Held: Monday, 9 September 2013

Back to Outline 05 - Drawings as Values. On to Outline 07 - Writing Your Own Procedures.

Summary

We continue our exploration of a simple model of drawings as values. What can we do with circles, squares, and simple ways to transform them?

Related Pages

• Reading: Drawings as Values
• Reading (unused): Procedures
• Lab: Drawings as Values
• EBoard (Source) (PDF)

Overview

• A bit more on reals.
• A bit more on drawings.
• Lab.
• Reflection.
• Looking ahead to procedures.

Administrivia

• We will continue the lab from Friday.
• I'm still working on getting HW1 reviewed. Time is a very limited commodity (as I think you all know from your own experience).
• Note: Sometimes the html versions of the eboards don't get updated right after class. You can always see what I typed by going to the original source.
• Study break, tonight, 8pm, CS commons
• Reminder: Mentor sessions are Thursdays and Sundays at 7:30 pm
• Readings for Tuesday
  • Reread: Writing Your Own Procedures
  • Add: How Scheme Evaluates Expressions (Take 2)
• EC Opportunities:
  • Humanities Center Speaker Sarah Hendron Wed., Sept. 11, 7:30 p.m., JRC101 Waking the Machines: Art, Design, and Adaptive Technology
  • CS Table Friday (Reflections on Trusting Trust; NYT on NSA)
  • Other?

A bit more about reals

• We're adding a fourth issue when you learn about new types: How are they represented internally?
• What we've started to see with real numbers is that the way they are represented internally can sometimes make them confusing to deal with.

A bit about drawings

Some of you are confused by various drawing issues. So we'll talk about some things I noticed.

• In general, you will need to create new drawings with something like

  (define *new-drawing* (*drawing-operation* *old-drawing* *val*))
  

• For example

  (define bigger-circle (drawing-scale drawing-unit-circle 200))
  (define bigger-red-circle (drawing-recolor bigger-circle "red"))
  

• Sometimes, we want to nest these so that we don't have to do so many define operations

  (define bigger-red-circle 
    (drawing-recolor (drawing-scale drawing-unit-circle 200)
                     "red"))
  

• Note that the operations do not change the underlying drawing. They are similar to

  (define x 100)
  (define y (square x))
  ; What's x at this point?
  

Lab

Continue the lab.

Reflection

Looking ahead: Procedures

• What's a procedure? Something that takes input and either does something or computes a value (or both).
  • We'll primarily focus on procedures that compute values.
• We've been using procedures that someone else has defined.
• What if we want to define our own?

• Scheme has a strange syntax for procedures. (Okay, it has multiple syntaxes. We've chosen one that's initially strange, but much easier over the long term.)

  (define *proc*
    (lambda (*inputs*)
      *body*))