Overview

• Preliminaries
  • Admin.
  • Connecting to MathLAN from your own computer.
• Why define your own procedures?
• How to define your own procedures.
• Lab.
• Reflection: What was the most confusing thing you dealt with in lab today?
  • I'll probably ask this question tomorrow, but think about it today.
  • "Sam" is not an acceptable answer.

Admin

• Reminder: Mentor sessions are Thursdays and Sundays at 7:30 pm.
• Reminder tomorrow's class is next door in 3815.
• HW3 will be distributed in class tomorrow.
• Reading for Wednesday
  • Documenting Procedures
• EC Opportunities:
  • Humanities Center Speaker Sarah Hendron Wed., Sept. 11, 7:30 p.m., JRC101 Waking the Machines: Art, Design, and Adaptive Technology
  • CS Table Friday (Reflections on Trusting Trust; NYT on NSA)
  • Others

How do I do CSC 151 work from my own computer?

• General instructions at http://www.cs.grinnell.edu/vnc
• I'll demo on my Mac. It's a bit harder if you use Microsoft Windows (and I don't know all of the details because I rarely use Windows).
  • I won't type the steps as I go

Why define your own procedures?

What is a procedure?

• A procedure is like a variable except it's an action (or series of actions) rather than just a simple value
  • That is, it automates a series of tasks
• On the scheme side, a procedure takes inputs and generates outputs
  • a square function might take a number as an input and produces a number as an output x -> x*x
  • make-a-circle: take as input x,y,radius, (number) etc.; output is a drawing taht represents a circle
  • um is a procedure that takes a question as input and gives as output the value "um" which is text (a string)

Why write them?

• Less redefinition
• More elegant
• More readable

• Easier to write

  (make-a-circle 10 10 100)

  is probably clear than (drawing-shift (drawing-scale drawing-unit-circle 100) 10 10)

How to define your own procedures

   (define *procname*
     (lambda (*inputs*)
       *expression*))

For example

    (define um
      (lambda (question)
        "um"))

    (define square
      (lambda (num)
        (* num num)))

    (define make-a-circle
      (lambda (x y radius)
        (drawing-shift (drawing-scale drawing-unit-circle (* 2 radius))
                       x y)))

Questions:

• Can procedures define other procedures?

  • Yes, but we won't cover that for a few weeks

• What's the delay?

  • You can't really evaluate the body of a procedure until you use the procedure?

• So how do you know it's correct?

  • You try it and see.
  • You try it a lot of times and see.
  • You do a formal proof

• How do I "read" a definition

  (define procname (lambda (inputs) expression))

I am defining a procedure named procname that takes as input inputs and computes its result by evaluating expression.

• When

Lab

• Remember to shift who is at the keyboard! (Unless you're one of those great groups that seems to function as one person.)
• Problems 5 and 6 will be your lab writeup (and yes, I'll email that out)