<?
* What cartoon pairs should I add for next year?