This is the archived website of SI 413 from the Fall 2012 semester. Feel free to browse around; you may also find more recent offerings at my teaching page.
This lab is due at 0900 on Wednesday, September 5.
(The schedule is a little different because Monday is Labor Day
and Tuesday is a Monday.)
It should be submitted as "413 lab 02", and should contain a single
Scheme file lab2.scm
with all of your solutions, plus
testing files in a subfolder called tests
, as discussed
below.
Full submission instructions are on the
submit page and
have been updated since last week, so be sure
to read them!
You will notice that exercises 8 and 14 are marked as "optional". Yes, that really means that they are totally optional. Sorry, no extra credit. Here's some more information on why we're doing it this way.
A major theme of this class is understanding careful specifications, and a consequence is that, for the programming parts at least, you should be able to know the quality of your solution before submitting it. To this end, you will be given the time and the credit for carefully testing your code. Remember that the purpose of testing is to find bugs in code. Therefore a good test is one that fails a piece of code. Don't feel bad about this; that's the point of testing! Easy tests might make you feel good, but are useless. Careful testing helps you find bugs before they cause problems.
Starting with this lab, you will write and submit (unit) tests with each of your labs. The full details of how these tests are submitted is outlined on the submit page, but here is a general overview.
You will write and submit two tests for each of the exercises below.
Each test is in two parts: First, there is a single Scheme expression
(possibly following some definitions) that will be evaluated in the
context of the file that's being tested. Then you have another
Scheme expression for the "expected" result. These are separated by
a single line that contains just a semicolon ;
and
nothing else.
For example, to test the to-celcius
function from
Lab 1, the following file would work:
(to-celcius 77) ; 25
Notice that the tests don't have to be long or complicated!
You write each of these tests into a different file, and put
all those files in the tests
subdirectory of your
submission.
An easy script is provided to test your solution against the tests you have written. There is also a collection of "public tests" (basically corresponding to the examples in the text of the lab) that I have written for you. There is another script provided to test your submission against all your tests as well as all the public tests. If you pass all these tests, you can feel reasonably confident that you have understood the basic intent of all the exercises and named your functions correctly. This "sanity check" against your tests and the public tests is also automatically run every time you submit.
After the deadline, I will test your code and I will test your tests! First, your tests are run against a correct sample solution, and against a very incorrect anti-solution. You will lose credit for any tests you write which my (correct) solution fails, or which my (incorrect) anti-solution passes. These bad tests are also discarded for the next round, in which your code is tested with everyone else's "good tests". Here, you will receive credit for other students' tests that you pass, as well as your tests that other students' code fails. So you will be rewarded for writing difficult tests, and for passing the difficult tests that everyone else writes!
My hope is that taking the time to do this testing makes you more confident in your own code, and encourages you to think carefully about what mistakes you might make before you make them. I would love to hear any feedback you have about the testing process in our labs.
Recall that a symbol is just a quoted identifier, such as
'si413
, or 'wombats
.
We can use the function symbol?
to find out whether
something is a symbol, and symbol=?
to compare two
symbols.
1 US dollar = .7988 Euros (euro) 1 US dollar = 78.71 Japanese Yen (yen) 1 US Dollar = 1134.75 South Korean Won (won)
(to-usdollar amt cur)
that takes
an amount of money amt
in some foreign currency cur
and returns that amount in US dollars.
The parameter cur
will be one of
the symbols
'euro
,
'yen
,
'won
, or
'usd
.(to-usdollar 500 'yen)
produces 500/78.71, which comes out to
6.3524...
.
(from-usdollar amt cur)
that
does the opposite: takes an amount in US Dollars and converts
it to the named currency.(convert amt fromCur toCur)
that takes an amount in currency fromCur
and converts it to currency toCur
. Use your functions from
parts 1 and 2 and life will be easy!
Recall "big 4" of list processing:
null
: the name for an empty list.(cons a L)
: Take an item a
and a list
L
and return the new list with a
inserted
in the front of L
.
(car L)
: Returns the first item in L
.
(cdr L)
: Returns the list containing all the
items in L
after the first item.
