Overview

• Preliminaries.
  • Admin.
  • Work to do.
  • Questions on the homework.
• Why define your own procedures?
• How to define your own procedures.
• Lab(s).

Preliminaries

Admin

• Continue partners from yesterday for the rest of this week!
• Since yesterday's lab went more smoothly than I expected, we're combining the drawings lab and procedure lab days. We'll continue with procedures tomorrow.
• Today is a changing of the guard, of sorts.
  • Thanks to our guest mentors, Toby B and Alex G
  • Welcome to our new Tuesday mentor, Ajuna K
• Extra credit:
  • Thursday at 4:15: Spencer Liberto, Lea Marolt Sonnenschein, and Daniel Torres on Ushahidi and more.
  • CS Table Friday at Noon: The ACM Code of Ethics.
  • Convo Feb. 5. (I'll give my "Why go to convo" lecture closer to the date.)
  • EC for attending tonight's climbing club meeting at 7:30 at the rock wall.
  • Others?

Work to do

• Finish HW 2 (due tonight).
• Read "How Scheme Evaluates Expressions (version 2)".
• Drawings lab writeup (due Friday): Exercise 4, parts a, c, d, f, and g
  • Email subject: CSC 151 Writeup 3: Drawings as Values (YOUR NAME(s) HERE)
• Procedures lab writeup (also due Friday): Exercises 3 and 6
  • Email subject: CSC 151 Writeup 4: Procedures (YOUR NAME(s) HERE)

Why define your own procedures?

• We often want to do the same (or similar) computations again and again and again.

  (* x x)

  (* y y)

  (* 2 2)

It would helpful to be able to name that code, and to take different inputs

How to define your own procedures

A fascinating syntax

(define NAME-OF-PROCEDURE (lambda (INPUTS) EXPRESSION)

(define square (lambda (val) (* val val)))

We can call this like any other procedure already in Scheme

(square 5)

Like

(define val 5)
(* val val)

Similarly,

(define weighted-average
  (lambda (grade1 grade2 grade3 grade4 grade5)
    (/ (- (+ grade1 grade2 grade3 grade4 grade5)
          (min grade1 grade2 grade3 grade4 grade5)
          (max grade1 grade2 grade3 grade4 grade5))
       3)))

Call

(weighted-average 100 90 90 10 90)

Remember:

• The form of procedures
• Evaluation is a lot like "(define PARAM VAL-YOU-PROVIDED)a" then body.

Questions

• Can I reuse names between procedures?

  • Yes. Scheme is smart.

  (define sq (lambda (val) (* val val)))

  (define dbl (lambda (val) (+ val val)))

  (+ (sq 5) (dbl 2)) 29 (sq (dbl 2)) 16

Lab

Lessons from the drawings lab

• The various procedures make new drawings, rather than modifying an existing drawing.

  (define d1 (recolor-drawing "yellow" (scale-drawing 50 drawing-unit-circle))) (drawing-hshift 20 d1) (image-show (drawing->image d1 200 200))

• Similar to

  (define x 5) (square x) x

• drawing-scale scales an image's offset as well as its size.

  • See the whiteboard for an explanation