Held: Friday, 3 October 2014

Back to Outline 20 - Anonymous Procedures. 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
• EBoard (Source) (HTML)

Overview

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

Administrivia

• New partners!
• Office hours
  • Friday 2:15-3:15
  • Next week back to normal (MThF 1:45-3:15; walking 1:15-1:45) plus 8:00-9:45 most days.
• Note: The labs assume that you've done (or at least thought about) the self checks.

Upcoming Work

• HW 5 due Tuesday.
• Exam 2 will be distributed on Wednesday.
• Reading for Monday
  • Building Images by Iterating Over Positions
  • One of Sam's favorite parts of the course.

Extra Credit Opportunities

Academic

• CS table TODAY: Serendipity and Computing (with Grinnell's Artist in Residence)
• Grinnell Prize events next week (particularly events related to the prize winners whose project involves Web tools)

Peer Support

• Football (???)
• Men's Tennis (???)
• Women's Tennis (???)
• Swimming (???)
• 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
• Anna Christie, Oct. 9-12 (SB plays Marthy)

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.