CSC151, Class 37: Input and Output

Overview:
* Procedures to generate output
* Procedures to read input
* Lab

Notes:
* Class Friday in Saints' Rest.
  Two classmates-on-crutches may need a ride.  Volunteers?
* Extra credit for attending "Fleeting Fictions".
* Read "Files in Scheme"
* Next year, should I make the Gimp a key subject of the class?
* Questions on exam 2?

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(define sum-of-number-tree
  (lambda (ntree)
    (if (pair? ntree)
        (+ (sum-of-number-tree (car ntree))
           (sum-of-number-tree (cdr ntree)))
        ntree)))

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Scheme does not behave like normal programs (not even
like normal command-line programs)

Moderately old interactive model of computation
1. User starts program
2. Program prompts for input
3. User gives input
4. Program computes
5. Return to step 2

Modern interactive model of computation
1. User starts program
2. User decides whether to click on a menu, button
   etc.
3. Program reacts

Scheme model
1. User enters a Scheme expression
2. Scheme evaluates it
3. Go back to step 1

We need to be able to prompt for input and then read the
input the user gives.

Prompting:
* (display val)
  Puts val on the screen
  If val is a string: Doesn't show quotation marks
  If val is a character: Doesn't show #\
* (newline)
  Starts a new line
* (write val)
  Puts val on the screen
  If val is a string: Shows quotation marks
  If val is a character: Shows the #\
* (write-char val)
  Puts val on the screen
  If val is a string: Complains that val is not a character
  Val must be a character: Doesn't show #\
Input:
* (read) 
  Gets and returns a value (from the user)
* (read-char)
  Gets and returns a character (from the user)

; Read n characters and return them as a list.
(define read-n-chars
  (lambda (n)
    (if (zero? n)
        null
        (cons (read-char) (read-n-chars (- n 1))))))