Held: Wednesday, 30 September 2015

Back to Outline 19 - Pause for Breath. On to Outline 21 - Anonymous Procedures, Revisited.

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 20 (Source 20)

Overview

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

Administrivia

• New partners!
• Review sessions
  • Wednesday at 8pm (Kumar)
  • Thursday at 10am (SamR)
  • Thursday at 8pm (Erin)

Upcoming Work

• Re-Reading for Friday: Anonymous Procedures
• HW 5 due Tuesday
  • To be released tonight
• Today's Lab writeup (due before class Monday
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 20 (Your Names)
  • Exercise:

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery
• EE's show in Smith (closing tonight at 8pm)
• Convocation tomorrow at 11am
• Peter Coyote talk 7pm Thursday
• Liz Queathem talk 4pm Friday
• MET Livecast Saturday at noon at Harris Auditorium

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included), Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance.
• PALS dinner Thursday night. (Sam will treat if you find it a financial burden.)
• Pun Club, Saturdays, 4pm, Way over Younker.
• Mid-Autumn Festival, Saturday, Oct. 3, 6-8 p.m. in Harris Concert Hall.

Other Good Things

• VBall Saturday at 1pm in Darby
• American Football Saturday at 1pm at the football field
• Women's Unamerican Football Saturday at 11am
• Men's Unamerican Football Saturday at 1pm
• Women's Soccer Sunday at 11am
• Men's Soccer Sunday at 1pm

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.