CSC153, Class 27: Input and Output in Scheme

WE'LL START A FEW MINUTES LATE SO THAT THE 151 STUDENTS
HAVE TIME TO FINISH THEIR EXAM!

Overview:
* Review: Output in Scheme
* Input in Scheme
* Lab

Notes:
* Read Files in Scheme
* Questions on writeup 3?
* Questions on yesterday's class?
* Questions on Exam 2?
  + Plus short discussion.
  + Problem one: Currying
  + Problem two: Recurrences
      Minimum bound: Five different recurrences
  + Problem three: Weird procedure for quicksort.
      Understand the procedure.
      Document the procedure.
      Use the procedure to implement vector-based quicksort.
  + Problem four: String to value
    "\"Hello\""
  + Problem five: Elegance Mantras
Questions on writeup 3:
Q Why does sequential-search only take a list and a predicate?
  What about the value you're searching for?
A You're searching for the first value that matches the
  *unary* predicate
(search odd? lst)
(search (left-section = 5) lst)

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Review: Output procedures in Scheme
* Display: Displays its parameter
* Print: Prints its parameter
* How do they differ?
  + Print requires an output port (no)
  + Print permits an output port (yes)
    But so does display
  + Print adds the newline (no)
    Neither does display
  + Display processes escaped characters (yes)
* Differ primarily in treatment of strings.
  + print prints strings as Scheme values.
  + display prints strings as humans would expect to read them
    (w/o quotes, with escaped characters processed)
* Error: Prints its parameters
  + More parameters
  + In DrScheme, prints pretty Bug and the text is in red.
  + Error stops your program
* write, which seems to be the same as print
* (newline), which prints a newline character

Key input procedures:
* read (converse of write/print): Read a Scheme value.
* read-char
* peek-char

In their base form, none of these take any parameters.

LAB

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Reflection:

* What's the difference between typing 'a and a in response
  to a read prompt?
* Explain
> (read)  ; Type in OGE
oge
> (read)  ; Type in (list 1 2 3)
(list 1 2 3)

* How can I enter a procedure?
> (define foo (read))  ; Type in (lambda (x) (+ x 1))
> foo
(lambda (x) (+ x 1))
> (eval foo)
#<procedure>
> ((eval foo) 3)
4

* How can I prompt for, read, and return a sequence of
  values?
(define prompt-and-read
  (lambda (prompt) (display prompt) (display " ") (read)))
(list (prompt-and-read "A:")
      (prompt-and-read "B:")
      (prompt-and-read "C:"))