If you're faced with nested lists, you sometimes need cars of
cdrs, and cdrs of cars, and so forth. The shortcut for
a bunch of these in a row is cXXXr
, where
each X
is either a
or d
,
corresponding to car
and cdr
:
There are two more extremely useful shortcuts:
list
: Takes an arbitrary number of items
and makes a single list containing them.
append
: Takes an arbitrary number of lists
and makes a single list containing all their items.
Make sure you understand the difference between these two!
In class we looked at the general pattern for a recursive function
on a list. When that function is also producing a list, it extends
the pattern by (usually) returning null
in the base case, and
(usually) returning a cons
in the general case. For example,
here is a simple recursive function that multiplies every number in a list
by 2:
; alon must be a list of numbers. (define (mulby2 alon) (if (null? alon) null (cons (* 2 (car alon)) (mulby2 (cdr alon)))))
See what's happening? Make sure you can identify the base case condition, the base case return value, and the recursive call. Notice what gets multiplied by 2, and where it happens. If it's confusing, now is the time to ask your able and handsome instructor!
squares
that takes integers i and j and
returns list of the squares i^2, (i+1)^2, ..., (j-1)^2, j^2.
> (squares 2 12) (4 9 16 25 36 49 64 81 100 121 144)
In class we saw that there is a built-in Scheme function length
which returns the length of a list. Using this, we could write a function
longer?
to test whether the first list is longer than the second:
; L1 and L2 must both be lists (define (longer? L1 L2) (> (length L1) (length L2)))
Now I want you to write your own version of longer?
that
doesn't use length calculation as a subroutine. Instead, you will have to
recursively process both L1
and L2
at the same time until one of them is empty.
(I really mean it - no length, even if you write it yourself! I will test you code using an infinite list as one of the arguments. Yes, those exist in Scheme. Look up "immutable cyclic data" in the manual if you're really really curious.)
sum-cash
that returns the
value in US dollars of a collection of amounts of different
currencies (same currencies as above). The amounts will be
given in a list L, such that each element of L is itself a
list, whose first element is an amount and whose second
element is a currency name. So, for example,
'((12.20 usd) (340 yen) (850 won))as an argument to
sum-cash
would mean adding
12.20 dollars, 340 yen and 850 won, and giving the total in
dollars. Here's an example:
> (sum-cash '((12.20 usd) (340 yen) (850 won))) 17.2687...
std-dev
that takes a list
of numbers and returns their standard deviation.
(Recall: std dev of x1, x2, ..., xn
is
__________________________________________ / /(x1 - u)^2 + (x2 - u)^2 + ... + (xn - u)^2 / ------------------------------------------ \/ n - 1... where u is the average of x1, x2, ..., xn. When you write this function, use top-down design. In scheme this means writing the
std-dev
function using functions you
wish were already defined, then going back and defining them
later. It can be useful to quote these function calls before
you implement them, so you can even do top-down testing!> (std-dev '(34 18 25 23 29 11 28 24 27 29)) 6.460134157533676
uniquefy
that takes a list of numbers,
and returns a list with all the distinct numbers in the original,
i.e., with all the duplicate numbers removed. For example:
> (uniquefy '(1 2 4 1 2 2 3 15)) (1 2 4 3 15)At the very least, you will probably want to also define at least one nice helper function. Try thinking about the efficiency of your function. Could you make it faster?
The let
construct in Scheme allows you to give a
name to a common subexpression. For example, consider the
expression
33 * (501 - 33) --------------------- 1 - (33 * (501 - 33))
The natural way to code this in Scheme is probably
(/ (- (* 33 501) 33) (- 1 (- (* 33 501) 33)))
But you could say "let a = 33 * (501 - 33), and return a / (1 - a)".
That's what let
allows you to do:
(let ((a (- (* 32 501) 33))) (/ a (- 1 a)))
Essentially, let
is the Scheme way of getting local
variable functionality. What you've got is the
let
keyword, followed by a list of
variable-name/value pairs, followed by an expression
(presumably using the new names) that provides the value of
the whole let
expression. For example, the following code
produces 12
as a result:
(let ((a 2) (b 4) (c 6)) (+ a b c))
There is power in using
let
in functions. Suppose I want to define a function called
shifted-cube
, which computes (x + 1)^3 for a
value x.
