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Monash University
Faculty of Information Technology
nd Semester 2022
Assignment 2
Regular Languages, Context-Free Languages, Lexical analysis, Parsing,
Turing machines and Quaternions
DUE: 11:55pm, Friday 7 October 2022
In these exercises, you will
• implement a lexical analyser using lex (Problems 2, 4);
• implement parsers using lex and yacc (Problems 1, 3–6);
• program a Turing machine (Problem 7);
• learn about quaternions, by applying our methods to calculations with them (Problems 3–6);
• practise your skills relating to context-free languages (Problem 8).
Solutions to Problem 7 must be implemented in the simulator Tuatara. We are providing version
2.1 on Moodle under week 8; the file name is tuatara-monash-2.1.jar. You must use exactly
this version, not some other version downloaded from the internet. Do not unpack this
file. It must be run directly using the Java runtime.
How to manage this assignment
• You should start working on this assignment now and spread the work over the time until it
is due. Do as much as possible before week 10. There will not be time during your prac class
to do the assignment from scratch; there will only be time to get some help and clarification.
• Don’t be deterred by the length of this document! Much of it is an extended tutorial to get
you started with lex and yacc (pp. 7–11) and documentation for functions, written in C, that
are provided for you to use (pp. 12–14); some sample outputs also take up a fair bit of space.
Although lex and yacc are new to you, the questions about them only require you to modify
some existing input files for them rather than write your own input files from scratch.
• The tasks required for the assignment are on pp. 3–6.
For Problem 1–5, read the background material on pp. 7–11.
For Problems 2–6, also read the background material on pp. 12–14.
For Problems 7–8, read the background material on p. 15.
Instructions are as for Assignment 1, except that some of the filenames have changed. To begin
working on the assignment, download the workbench from Moodle. Create a new Ed
Workspace and upload this file, letting Ed automatically extract it. Edit the student-id file to
contain your name and student ID. Refer to Lab 0 for a reminder on how to do these tasks.
Open a terminal and change into the directory asgn2. You will find three files already in the
directory: plus-times-power.l, plus-times-power.y, and quat.h. You will not modify these files
directly; you will make copies of the first two and modify the copies, while quat.h must remain
unaltered in the directory where you do this work.
You need to construct new lex files, using plus-times-power.l as a starting point, for Problems
1, 4 & 5, and you’ll need to construct a new yacc file from plus-times-power.y for Problem 5.
Your submission must include:
• a lex file prob1.l which should be obtained by modifying a copy of plus-times-power.l
• a text file prob2.txt which should contain a single line with a regular expression in lex syntax
• a PDF file prob3.pdf which should contain your solution to Problem 3
• a lex file prob4.l which should also be obtained by modifying a copy of plus-times-power.l
• a lex file prob5.l which should be obtained by modifying a copy of prob4.l
• a yacc file prob5.y which should be obtained by modifying a copy of plus-times-power.y
• a text file prob6.txt which should contain two lines, being your solution to Problem 6
• a Tuatara Turing machine file
• a PDF file prob8.pdf which should contain your solution to Problem 8.
Each of the problem directories under the asgn2 directory contains empty files with the required
filenames. These must each be replaced by the files you write, as described above. Before submission,
check that each of these empty files is, indeed, replaced by your own file.
To submit your work, download the Ed workspace as a zip file by clicking on “Download All” in
the file manager panel. The “Download All” option preserves the directory structure of the zip file,
which is required to aid the marking process. You must submit this zip file to Moodle by the
deadline given above.
Assignment tasks
For each problem, the files you are submitting must be in the corresponding subdirectory, i.e.
problemx for Problem x.
First, read about “Lex, Yacc and the PLUS-TIMES-POWER language” on pp. 7–11.
Problem 1. [2 marks]
Construct prob1.l, as described on pp. 9–11, so that it can be used with plus-times-power.y
to build a parser for PLUS-TIMES-POWER.
Now refer to the document “Quaternions and the language QUAT”, pages 12–14.
Problem 2. [2 marks]
Write a regular expression, using the regular expression syntax used by lex, that matches any
finite decimal representation (of the type specified on p. 12) of a nonnegative real number. Save
it as a file prob2.txt.txt
Problem 3. [7 marks]
Write a Context-Free Grammar for the language QUAT over the fifteen-symbol alphabet
{i, j, k, +, -, *, /, ^, |, (, ), NUMBER,WHOLENUMBER, ROTATION, , }. It can be typed or
hand-written, but must be in PDF format and saved as a file prob3.pdf.pdf.
