CSC 533: Programming Languages
Spring 2015
HW6: Scheme Programming II
For this assignment, you are to define numerous Scheme functions. For
simplicity, place all of your function definitions in a single file named
hw6.ss. Be sure to include a comment block at the top of the file containing you rname and also comment each function as to its behavior.
Part 1: TailRecursive Simulations
 The
randomwalk
function below simulates a random 1dimensional walk. Initially,
the
walker is assumed to be at position 0. Depending on the flip of a coin, the
walker
either moves to the right (in the positive direction) or to the left (in the
negative direction). The simulation ends when the walker reaches the
specified goal
distance, in
either direction.
(define (coinflip)
(if (< (random) 0.5)
'heads
'tails))
(define (randomwalk goaldist)
(define (randomwalkhelp position)
(begin (display "position: ") (display position) (newline)
(cond
((= (abs position) goaldist) #t)
((equal? (coinflip) 'heads) (randomwalkhelp (add1 position)))
(else (randomwalkhelp (sub1 position))))))
(randomwalkhelp 0))
As is, this function displays each step in the walk and returns #t
when the walk ends. Modify the function so that it keeps track of the number of
steps in the walk and returns this value instead. Your modification should only
utilize tailrecursion.
 An interesting unsolved problem in mathematics concerns what is
called the hailstone sequence of integers. This sequence is
defined as follows: start with any integer. If that number is odd, then
multiply it by 3 and add 1. If it is even, then divide it by 2. Now,
repeat. For example, if we start with the number 5, we get the following
sequence: 5, 16, 8, 4, 2, 1, 4, 2, 1, 4, 2, 1, . . .
Here, the subsequence 4, 2, 1 is reached which produces a loop. It was
conjectured (by mathematician Lothar Collatz) that no matter what number you start with, you will always
end up stuck in the 4, 2, 1 loop. It has, in fact, been shown to hold
for all starting values less than 5 x 2^{60} ≈ 5.764 x 10^{18}. However, it still has not
been proven for all numbers. Define a function named hailstone
which has one input, the starting number, and which displays the hailstone
sequence
starting with that number and ending with 1. The function should return
the
length of the sequence. For example,
(hailstone 1) should display the lone number 1 and evaluate to 1,
while
(hailstone 5) should display 5 16 8 4 2 1 and evaluate
to 6.
Your function should only utilize tailrecursion
(perhaps a help function will be required).
 Write a function named
head%
that simulates a specified number of coin flips and returns the percentage of those flips that were heads. For example, the call (head% 1000)
would simulate 1,000 flips and return the percentage of heads, e.g., 50.8. Since the desired number of flips could be extremely large, your function should only utilize tailrecursion.
Part 2: Association Lists
In biology, the four bases that make up messenger RNA (mRNA) are adenine (A),
cytosine (C), guanine (G), and uracil (U). Within a cell, an mRNA sequence
is processed by a ribosome and translated into amino acids. Each grouping of 3
bases is known as a codon, and is translated to a particular amino acid.
For example, the codon GCU translates to Alanine.
The following Scheme association list
maps each codon (represented by a triplet of
base symbols) to the abbreviation for the corresponding amino acid.
(define ACIDS
'( ((U U U) F) ((U U C) F) ((U U A) L) ((U U G) L)
((U C U) S) ((U C C) S) ((U C A) S) ((U C G) S)
((U A U) Y) ((U A C) Y) ((U A A) *) ((U A G) *)
((U G U) C) ((U G C) C) ((U G A) *) ((U G G) W)
((C U U) L) ((C U C) L) ((C U A) L) ((C U G) L)
((C C U) P) ((C C C) P) ((C C A) P) ((C C G) P)
((C A U) H) ((C A C) H) ((C A A) Q) ((C A G) Q)
((C G U) R) ((C G C) R) ((C G A) R) ((C G G) R)
((A U U) I) ((A U C) I) ((A U A) I) ((A U G) M)
((A C U) T) ((A C C) T) ((A C A) T) ((A C G) T)
((A A U) N) ((A A C) N) ((A A A) K) ((A A G) K)
((A G U) S) ((A G C) S) ((A G A) R) ((A G G) R)
((G U U) V) ((G U C) V) ((G U A) V) ((G U G) V)
((G C U) A) ((G C C) A) ((G C A) A) ((G C G) A)
((G A U) N) ((G A C) N) ((G A A) E) ((G A G) E)
((G G U) G) ((G G C) G) ((G G A) G) ((G G G) G) ))
Copyandpaste this association list into your file and add the following definitions.
 Define a function named translate that takes one input, a list
representing a codon, and returns the abbreviation for the corresponding
amino acid. For example, (translate '(G G A)) should evaluate to G.
 Define a function named translatesequence that takes one input, an
arbitrarly long sequence of bases, and returns the corresponding list of amino acid
abbreviations. That is, the first symbol in the returned list should be the amino acid
corresponding to the first three bases, the second symbol should be the amino
acid corresponding to the next three bases, etc. If the length of the sequence
is not a multiple of three, any trailing bases should be ignored. For example,
(translatesequence '(G G A U U C A C C C)) should evaluate to (G F T).
 When scientists extract mRNA samples from a cell, they invariably obtain
partial sequences. Since the start of the sequence may be unknown, it is unclear
exactly how the codons will be grouped in an actual translation. For example, in the sequence
GGAUUCACCC the codons could be grouped starting at the beginning
(yielding codons GGA UUC ACC), at the second base (yielding codons GAU UCA CCC),
or at the third base (yielding codons AUU CAC). In addition, it is possible that the
sequence has been read backwards, so the actual sequence to be translated should be
CCCACUUAGG.
Define a function named translateallpossible that takes one input, an
arbitrarly long sequence of bases, and returns a list containing all six possible
translations of that sequence. Any extra bases at the beginning or end of the sequence
should be ignored in the translation. For example,
(translateallpossible '(G G A U U C A C C C)) should evaluate to
((G F T) (N S P) (I H) (P T *) (P L R) (H L)).
Part 3: Binary Trees
In class, we discussed how structured lists could be used to represent
binary trees. For example, the following binary tree is represented by the
list to the right:

(define ANIMALS
'(dog
(bird (horse () ()) (cat () ()))
(possum (dog () ()) ())))

Several utility functions (emptytree?, root, leftsubtree,
and rightsubtree) were provided for you in the lecture notes.
Copyandpaste these functions into your file and add the following functions for manipulating binary trees.
 Define a function named height that takes one input, a list
representing a binary tree, and returns the height of that tree. For example, given the ANIMALS
structure above, (height ANIMALS) should evaluate to 3.
 Define a function named contains? that takes two inputs, a list
representing a binary tree and an atom, and returns #t if the list
contains the atom (otherwise #f). For example, given the ANIMALS
structure above, (contains? ANIMALS 'emu) should evaluate to #f
while (contains? ANIMALS 'cat) should evaluate to #t.
 Define a function named numleaves that takes one input, a list
representing a binary tree, and returns the number of leaf nodes in that tree.
For example, given the ANIMALS structure above, (numleaves ANIMALS) should
evaluate to 3.