CSC151.01 2015S, Class 04: Computing with Symbols and Numbers

Overview

Preliminaries

Office Hours:
- Normally: 10:00-11:00 MTWF, 3:15-4:00 M
- Currently booked: 10:30-11:00 M, 10:45-11:00 Tu, 10:00-11:00 F (well, it will be).
- Reserve a slot at https://rebelsky.youcanbook.me/.
Welcome to our Prospie!
Mentor sessions Monday at 7pm and Thursday at 8pm in the Commons.
No, the "Current Reading" link doesn't always work. You can use the syllabus or outline or eboard to get to the reading.
How many people would use lab tutors on Sundays from 3-5 pm if we added a shift then? Yes.
Homework comments:
- My goal is that homework should be no more than three hours. If you're finding that it's taking much more (or that you're on track to take much more), you should talk to me.
- I hear that getting three Grinnellians together is hard. I'll shoot for partners on most remaining assingnments.
- Submit ONE assignment for each group, CSC 151.00 Assignment 2 (Person One, Person Two, Person Three)

Assignment 2.
- Due Tuesday at 10:30 p.m.
Lab writeup for lab 4, due Wednesday at 10:30 p.m.: Problem 2, parts d,e,f. http://bitly.com/151-2015S-lab04
Reading for Tuesday:
- Drawings as Values

Friday, 12:10 p.m., JRC 224A (Day PDR), CS Table, "Redundant Arrays of Inexpensive Disks" (copies outside my office)

Swimming, Friday, January 30, 6:30 p.m., Osgood Natatorium
Swimming, Saturday, January 31, noon, Osgood Natatorium
Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.

What were the key points of the reading?

A "word" can mean/be three different things
- The name for something: No extra typography teacher
- "Symbol" - Stands for itself, preceded by a single-quotation mark 'teacher
- "String" - Stands for itself, surround by double-quotation marks "teacher"
- The latter two are more specific, the former is more general.
- Variables have values attached to them; symbols are their own value
- Symbols are atomic; you can't split them up
Scheme lets you do lots of computation with numbers
- Both exact and inexact
- Many different operations
- Often variants of the same operation
You need to know the basic operations anything/one can do and the kinds ("types") of values it works with.

What was troublesome?

Modulo
- Like remainder, except always the same sign as the divisor.
- For positive divisor, think of it as a circle/cycle/clock
- Sam is hopeless at clarifying. Forotunately he has tenure. We'll explore in lab.
Why use inexact vs. exact numbers?
- Some things can't be represented exactly (at least the way the computer works: pi, (sqrt 2), ...). We have to use inexact there.
- Exact computation is expensive.
- Inexact exists for both purposes.
Why do we use symbols and strings and how do they differ?
- Week ten
Why do we have so many kinds of division?
- Same as math: Sometimes you want a whole result, some

Notes from watching students:

For problem 2, you definitions window should start out something like

#lang racket
(define lower 0)
(define upper 100)
(define val 25)
(define bounded-val ...)

As it says "suppose ...", you can change any or all of the first three definitions.
Control-up-arrow brings back the previous command, as does escape-p
I seem to have been correct, modulo is becoming more clear for many of you ask you try experiments.
Over the longer term, you'll find it useful to have the lab and the writeup on the same machine as DrRacket. That way, you can copy and paste.
People work at different speeds. That's okay. Speed corresponds to neither understanding nor success.
Note that (modulo 3 8) is asking questions about 3/8, not vice versa.