Now we use regular expressions (in the lex file, prob4.l) and a grammar (in the yacc file,
prob5.y) to construct a lexical analyser (Problem 4) and a parser (Problem 5) for QUAT.
Problem 4. [6 marks]
Using the file provided for PLUS-TIMES-POWER as a starting point, construct a lex file,
prob4.l, and use it to build a lexical analyser for QUAT.
You’ll need to change the regular expressions associated with the NUMBER, WHOLENUMBER and some other tokens, among other things.
Sample output:
$ ./a.out
Rotation(120.0,2.0i+2.0j+2.0k)^2 * i / Rotation(120.0,2.0i+2.0j+2.0k)^2
Token: ROTATION; Lexeme: Rotation
Token and Lexeme: (
Token: NUMBER; Lexeme: 120.0
Token and Lexeme: ,
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: i
Token and Lexeme: +
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: j
Token and Lexeme: +
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: k
Token and Lexeme: )
Token and Lexeme: ^
Token: WHOLENUMBER; Lexeme: 2
Token and Lexeme: *
Token and Lexeme: i
Token and Lexeme: /
Token: ROTATION; Lexeme: Rotation
Token and Lexeme: (
Token: NUMBER; Lexeme: 120.0
Token and Lexeme: ,
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: i
Token and Lexeme: +
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: j
Token and Lexeme: +
Token: NUMBER; Lexeme: 2.0
Token and Lexeme: k
Token and Lexeme: )
Token and Lexeme: ^
Token: WHOLENUMBER; Lexeme: 2
Token and Lexeme:
Problem 5. [11 marks]
Make a copy of prob4.l, call it prob5.l, then modify it so that it can be used with yacc.
Then construct a yacc file prob5.y from plus-times-power.y. Then use these lex and yacc
files to build a parser for QUAT.
Note that you do not have to program any of the quaternion functions yourself. They have
already been written: see the Programs section of the yacc file. The actions in your yacc file
will need to call these functions, and you can do that by using the function call for pow(. . . )
in plus-times-power.y as a template.
The core of your task is to write the grammar rules in the Rules section, in yacc format,
with associated actions, using the examples in plus-times-power.y as a guide. You also need
to do some modifications in the Declarations section; see page 10 and further details below.
When entering your grammar into the Rules section of prob5.y, it is best to leave the existing
rules for the nonterminal start unchanged, as this has some extra stuff designed to allow you to
enter a series of separate expressions on separate lines. So, take the Start symbol from your grammar
in Problem 2 and represent it by the nonterminal line instead of by start.
The specific modifications you need to do in the Declarations section should be:
• You need a new %token declaration for the ROTATION token. It has the same structure as the
line for the NUMBER token, except that “num” is replaced by “str” (since ROTATION represents
a string, being a name for a function, whereas NUMBER represents a number).
• For symbols that represent a binary (i.e., two-argument) arithmetic operation, it is worth
including them in an appropriate %left statement. Each of these statements makes the parser
treat these operations as left-associative, which helps it determine the order in which to do the
operations and removes some sources of possible ambiguity. When using %left, operations
having the same precedence are listed on the same line with spaces between them. So for +
and - you can use the following statement:
%left ’-’ ’+’
A similar line can be used for multiplication and division. For operations whose %left statements are on different lines, the operations with higher precedence are those with higher line
numbers (i.e., later in the file). Right-associative operations can be handled similarly with a
%right statement. Treat exponentiation as having higher precedence than multiplication and
• For every nonterminal symbol, you need a %type line that declares its type, i.e., the type of
value that is returned when an expression generated from this nonterminal is evaluated. For
%type start here
Here, “qtn” is the type name we are using for quaternions. The various type names can be
seen in the %union statement a little earlier in the file. But you do not need to know how that
works in order to do this task.
• You should still use start as your Start symbol. If you use another name instead, you will
need to modify the %start line too.
Sample output:
$ ./a.out
Rotation(120,2i+2j+2k) * i / Rotation(120,2i+2j+2k)
0.000000 + 0.000000 i + 1.000000 j + 0.000000 k
Now refer to the explanation of quaternions and 3D rotations, page 14.
