Held: Wednesday, 2 October 2013

Back to Outline 19 - Iteration. On to Outline 21 - Boolean Values and Predicate Procedures.

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

Overview

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

Administrivia

The Problem: Naming Values

• 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 reasons 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 ((name₁ exp₁) (name₂ exp₂) ... (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.

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