Held: Tuesday, 24 February 2015

Back to Outline 20 - Anonymous Procedures, Revisited. On to Outline 22 - Images as Functions from Position to Colors.

Summary

We consider why and how you name values within a procedure. We also consider why and how you limit access to those names to within the procedure.

Related Pages

• Reading: Naming Values wiht Local Bindings
• Lab: Naming Values with Local Bindings
• Morning EBoard 21 (Source a.m. 21)
• Afternoon EBoard 21 (Source p.m. 21)

Overview

• Why name things.
• Naming things with let.
• Naming things with let*.
• Naming procedures.

Administrivia

• Retain partners!
• When you send questions on homework, please carbon-copy your partner.
• Review sessions Thursday night and Thursday day.
• Office hours today at 10:00 a.m. and 2:45-3:30 p.m. Office hours Wednesday at 10:00 a.m.
  • I am trying to make office hours available for any time that I am not booked with other activities. You will see more appear as time goes on.
  • http://rebelsky.youcanbook.me
• I have a few people who won't be able to work on the assignment until Sunday night at the earliest. Does anyone want to partner with them? (Is anyone one of them?) If so, send me an email message.

Upcoming Work

• Homework 5.
  • Due TONIGHT at 10:30 p.m.
• Homework 6 to be assigned tomorrow.
• Exam 2 to be assigned in a week.
• Lab writeup for today: http://bit.ly/151-2015S-lab21
• Reading for Wednesday
  • Images as Functions

Extra Credit Opportunities

Academic

• Pioneer Diversity Council Diversity Dialogue, 8:30 p.m. 26th, Main Quad "Academic Athletes and Academic Students".
• CS Table Friday - Art, CS, and Artificial Intelligence.

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• Host a prospie.
• Indoor Track this Saturday
• Neverland Players Friday and Saturday at 7:30, Sunday at 1:30 and 7:30
• Nobody Gets Hurt, a show with an audience of one. Info on trying to get one of the tickets is on the posters.

Peer Support (Afternoon Section)

• Indoor Track this Saturday

Other Good Things

• Neverland Players this weekend.

The Problem: Naming Values

• You may recall that naming values is one of the key steps in algorithm design.
• We've named values outside of procedures, and as parameters to procedures.
• Today, we name values within procedures.
• As we've seen in many problems, it helps to name the values that we use within our procedure. Why?
  • It can make the code more readable because the name tells us something about the role the value plays.
  • It can make the code more efficient, because it allows us to avoid recomputing a value.
• Another reason to name things is that we might want to create helper procedures and only make them available to the current procedure.

Naming Things with let

• You name things with let.

• let has the form

  (let ((name_1 exp_1) (name_2 exp_2) ... (name_n exp_n)) body)

• let has the meaning:

  • Evaluate all the expressions.
  • Update the binding table to associate each name with the corresponding value.
  • Evaluate body using the updated binding table.
  • Eliminate all the bindings just created.

• You can use let in a simple expression:

  (define values (list 1 4 2 4 1 5 9)) (let ((largest (max values)) (smallest (min values))) (/ (+ largest smallest) 2))

• More frequently, we use let within a procedure.

Sequencing Bindings with let*

• If we want to bind some things in sequence, we need to use let* rather than let.
• let* has the form (let* ((name_1 exp_1) (name_2 exp_2) ... (name_n exp_n)) body)
• let* has the meaning:
  • Evaluate exp1.
  • Update the binding table to associate name1 with that value.
  • Evaluate exp2.
  • Update the binding table to associate name2 with that value.
  • ...
  • Evaluate expn.
  • Update the binding table to associate namen with that value.
  • Evaluate body using the updated binding table.
  • Eliminate all the bindings just created.