Problem 6. [6 marks]
Convert your eight-digit student ID number into an angle and direction as follows. Let
be the digits of your student ID number. Divide this into six single-digit numbers followed
by one two-digit number: d1, d2, d3, d4, d5, d6, d7d8. The point to be rotated
is (d1, d2, d3), which can be represented by the pure quaternion d1i + d2j + d3k. The axis of
rotation is the line whose direction is given by d4i + d5j + d6k. (If this is the zero vector, then
use (d1 +d4)i+d5j +d6k instead.) Then work out the sum of the first digit d1 and the two-digit
number d7d8, and use it for your angle of rotation, θ

For example, if your ID number is 12345678, then your point to be rotated is (1, 2, 3), your
axis of rotation is 4i + 5j + 6k, and your angle is 78 + 1 = 79◦
(a) Write down the quaternion expression in QUAT that represents the calculation required
to rotate the pointR (d1, d2, d3) by θ degrees clockwise around the axis whose direction is given
by d4i + d5j + d6k.
(Your expression must use the actual numbers derived from your student ID number as
specified, not the algebraic quantities used above.)
Append the string “Hamilton”, out of respect for the person who invented quaternions.
(b) Run your parser on your expression from (a), and report the result of evaluating it.
The answers to (a) and (b) should be copied into a single line each in the file prob6.txt.
Turing machines
Now refer to the description of walks on page 15. Let CW be the language of closed walks using
alphabet {N, S, E, W}.
Problem 7. [8 marks]
Build, in Tuatara, a decider for CW and save it as a file
There is no restriction on the contents of the output tape at the end of the computation.
Context-Free Languages
Problem 8. [8 marks]
Prove or disprove: The language CW is context-free.
Please mention the Cocke-Younger-Kasami algorithm (but there is no need to demonstrate it).
Your submission can be typed or hand-written, but it must be in PDF format and saved as a
file prob8.pdf.
Lex, Yacc and the PLUS-TIMES-POWER language
In this part of the Assignment, you will use the lexical analyser generator lex, initially by itself,
and then with the parser generator yacc1
Some useful references on Lex and Yacc:
• T. Niemann, Lex & Yacc Tutorial,
• Doug Brown, John Levine, and Tony Mason, lex and yacc (2nd edn.), O’Reilly, 2012.
• the lex and yacc manpages
We will illustrate the use of these programs with a language PLUS-TIMES-POWER based on
simple arithmetic expressions involving nonnegative integers, using just addition, multiplication and
powers. Then you will use lex and yacc on a language QUAT of expressions based on quaternions,
which we describe later.
The language PLUS-TIMES-POWER consists of expressions involving addition, multiplication and
powers of nonnegative integers, without any parentheses (except for those required by the function
Power). Example expressions include:
5 + 8, 8 + 5, 3 + 5 ∗ 2, 13 + 8 ∗ 4 + Power(2,Power(3, 2)), Power(1, 3) + Power(5, 3) + Power(3, 3),
Power(999, 0), 0 + 99 ∗ 0 + 1, 2014, 10 ∗ 14 + 74 + 10 ∗ 13 ∗ 73, 2 ∗ 3 ∗ 5 ∗ 7 ∗ 11 ∗ 13 ∗ 17 ∗ 19.
In these expressions, integers are written in unsigned decimal, with no leading zeros or decimal point
(so 2014, 86, 10, 7, and 0 are ok, but +2014, −2014, 86.0, A, 007, and 00 are not).
For lexical analysis, we treat every nonnegative integer as a lexeme for the token NUMBER.
An input file to lex is, by convention, given a name ending in .l. Such a file has three parts:
• definitions,
• rules,
• C code.
These are separated by double-percent, %%. Comments begin with /* and end with */. Any
comments are ignored when lex is run on the file.
You will find an input file, plus-times-power.l, among the files for this Assignment. Study
its structure now, identifying the three sections and noticing that various pieces of code have been
commented out. Those pieces of code are not needed yet, but some will be needed later.
We focus mainly on the Rules section, in the middle of the file. It consists of a series of statements
of the form
pattern { action }
where the pattern is a regular expression and the action consists of instructions, written in C,
specifying what to do with text that matches the pattern.
In our file, each pattern represents a set
of possible lexemes which we wish to identify. These are:
1actually, Linux includes more modern implementations of these programs called flex and bison.