(define (shifted-cube x) (let ((a (+ x 1))) (* a a a)))
Then running (shifted-cube 2)
produces the value 27
.
test-sin
that computes
1/((sin x)^2 + 1) + sqrt((sin x)^2 + 1) - ((sin x)^2 + 1)^2
dist
that computes the
difference (in inches) between two lengths (given in feet
and inches). Example:
> (dist 3 7 2 11) ;;; difference between 3'7'' and 2'11''
8
Write this function using a let
expression
to create values L1
and L2
for
the lengths in inches of the input feet-and-inches lengths.
As you have perhaps noticed, you never actually tell scheme
what the type of a function parameter is. Thus, if I
define some function (f x y)
, there's nothing to stop me from
calling it like this: (f sqrt 4)
. In other words, there's
nothing to stop me from passing f a function as one of its
arguments. This is something very special about functional programming
languages: functions can be used like any other kind of data.
Take a moment to allow this to sink in.
Now check out this example. f
uses its first argument,
x
, as a function. So x
should be a function!
> (define (f x y) (* y (x y))) > (f sqrt 4) 8
What happened here? Well since
x
is sqrt
and y
is 4
,
the evaluation produces
(* 4 (sqrt 4))
which is 8. So f is the
"apply
function x to argument y and then multiply by y" function.
Passing functions
to functions like this is very powerful. This is part of what
we mean when we say that "functions are first class objects"
in a functional language.
(fd-at g n)
that takes a function g
and a value n
and returns the finite
difference of g at n. For example:
> (define (f x) (* x x)) > (fd-at f 3) 7
map
is a really useful function that takes functions as arguments.
The expression
(map f L)
applies the function f
to each element of the list L
and puts the results together in a new list. For example:
> (map abs '(-4 12 -3 -8 11 0)) (4 12 3 8 11 0)
If you have a function with k arguments, then you give map
k
lists, and it will take the first argument from list1, the
second from list2, etc.
> (map * '(2 3 4) '(6 5 4)) (12 15 16)
Another useful function of this type is apply
.
The expression (apply f L)
calls the function f
with arguments the elements of
L
. Here
are some examples:
> (apply max '(4 6 2)) ; same as (max 4 6 2) 6 > (apply - '(3 7)) ; same as (- 3 7) -4
Combining map
and apply
can be very interesting.
> (define (sqr x) (* x x)) > (map sqr '(1 2 3 4 5 6 7 8 9 10)) (1 4 9 16 25 36 49 64 81 100) > (apply + (map sqr '(1 2 3 4 5 6 7 8 9 10))) 385
Yet another useful function-that-takes-a-function is filter
.
The expression (filter f? L)
applies the predicate function
f?
to each element in the list L
, and returns the
list containing only the elements for which the predicate returns true.
For example:
> (filter number? '(a b 2 #t + 4)) (2 4)
For these, you will probably want to use this helper function. Just copy its definition into your code.
; Returns a list containing integers a, a+1, ..., b. (define (range a b) (if (> a b) null (cons a (range (+ a 1) b))))
sqrt-prod
that takes a number
n
and computes the product of the square roots of the
integers from 1 up to n. For example:
> (sqrt-prod 10) 1904.94...
special-nums
that takes an integer
n
and computes a list of all integers between 1 and n that
are both divisible by 3
and are perfect squares. For example:
> (special-nums 100) (9 36 81)Hint: you should make some helper functions that are predicates. The built-in function
sqr
might also be useful.
(define P1 '((3 5) (9 2) (11 6) (8 8) (4 6)))P1 represents a polygon whose vertex coordinates are given by the pairs in P1. Write a function
(translate p d)
that takes
a polygon (in the sense of P1, i.e., a list of coordinate pairs)
and an offset d
(represented by a pair, the x offset and the y offset) and
translates the given polygon by the given offset.
> (define P1 '((3 5) (9 2) (11 6) (8 8) (4 6))) > (translate P1 '(1 2)) ((4 7) (10 4) (12 8) (9 10) (5 8)) >Depending on your approach, the following function might be useful to you:
;Creates a list of k copies of x (define (repeat x k) (if (= k 0) '() (cons x (repeat x (- k 1)))))For full credit, your function should also work in higher dimensions. By this I mean, if each point has x, y, and z coordinates, for example, everything should still work, by providing a list of 3 offsets to the
translate
function.