CSC151, Class 23: Lab: Local Bindings

Overview:
* Lab
* Reflection

Notes:
* Any final questions on the exam?
* Writing a variant of round.
  "I want round to always round up and not do the 
   weirdo "round towards even" thing that it does."
  2.5 => 3
  3.5 => 4
  4.5 => 5
* Questions on let?

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Exam questions:

Q For number of quadratic roots, can I just find the two roots
  and see if they're both real and equal?
A No.  Be smarter.

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Reflection:

* For exercise 2:

(let ....
   (+ alpha beta gamma delta epsilon))

* Why did I have you do exercise 2?
  So that you'll never forget the format of lets
(let ((NAME EXP) ...)
  BODY) 

* Why is let like a liberal arts college?
* Because it lets you define your own little world which
  need bear no resemblance to the outside world.

Here's a very silly procedure

(define silly
  (lambda (val)
    (display val)
    (newline)
    val))

(define multiply-by-two-a
  (lambda (lst)
    (let ((multiplier (silly 2)))
      (if (null? lst) 
          null
          (cons (* (car lst) multiplier)
                (multiply-by-two-a (cdr lst)))))))

(define multiply-by-two-b
  (let ((multiplier (silly 2)))
    (lambda (lst)
      (if (null? lst) 
          null
          (cons (* (car lst) multiplier)
                (multiply-by-two-b (cdr lst)))))))

How do I generate really long lists?

(vector->list (make-vector 1000 5))