Class 30: Local Procedure Bindings

Held Wednesday, March 14, 2001

Summary

Today we consider techniques for defining local procedures, procedures that are only available to select other procedures.

Notes

• There is no reading for tomorrow. Tomorrow's topic is sufficiently complex that I would prefer to introduce it with a lecture.
• I still haven't finished updating the additional notes on let. I hope to have them ready for tomorrow.
• There was some misunderstanding of how to use Blackboard. You submit files in a two-big-step process.
  • First, you add the file to your drop box. This makes it available to you, but not to me. (Some people like to keep their temporary files in a drop box.)
  • Second, you submit the file. You can tell if something in your drop box is submitted because it no longer has a "Remove" button and because the text reads "Submitted ...".
• I thought it would be helpful to look at one inelegant but primarily correct solution to the count-odd-rolls question. This person, like many of you, reflected on the possibility of using roll-die as a helper. Unfortunately, the solution does much more work than it needs to

  (define count-odd-rolls
    (lambda (n)
      ; If you roll 0 dice, then you can have 0 odd numbers.
      (if (= n 0) 0
          ; Otherwise ...
          (let ((dice (roll-dice n))) ; Roll some dice
               ; See if the first one is odd
               (if (odd? (car dice))
                   ; If so, count it and then do the rest
                   (+ 1 (count-odd-rolls (- n 1)))
                   ; Otherwise, just cound the remaining odd
                   ; rolls
                   (count-odd-rolls (- n 1)))))))
  

  • No, I won't tell you what's wrong with it.
  • I will suggest that you can observe one problem if you update roll-dice to print a short note each time it is called.
  • We can improve the procedure with an appropriate let.

Overview

• Why have local procedures
• Creating local procedures
• Lab

Local Procedure Bindings

• We frequently want to define procedures that are only available to certain other procedures (typically to one or two other procedures). We call such procedures local procedures
• Most local procedures can be done with let and let*.
• However, neither let nor let* works for recursive procedures.
• When you want to define a recursive local procedure, use letrec.
• The introductory example (from the notes) will serve as our first test case.

Lab

• Do the lab.