CS3 Fall 2005
Lecture 11:
Finish higher order functions
Midterm Prep
Oct 31
More HOF
– "tic-tac-toe" program: SS chapter 10!
– "Change Making" case study, in the reader
Nov 7
Miniproject #3: Election-processing
– Thursday a catch-up day, Friday is a holiday
– MiniProject #3 due Thur/Fri: plan ahead!
Nov 14
Midterm #2
Lab: Lists (extending words and sentences)
Nov 21
Nov 28
Dec 5
Start on the project: check-off #1
Work on the project: checks #2
Checkoff #3, finish the project
Dec 17
Final Exam (midterm #3)
Programming Style and Grading
• During grading, we are going to start
becoming “more strict” on style issues
– MiniProject #3 will be the start
– For the big project, style is important
• Why?
– Program maintenance: 6 months later, will
you know what your code does?
– Code “literacy”: sharing code
What issues of style matter?
• Keep procedures small !
• Good names for procedures and
parameters
• Adequate comments
– Above and within procedures
• Avoid nesting conditional statements
• Put tests cases in a comment block
• Indent to aid program comprehension
Midterm 2
• Midterm 2: Nov. 14th (next Monday).
– In the lecture slot plus 30 minutes
(4-5:30 pm, 120 Latimer)
– Practice exam in reader (do this all at once)
– Check announcements for more practice
items and solutions.
• Review session
– this Wednesday (Nov. 9), 7-9 p.m.
– 306 Soda Hall (HP Auditorium).
What does midterm #2 cover?
– Advanced recursion (accumulating, multiple
arguments, etc.)
– All of higher order functions
– Those "big" homeworks (bowling, compress, and
occurs-in)
– Elections miniproject
– Reading and programs:
• Change making,
• Difference between dates #3 (HOF),
• tic-tac-toe
– SS chapters 14, 15, 7, 8, 9, 10
– Everything before the first Midterm (although, this
won't be the focus of a question)
When do you NEED lambda?
1. When you need the context
(add-suffix '-is-great '(nate sam mary))
 (nate-is-great sam-is-great
mary-is-great)
2. When you need to make a function on
the fly
Procedures that make procedures
• Generally, name procedures that create
procedures "make-XXX"
(make-bookends 'o)
 #[closure arglist=(inner-wd) d7d0e0]
((make-bookends 'o) 'hi)  ohio
((make-bookends 'to) 'ron)  toronto
(define tom-bookend
(make-bookends 'tom))
(tom-bookends "")  tomtom
Write successive-concatenation
(sc '(a b c d e))
 (a ab abc abcd abcde)
(sc '(the big red barn))
 (the thebig thebigred thebigredbarn)
(define (sc sent)
(accumulate
(lambda ??
)
sent))
make-decreasing
• make-decreasing
– Takes a sentence of numbers
– Returns a sentence of numbers, having
removed elements of the input that were not
larger than all numbers to the right of them.
(make-decreasing '(9 6 7 4 6 2 3 1))
 (9 7 6 3 1)
(make-decreasing '(3))  (3)
Chips and Drinks
"I have some bags of chips and some drinks.
How many different ways can I finish all of
these snacks if I eat one at a time?
(snack 1 2)  3
– This includes (chip, drink, drink), (drink, chip,
drink), and (drink, drink, chip).
(snack 2 2)  6
– (c c d d), (c d c d), (c d d c)
(d c c d), (d c d c), (d d c c)
Lists (after the midterm)
• Lists are containers, like sentences where each
element can be anything
– Including, another list
((beatles 4) (beck 1) ((everly brothers) 2) … )
((california 55) (florida
(#f
#t
#t
#f
#f
23) ((new york) 45) )
…)
List constructors
• cons
– Takes an element and a list
– Returns a list with the element at the front, and the list
contents trailing
• Append
– Takes two lists
– Returns a list with the element of each lists put
together
• List
– Takes any number of elements
– Returns the list with those elements
List selectors
• car
– Like first
• cdr
– Like butfirst
Common list procedures
• Map = every
• Filter = keep
• Reduce = accumulate
• Null? = empty?
• Recursion is just the same!