2This may seem reminiscent of awk, but note that: the pattern is not delimited by slashes, /. . . /, as in awk; the
action code is in C, whereas in awk the actions are specified in awk’s own language, which has similarities with C but
is not the same; and the action pertains only to the text that matches the pattern, whereas in awk the action pertains
to the entire line in which the matching text is found.
• a decimal representation of a nonnegative integer, represented as described above;
– This is taken to be an instance of the token NUMBER (i.e., a lexeme for that token).
• the specific string Power, which is taken to be an instance of the token POWER.
• certain specific characters: +, *, (, ), and comma;
• the newline character;
• white space, being any sequence of spaces and tabs.
Note that all matching in lex is case-sensitive.
Our action is, in most cases, to print a message saying what token and lexeme have been found.
For white space, we take no action at all. A character that cannot be matched by any pattern yields
an error message.
If you run lex on the file plus-times-power.l, then lex generates the C program lex.yy.c.
This is the source code for the lexical analyser. You compile it using a C compiler such as cc.
For this assignment we use flex, a more modern variant of lex. We generate the lexical analyser
as follows.
$ flex plus-times-power.l
$ cc lex.yy.c
By default, cc puts the executable program in a file usually called a.out4 but sometimes called
a.exe. This can be executed in the usual way, by just entering ./a.out at the command line. If
you prefer to give the executable program another name, such as plus-times-power-lex, then you
can tell this to the compiler using the -o option: cc lex.yy.c -o plus-times-power-lex.
When you run the program, it will initially wait for you to input a line of text to analyse. Do
so, pressing Return at the end of the line. Then the lexical analyser will print, to standard output,
messages showing how it has analysed your input. The printing of these messages is done by the
printf statements from the file plus-times-power.l. Note how it skips over white space, and only
reports on the lexemes and tokens.
$ ./a.out
13+8 * 4 + Power(2,Power (3,2 ))
Token: NUMBER; Lexeme: 13
Token and Lexeme: +
Token: NUMBER; Lexeme: 8
Token and Lexeme: *
Token: NUMBER; Lexeme: 4
Token and Lexeme: +
Token: POWER; Lexeme: Power
Token and Lexeme: (
Token: NUMBER; Lexeme: 2
Token and Lexeme: ,
Token: POWER; Lexeme: Power
Token and Lexeme: (
Token: NUMBER; Lexeme: 3
Token and Lexeme: ,
Token: NUMBER; Lexeme: 2
Token and Lexeme: )
Token and Lexeme: )
Token and Lexeme:
3The C program will have this same name, lex.yy.c, regardless of the name you gave to the lex input file.
4a.out is short for assembler output.
Try running this program with some input expressions of your own. You can keep entering new
expressions on new lines, and enter Control-D to stop when you are finished.
We now turn to parsing, using yacc.
Consider the following grammar for PLUS-TIMES-POWER.
S −→ E
E −→ I
E −→ POWER(E, E)
E −→ E ∗ E
E −→ E + E
In this grammar, the non-terminals are S, E and I. Treat NUMBER and POWER as just single
tokens, and hence single terminal symbols in this grammar.
We now generate a parser for this grammar, which will also evaluate the expressions, with +, ∗
interpreted as the usual integer arithmetic operations and Power(. . . ,. . . ) interpreted as raising its
first argument to the power of its second argument.
To generate this parser, you need two files, prob1.l (for lex) and plus-times-power.y (for
• Change into your problem1 subdirectory and do the following steps in that directory.
• Copy plus-times-power.l to a new file prob1.l, and then modify prob1.l as follows:
– in the Declarations section, uncomment the statement #include "";
– in the Rules section, in each action:
∗ uncomment the statements of the form
· yylval.str = ...;
· yylval.num = ...;
· return TOKENNAME;
· return *yytext;
· yyerror ...
∗ Comment out the printf statements. These may still be handy if debugging is
needed, so don’t delete them altogether, but the lexical analyser’s main role now is
to report the tokens and lexemes to the parser, not to the user.
– in the C code section, comment out the function main(), which in this case occupies
four lines at the end of the file.
• plus-times-power.y, the input file for yacc, is provided for you. You don’t need to modify
this yet.
An input file for yacc is, by convention, given a name ending in .y, and has three parts, very loosely
analogous to the three parts of a lex file but very different in their details and functionality:
• Declarations,
• Rules,
• Programs.
