Same partners!

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Questions on letrec and named let.
• Brief overview of reading.
• Lab.
• Debriefing.

Preliminaries

Admin

• This group signup was particularly fascinating
  • It was very interesting to see some student names repeated in multiple groups. You were supposed to check with your partners to see if they were willing to work with you.
  • One of you said that you wanted two partners, but you didn't tell me your name.
  • Three groups sent their names multiple times.
• Review sessions tonight at 8pm, Thursday at 10am, and Thursday at 8pm.
• Quiz topics:
  • Recursion! (list, helper, numeric)
  • Preconditions, husk, kernel
  • Letrec and named let
  • Characters and strings

Upcoming Work

• Sign up to meet with me if you had difficulty on the exam (or even if you just want to discuss a few more problems or need help interpreting my scrawls).
• Lab writeup: Problems 5 and 6, http://bit.ly/151-2014F-w32.
• Reading for Friday: Turtle Graphics
• Homework 6, due Tuesday.
• Exam 3, distributed a week from today, due two weeks from yesterday.

Extra Credit Opportunities

Academic

• CS Extra Thursday: Randomness in Computer Science.

Peer Support

• Noteworthy is going to be performing at 7:30 PM at Herrick Chapel on Halloween night (Friday the 31st).
• Football (Saturday at 1pm)
• Swimming scrimmage nine on Saturday
• Karan's radio show 11pm Thursday nights on KDIC
• Evan's radio show 5pm Friday nights on KDIC
• Donna's radio show Sunday midnight on KDIC
• Wear a costume on Friday

Questions

What encoding does DrRacket use?

Unicode.

Could you explain this part of the reading? It makes sense that you can compare characters together to see which comes first in integer collation, but I don't understand how that concept can apply to entire strings.

Strings can be compared for "lexicographic order", the extension of alphabetical order that is derived from the collating sequence of the local character set. Once more, Scheme provides both case-sensitive and case-insensitive versions of these predicates: string<?, string<=?, string=?, string>=?, and string>? are the case-sensitive versions, and string-ci<?, string-ci<=?, string-ci=?, string-ci>=?, and string-ci>? the case-insensitive ones.

This is mostly what you expect for alphabetical order. Say we have two strings, which I'll call string1 and string2.

If the collating sequence number of the first letter of string1 is smaller than the collating sequence number of the first letter of string2, string1 precedes string2 lexicographically.

If the collating sequence number of the first letter of string1 is larger than the collating sequence number of the first letter of string2, string1 follows string2 lexicographically.

If the two collating sequence numbers are equal, we go on to the next character and repeat that whole process again.

If we run out of letters in one string before running out of letters in the other string, the first string is lexicographically first.

It's basically "dictionary order".

Questions on letrec and named let

Students are too overwhelmed to have any questions.

Brief overview of reading

Nevermind, Sam, we think we understand it.

Lab

Why is the last parameter to substring the index after the last character that substring extracts?

1. It means that we can use the same value for the end of one substring and the start of the next substring.

2. It means that we can use (string-length str) as the last parameter to substring when we want "everything until the end".

Something I'd forgotten about Scheme

    > (substring "Hello" 1)
    "ello"

Debriefing

How did you make "???" using list->string?

(list->string (list #\? #\? #\?))
(list->string (make-list 3 #\?))

If make-string did not exist, how would you define it?

(define make-string
  (lambda (num char)
    ...))

How did you write string-sum?