Held: Wednesday, 24 February 2016

Back to Outline 18 - Conditionals. On to Outline 20 - Collage - Copy and Paste.

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 with Local Bindings
• Lab: Naming Values with Local Bindings
• EBoard 19 (Source 19)

Overview

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

Administrivia

• Continue partners.
• Quizzes returned.
• We'll continue to talk a little bit about testing.

Reminders

• Office hours: MTWF 10-11, Tu 1-2.
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  • Also feel free to stop by when my door is open.
  • Or to email me for an appointment.
• Tutor hours
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  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with Sarah
  • Thursday at 10am in this room with SamR
  • Thursday at 8pm in the CS Commons with Alex

Upcoming Work:

• Reading for Friday
  • Collage (not yet written)
• Assignment 5 due next Tuesday at 10:30 p.m.
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 19 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic

• Town Hall, Wednesday 7:30 pm in JRC 101
• CS Extras, Thursday: ????

Peer

• Go to Wednesday's SGA meeting at 8:30 p.m in JRC 209 to make noise about the reserve.
• Pun Club Tournament Friday the 26th in the evening.
• Contra Dance Friday the 26th in the evening.
• Improv Show the 27th in the evening in Loose Lounge.
• AppDev Pub Night, Friday, 8pm, Lyle's (JRC Basement).
• Black History Month Showcase, Friday, Harris, 7:30 p.m.
• Vote for SGA positions.

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Pun club Saturdays at 4pm in Younker
• Electronic Potpourri on KDIC Fridays at Five
• Space Odyssey KDIC Fridays at Six

Misc

• Christine Tran show in Smith Gallery.

No Extra Credit, But Still Good

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.