Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Lab!
• Debrief.

Preliminaries

Admin

• Apologies about occasional lab confusion yesterday.
• You now know how to get online, so I won't be providing printed copies of the lab manuals.
• My normal office hours are MWF 11:00-11:50. You sign up for those at https://rebelsky.youcanbook.me. I will also schedule meetings in other times. My paperwork times are probably the best, M-F, 8-9 am. Check my schedule for more. (I'll give up the occasional class prep time if necessary, but I prefer advance notice.)
• The mentors will be surveying you about preferred time for mentor sessions (probably Wednesday and Thursday evenings)

Upcoming Work

• Two readings for Wednesday: Numeric Values and Symbolic Values
• HW 2 to be distributed tomorrow.
• Quiz 1 on Friday.

Extra Credit Opportunities

Academic

• TODAY, noon, CS Table, JRC 224C. How Google Can Manipulate Elections. Readings available in the back of the classroom. If you are not on a meal plan, we'll pay for your meal.

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 1:30 and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker

Leftover Questions on Algorithms

Thanks very much to the students who provided these questions!

Can you give an example of a subroutine from the nut butter and preserve algorithm?

Sure. There was an "open a jar" algorithm that I could have applied to both the nut butter jar and the preserve jar. Given that the preserve jar is glass, I don't think it would have been a good idea to use the "hit the top of the jar on the counter" algorithm. Here's an alternative. (Yes, you can probably find flaws.)

To open a jar

1. grasp the body of the jar with your non-dominant hand holding the top of the jar upward.

2. grasp the lid of the jar with your dominant hand

3. if the jar appears to be a screw-top jar, rotate the lid of the jar counter-clockwise while continuing to hold the bottom steady. Keep rotating the lid counter-clockwise until it separates from the jar.

4. if the jar appears to be a pop-top jar, pull the lid upward, applying force at the edge of the lid while holding the body of the jar steady

5. if the jar appears to be neither a screw-top jar nor a pop-top jar, report an inability to open the jar and stop.

Can you give examples of the where parts of an algorithm appear in the nut butter and preserve algorithm?

• "Untwist" and "grab" are basic operations. (Many of you assumed that "open" is a basic operation. But it didn't seem to be.)
• The jars, utensils, bread, plates, and napkins are the parameters to the algorithm.
• Similarly, the jar is the parameter to the "open a jar" algorithm.
• Within the broader algorithm, "the slices of bread" (meaning the ones that I removed from the bag) are variables.
• Similarly, the "dominant hand" is a variable within the "open a jar" algorithm.
• Opening the bag of bread before removing the bread is an example of sequencing
• Questions about the type of jar are examples of conditionals
• "Keep rotating the lid counter-clockwise until it separates from the jar" is an example of repetition.
• And as we just saw, the "open a jar" steps are a subroutine.

Additional Questions

What is the difference between basic values and parameters?

Parameters name the inputs to the algorithms. "jar of fruit-based substance"

Basic values are the kinds of values we can provide, such as "jar of Smuckers(r) raspberry preserves"

In math, we think about things like f(x). x is a parameter to the function f. Numbers, like 42, are basic values.

Lab

Why can't I use square?

square is not defined by default in DrRacket. That was a trick to help you remember that not every natural operation is defined. (Don't worry, we'll define it soon.)

What are the shortcuts that Erin, Zachary, and Sam provided to folks?

In the interactions pane, control-uparrow brings back the previous command for you to edit

If you select with the left mouse button, you can (usually) paste with the middle mouse button.

Debrief

Where can I do work for this class?

For the first few weeks of class, please work on Noyce 3rd.

After that, we will provide a few additional options

1. Use a virtual machine.

2. Connect from your computer to ours. (Harder, sometimes slower, but gives the same files.)

3. If you run Linux, you can install the class software (I think).

But working on Noyce 3rd is always good.

Do we need to define something before we use it?

Yes, in general.

How did you know that the square root was correct?

Lab Writeup

Email an answer to the following question to rebelsky@grinnell.edu.

What did you see as the learning outcomes for exercise 3?

Due Friday.

Submit as a joint assignment by sending an email with all of your names..

Subject: CSC 151 Lab Writeup Day 3 (Z. Livingston Seagull and E. Gotshot)

Please substitute your own names.