Class 9: Writing Your Own Procedures, Continued

Held: Monday, 6 February 2017

We continue to consider new ways to write your own procedures and why you might do so.

Preliminaries

Overview

• Evaluation in Scheme
• More about procedures
• Lab
• Questions and reflection.

Related Pages

• Eboard 9 (Source)

• Reading: Writing Your Own Procedures

• Reading: How Scheme Evaluates Expressions (take 2)

• Lab: Writing Your Own Procedures

Updates

News / Etc.

• New partners!
• Quiz 1 returned.
  • Note that (irgb-lighter c1) does not modify c1. Instead, it returns a new color.
  • That should not be surprising. You would not expect (square x) to change x.
  • Some of you missed that since the result of compose is a procedure, it implicitly has a parameter. For example, we would describe the result of (define irgbll (compose irgb-lighter irgb-lighter)) as “A procedure, (irgbll color), that computes (irgb-lighter (irgb-lighter color)).
  • That means that the solution to problem 1 should have been something of the form (define myfun (compose ....)).
• Our graders’ account got locked last night. (Even I don’t have access right now.) I’m not sure where we stand in grading.
• Exam 1 will be distributed on Wednesday.

Reminders

• I’m always happy to (try to) answer questions via email. There is no need to apologize when sending me questions. If I take too long to answer, send another email (or even text, if it’s a reasonable hour).
• We have tutors available Sunday through Thursday evening from 7-10 p.m. in Science 3813/15.
• We have mentor sessions on Wednesday and Thursday evenings from 8:00-9:00 p.m. in the CS Commons. Wednesdays will be more Q&A, Thursdays will include sample quizzes.
• I run review sessions on Thursdays at 9am in this room.
• We have individual tutors available for those who take advantage of the above and find that it’s not enough.

Upcoming Work

• Assignment 3. Due Tuesday!
• Reading: Output in Scheme.
• Reading: Simple input in Scheme.
• Lab writeup: Exercises 4, 5, and 6. Due Wednesday before class.

Extra credit (Academic/Artistic)

• CS Table, Tuesday, 7 Feb 2017. Something on privacy. See email.
• Thursday extras, Thursday, 9 Feb 2017, 4:15 p.m., Science 3821: Something on computer graphics (visitor from UMN). Let me know if you want the email.
• Scholars Convocation, Thursday, 9 Feb 2017, 11:00 a.m., JRC 101.

Extra credit (Peer)

Good things to do

Evaluation in Scheme

• Two main issues:
  • Table that associates names and values.
  • Where we are in the expression we are evaluating.
• General process
  • Evaluate from inside out
  • If you see a name, look it up in the table
  • If you see a value, leave it as is
• Complication from procedures
  • We also look up procedures in the table.
  • When we run a procedure, we temporarily update the table, associating the values of parameters for their names.

More About Procedures

• Three mechanisms for making procedures.
• Composition: Given multiple existing one-parameter procedures, string them together to make a new procedure.
• Sectioning: Given an existing multi-parameter procedure, make a new procedure that fills in some parameters with constant values.
• General/Lambda: Parameterize a chunk of code by writing (lambda (inputs) ...).
• For composition and sectioning, we don’t name the parameters.
• For lambda, we name the parameters, but Scheme doesn’t really care what we name them.

Lab!

• Work on the procedures lab.
• We’ll probably try to reflect a little bit at the end.