These are separated by double-percent, %%. Comments begin with /* and end with */.
Peruse the provided file plus-times-power.y, identify its main components, and pay particular
attention to the following, since you will need to modify some of them later.
• in the Declarations section:
– lines like
int printQuaternion(Quaternion);
Quaternion newQuaternion(double, double, double, double);
Quaternion rotation(double, Quaternion);
which are declarations of functions (but they are defined later, in the Programs section);5
– declarations of the tokens to be used:
%token NUMBER
%token POWER
– some specifications that certain operations are left-associative (which helps determine the
order in which operations are applied and can help resolve conflicts and ambiguities):
%left ’+’
%left ’*’
– declarations of the nonterminal symbols to be used (which don’t need to start with an
upper-case letter):
%type start
%type line
%type expr
%type int
– nomination of which nonterminal is the Start symbol:
%start start
• in the Rules section, a list of grammar rules in Backus-Naur Form (BNF), except that the
colon “:” is used instead of →, and there must be a semicolon at the end of each rule. Rules
with a common left-hand-side may be written in the usual compact form, by listing their
right-hand-sides separated by vertical bars, and one semicolon at the very end. The terminals
may be token names, in which case they must be declared in the Declarations section and also
used in the lex file, or single characters enclosed in forward-quote symbols. Each rule has
an action, enclosed in braces {. . . }. A rule for a Start symbol may print output, but most
other rules will have an action of the form $$ = . . . . The special variable $$ represents the
value to be returned for that rule, and in effect specifies how that rule is to be interpreted for
evaluating the expression. The variables $1, $2, . . . refer to the values of the first, second, . . .
symbols in the right-hand side of the rule.
• in the Programs section, various functions, written in C, that your parsers will be able to use.
You do not need to modify these functions, and indeed should not try to do so unless you are
an experienced C programmer and know exactly what you are doing! Most of these functions
are not used yet; some will only be used later, in Problem 4.
After constructing the new lex file prob1.l as above, the parser can be generated by:
$ yacc -d plus-times-power.y
$ flex prob1.l
5These functions for computing with quaternions are not needed by plus-times-power.y, but you will need them
later, when you make a modified version of plus-times-power.y to parse expressions involving quaternions.
$ cc lex.yy.c -lm
The executable program, which is now a parser for PLUS-TIMES-POWER, is again named a.out
by default, and will replace any other program of that name that happened to be sitting in the same
$ ./a.out
13+8 * 4 + Power(2,Power (3,2 ))
Run it with some input expressions of your own. You can keep entering new expressions on new
lines, as above, and enter Control-D to stop when you are finished.
Quaternions and the language QUAT
The quaternions are a system of four-dimensional numbers that are used in computer graphics to
describe rotations in three-dimensional space, beginning with Tomb Raider in 1996 [1]. They were
discovered by William Rowan Hamilton in Dublin in 1843.
Every quaternion has the form
w + xi + yj + zk,
where w, x, y, z ∈ R and i, j, k are special quantities, called quaternion units, satisfying
2 = −1,
2 = −1,
2 = −1,
ijk = −1.
Other properties that follow from these equations include:
ij = k, ji = −k,
jk = i, kj = −i,
ki = j, ik = −j.
These properties can be used to compute any sum or product of quaternions, since the usual associative and distributive laws still apply. Note, though, that multiplication of quaternions is not
commutative: the order of multiplication affects the outcome, in general.
You may have already met the complex numbers. These have the form x+yi, where x, y ∈ R and
2 = −1, and may be used to describe rotations in two-dimensional space. It is an intriguing mathematical fact that, in order to extend complex numbers to describe rotations in three-dimensional
space, a four -dimensional system is needed.
The set of quaternions is denoted by H, just as the sets of real and complex numbers are denoted
by R and C respectively.
In this assignment, you will construct a quaternionic calculator for parsing and evaluating
arithmetic expressions involving quaternions.
Quaternions: representation
We assume that, when representing the quaternion w + xi + yj + zk, the numbers w, x, y, z are
represented either as integers (without a decimal point) or as numbers with a decimal point and
at least one digit before the decimal point. It’s ok to have a decimal point with no digits after
it. Trailing zeros are always allowed, but leading zeros are only allowed for the integer 0 or if 0 is
the only digit before the decimal point. So the number −3/4 may be represented as any of −0.75,
−0.750, −0.7500, etc, and all these possibilities must be accepted as valid representations of the
same real number. But it must not be represented as −.75 or −.750, etc. The numbers 42, 42., 42.0,
42.00 are all valid, but 042, 0042, 042.0, etc are invalid.
