University of California, Berkeley
EECS Department - Computer Science Division

CS3 Lecture 15 : Lambda (cont'd)

---

Overview of today's lecture

• Review
• Lambdas (cont'd)
  • More Lambda Coolness
• Next Time
• Puzzle : Pairing Up Students
• Game : Order vs. Disorder

Review

• Lambda expressions are a way of creating nameless functions.
  • They are most often used to create specialized functions to use with higher-order functions.
  • You sometimes NEED lambdas when the function you are passing in as an argument to the functional needs more information than just each list element provides.
  • They are the easiest way to return a function

---

Lambdas (cont'd)

More Lambda Coolness

• You can make a procedure that returns a procedure yourself!
• Read the following code carefully to make sure you understand it. It's subtle, but important.

: (define (make-bookends bookend-word)
      (lambda (w) (word bookend-word w bookend-word)))
make-bookends

: ((make-bookends 'o) 'hi)
ohio

: ((make-bookends 'to) 'n)
tonto

• Now we can define another procedure to be the result of the call to make-bookends!

: (define to-bookend (make-bookends 'to))
to-bookend

• And now to-bookends is just like any other procedure!!

: (to-bookend 'ron)
toronto

• Howabout writing a procedure oo that takes no arguments and returns a procedure that takes no arguments that returns infinity?

: (define (oo)
    (lambda () 'infinity))

• How do we invoke it?

: ((oo))
infinity

Summary

• Read chapter 9 carefully and make sure you understand it all.

Next Time

• Tic Tac Toe project (read it before coming to lecture!)

Puzzle : Pairing-up Students

• Assume you have n students (n is even). You would like to have lab sections in which every student chooses a partner to work together for that particular lab section. Over the course of the semester, you'd like to have every student work with every other student exactly once. As there are n students, this can be done in n-1 days optimally. How is it done? (e.g. show how to pair 20 students up)
• Example of dead-end solution with 6 people:
  • Day 1 : [1 pairs with 2] [3 pairs with 4] [5 pairs with 6]
  • Day 2 : [1 pairs with 3] [2 pairs with 4] ...uh oh, 5 already paired with 6...

Game : Order vs. Disorder

• On a 6 x 6 grid, one person chooses order and one person chooses disorder and they take turns moving.
• On their turn, each person places either an X or an O on the board.
• Order wins if they achieve 5-in-a-row (horizontally, vertically or diagonally)
• Disorder wins if they achieve a full board with no 5-in-a-row present.

WWW Maven: Dan Garcia (ddgarcia@cs.berkeley.edu) Send me feedback