CSC 151.01, Class 30: Other Forms of List Recursion

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Lab

News / Etc.

• Continue partners
• Happy pi day!

Rotating reminders

• Use our tutors! We have tutors available Sunday through Thursday evening from 7-10 p.m. in Science 3813/15.

Upcoming Work

• HW 6 due TONIGHT!
• Lab writeup: Exercise 3. Due Friday.
• Reading for tomorrow: Numeric Recursion
• No homework over break.
• Friday’s quiz: Identify your classmates

Extra credit (Academic/Artistic)

• CS Table, Tuesday at noon, SIGCSE Debrief
• CS Extras, Thursday at 4:15 pm: ???

Extra credit (Peer)

You do not get extra credit for supporting yourself.

• Singers Concerts during spring break.
  • If you go and there is an admission charge, Sam will reimburse (up to $20)
• Baseball games April 1 and 2, all day. (1 hour suffices, no more than two units for the weekend)
• Baseball games in Arkansas and Arizona during break.

Extra credit (Misc)

Good things to do

Questions

Lab

Can we use direct recursion for exercise 1?

Yes.

Can we use a helper procedure and tail recursion for exercise 1?

Yes.

Can we use direct recursion for exercise 2?

Yes, I’d recommend it.

Can we use a helper procedure and tail recursion for exercise 2?

Yes, but I think it’s harder than direct recursion.

How many parameters to lists-join take?

One, a list of lists.

Writeup

Exercise 3! (It’s class 30.)

Notes for next class

1. Sam should talk about the benefit of building tables for tail-recursive procedures.

2. Sam should discuss the following code.

#lang racket

(define iota
  (lambda (n)
    (iota-helper 0 n)))
(define iota-helper
  (lambda (start end)
    (if (>= start end)
        null
        (cons start (iota-helper (+ start 1) end)))))

(define smallest
  (lambda (lst)
    (display (list 'smallest lst)) (newline)
    (cond
      [(null? (cdr lst)) 
       (car lst)]
      [(< (car lst) (smallest (cdr lst)))
       (car lst)]
      [else
       (smallest (cdr lst))])))

(define minimal
  (lambda (lst)
    (if (null? (cdr lst))
        (car lst)
        (min (car lst) (minimal (cdr lst))))))

(define la
  (lambda (lol)
    (lala lol null)))

(define lala
  (lambda (remaining so-far)
    (if (null? remaining)
        so-far
        (lala (cdr remaining) (append so-far (car remaining))))))

(define example
  (lambda (lol)
    (time (la lol))
    1))