EBoard 05 (Section 1): Expressions and Types

Approximate overview

• Administrative stuff [20 min]
• Questions [?? min]
• Lab [Approximately 60 min]

Administrivia

Introductory notes

• I am not sure why recording is not working today; follow along on the eboard.
• Please say your name when you ask or answer a question (even if I’ve just called you by name).
• Don’t forget that we have evening tutors available 7–10 p.m. Sunday through Thursday, as well as 3–5 p.m. on Sunday.
• The readings appear to have been a bit long. I apologize. We’re down to two readings for Friday (and none for Monday).
• Thanks for the good questions. And don’t apologize for not understanding!

Racket notes / Debrief from last lab

• Save early and often! (insert obligatory joke)
• Use Ctrl-I to reindent (Cmd-I on Mac). Understanding indentation will help you find some problems, particularly with regard to number of parameters and parenthesization.
• Remember that “How do you know your answer is right?” Many of you neglected to check whether snowman-revisited is the right height. (Or other procedures.)
• It would be nice if you decomposed in your procedures, which is why we had the top-hat procedure in the lab, but we understand that it will take a little time to get used to it.
  • fancy-house should probably call painted-house and door procedures, especially since you’d already written painted-house.
  • snowman-with-hat should probably call …

• There are other ways to think about reusing your prior code. Could you define (snowperson-revisited height) in terms of (snowperson size)?

    ;;; (snowball size) -> image?
    ;;;   size : positive-real?
    ;;; Make a snowball of the given size.
    (define snowball
      (lambda (size)
        (overlay (circle size 'outline 'black)
                 (circle size 'solid 'grey))))
  
    ;;; (snowperson size) -> image?
    ;;;   size : positive-real?
    ;;; Make a snowperson whose base is size.
    ;;; Height is 260/60 or 26/6 or 13/3 * size
    (define snowperson
      (lambda (size)
        (above (snowball (* 1/2 size))
               (snowball (* 2/3 size))
               (snowball size))))
  

  If size is 60, we get snowballs of radii 30, 40, and 60 (diameters 60, 80, and 120). The total height would then be 260. So we could define snowperson-revisited as

    (define snowperson-revisited
      (lambda (height)
        (snowperson (* 13/3 height))))
  

• This example illustrates part of the difference between correct (M) and exemplary (E).
• Once you’ve written a procedure, think of how it would simplify your life in future computations.

Notes on Numbers in Racket

Important issue: Exact numbers are stored precisely; inexact numbers can be approximated. For integers, you’ll see problems when you get to large numbers. For smaller numbers with a fractional part, you’ll see a variety of issues.

Exact integers are stored exactly. Inexact integers are approximated. For large enough values, you’ll see a difference.

    > (- (+ 1 (expt 10 50)) (expt 10 50))
    > (- (+ 1 (expt 10.0 50)) (expt 10.0 50))

We generally prefer exact numbers. However, inexact numbers naturally crop up in our calculations. There are also some storage and speed advantages to using inexact numbers.

Notes on Strings in Racket

Syntax vs. Semantics

We’re beginning to see the difference between the conceptual (semantics) and how we express concepts (syntax). Consider, for example, a string that contains two quotation marks.

Strings must begin and end with quotation marks. That is, the take the form “…”. How do we signify a quotation mark within the string? If we just write a quotation mark, it’s indistinguishable from the quotation mark that ends the string. Hence, we must precede it with a backslash. "\"\"".

Collating Sequences

Behind the scenes, computers essentially store everything as numbers. The collating sequence tells you what number corresponds to each letter (or vice versa). It’s helpful to see that, say, the uppercase letters precede the corresponding lowercase letters by exactly 32, or that the digit characters appear in order. We’ll use them for those kinds of reasons.

Gradescope notes

• For S/N, S is 1 and N is 0.
• For E/M/R/I, E is 3, M is 2, R is 1, N is 0.
• For everything, ignore the autograder computations (other than to see what you might have had problems with).
• We’ll look at how you check grades on Gradescope.

Upcoming work

• Readings for Friday due Thursday at 10:00 p.m.
  • Double-dagger problems are Booleans Self-Check #2 and Conditionals Self-Check #1.
• Lab writeup from today due Thursday at 10:30 p.m.
  • Ideally, you’ll finish that in class today.
  • If not, set up a time to meet with your partners.
  • If that’s not possible, finish (and submit) separately, but cite your partner.
• Mini-Project 1 due Thursday at 10:30 p.m.
  • On gradescope (after lunch).
• Friday’s quiz will be on procedural abstraction.

Sample quiz problem

Take a concrete implementation in Racket and create a new function that generalizes the behavior.

As you may recall in building snowmen, we found it useful to be able to create hats for the snowmen. A hat consists of three parts: a box, a brim, and a ribbon. The brim is at the bottom. The ribbon appears to be on the box, directly above the brim, although we will create it by stacking a rectangle over the box.

We’ve decided on a 50x50 grey hat with a purple ribbon. We’ve decided that brims will always be 5 units high and that ribbons will take 1/5 the height of the box. Rather than doing the calculations by hand, we’ll make Racket do the computations.

(above (rectangle (* 50 1/2) (* (- 50 5) 4/5) "solid" "grey")
       (rectangle (* 50 1/2) (* (- 50 5) 1/5) 192 "purple")
       (rectangle 50 5 "solid" "grey"))

Write a procedure, colorful-hat, that generalizes this approach by taking the width, height, hat color, and ribbon color as parameters and producing an appropriate image as output.

Upcoming Token-Generating Activities

• CS Extras: The 4+1 (Undergrad to Grad) program in CS w/UIowa. 4 p.m., Thursday, 3 February 2022, Noyce 3821.
• Men’s Tennis 9am and 5pm Saturday in the Field House. (30 min of watching is enough)
• Swim meet Saturday at 1pm. (30 min of watching is enough)

Other Upcoming Activities

• Men’s Basketball Saturday at 1 p.m.
• Women’s Basketball Saturday at 3 p.m.

Reading Questions

I’ve started putting answers to the reading questions in the readings. These are ones that didn’t naturally fit in a reading

Why do we sometimes use a symbol and sometimes use a string when defining something like a square? For instance, both (square 40 ‘solid ‘blue) and (square 40 “solid” “blue”)` are valid and produce the same image.

The designers of the 2htdp/image library decided it was nicer to support either symbols or strings. I assume they are using symbols as the primary mode, though.

Other Questions

How do I do Ctrl-uparrow on a Mac?

Esc-P (“previous”). You can also use Esc-N if you’ve gone back too far. We normally hit the escape and the letter in sequence.

Ctrl-Command-Uparrow also appears to work.

Lab

Preparation

Make sure to update the csc151 library. (It’s always good to check.)

Make sure to introduce yourself to your partner.

Problems are labeled A and B.

The partner closer to the front of the room is Partner A. The partner further from the front of the room is Partner B. Partner A should be at the keyboard for A problems. Partner B should be at the keyboard for B problems.

Make sure to grab the basic-types.rkt file and put it into DrRacket. All the instructions are in the file.

Stopping

Today is an as far as you got is far enough day. When it’s time to submit, write

    ; THIS IS AS FAR AS WE GOT!  SAM SAID WE COULD STOP HERE.

And then submit. You should read through the rest of the lab, though.