Quaternion operations
We allow all the four arithmetic operations on quaternions, denoted by the usual symbols, +, -, *,
/ — and grouping by parentheses, (. . . ). We also allow:
• exponentiation, denoted by ^, with an integer exponent. So, e.g., (1-2i+3j-4k)^3 represents
(1−2i+ 3j −4k)
, the cube of 1−2i+ 3j −4k, while (1-2i+3j-4k)^-3 is (1−2i+ 3j −4k)
• absolute value, denoted by | · · · | (just as for real and complex numbers). We treat |q| as giving
a quaternion with zero imaginary part. The real part of |q| is nonnegative and gives the length
of the quaternion q.
• the special operation Rotation(. . . ,. . . ), written as a function, which creates a quaternion
that represents a 3D rotation. The Rotation function takes two arguments, the first being a
number, and the second being any quaternion expression.
The functions to do these operations have all been written for you and provided in the file
plus-times-power.y. You only need to modify a copy of that file, using the guidance below, to
build your calculator.
Quaternion expressions
These operations can be combined in the same way in which they are combined when used for
“normal” numbers (real, complex, etc.) to make expressions. Any valid expression can be given as
the second argument of Rotation(. . . ,. . . ), to give another valid expression, and expressions using
Rotation(. . . ,. . . ) can be combined using arithmetic operations.
We formalise the concept of a quaternion expression with the following inductive definition:
1. Each of i, j, k is a quaternion expression.
2. If r is any nonnegative integer or any nonnegative real number, then each of r, ri, rj, rk is a
quaternion expression.
3. If p and q are quaternion expressions, then so are: (q), |q|, −q, p+q, p−q, p∗q, p/q.
4. If q is a quaternion expression and n is a nonnegative integer, then each of the following is a
quaternion expression: qˆn, qˆ-n.
5. If r is a nonnegative integer or a nonnegative real number and q is a quaternion expression,
then the following is a quaternion expression: Rotation(r, q)
• Negative numbers can be represented by negating positive numbers.
• We allow juxtaposition of any number r with any of i, j, k to form the simple expressions ri,
rj and rk. This enables the succinct quaternion notation ±w ± xi ± yj ± zk. However, apart
from that, multiplication in our quaternion expressions is always denoted by a star, ∗.
The language QUAT
Let QUAT be the language of valid quaternion expressions in which all numbers are finite decimal
representations. Here are some examples of valid quaternion expressions (i.e., members of QUAT):
expression evaluates to
(0.5 − 1.618j − 32i ∗ j − k/(3.5 + i ∗ j))/(i/j − k ∗ 48) 0.658452 + −0.033020i + 0.000000j + 0.008664k
3i ∗ j ∗ k −3
−1 + 2i − 0.30 ∗ j + 4. ∗ k −1 + 2i − 0.3j + 4.0k
0 0
(−0.5 + 0.866j)ˆ−3 1.000066 + −0.000038j
|j − 1.732k| 2
Rotation(120, 2i + 2j + 2k) ∗ i/Rotation(120, 2i + 2j + 2k) j
Some examples of invalid quaternion expressions (i.e., not members of QUAT):
expression comment
3i ∗ j k The product of j and k should use ∗ instead of juxtaposition.
−1 + 2i − .3 ∗ j + 4.0 ∗ k Need at least one digit before decimal point.
Power(−0.5 + 0.866j, 3) Power is not valid in QUAT; we only use it in PLUS-TIMES-POWER.
We treat QUAT as a language over the fifteen-symbol alphabet {i, j, k, +, -, *, /, ^, |, (, ),
6 Here,
• NUMBER is a token representing any finite decimal representation of a nonnegative real number,
• WHOLENUMBER is a token representing any nonnegative integer,
• ROTATION is a token representing the name of the Rotation function.
Quaternions and 3D rotations
We can use quaternions to rotate a point around an axis in 3D space. Throughout, we assume the
axis goes through the origin.
Points in 3D space are represented as pure quaternions, which means quaternions of the
form xi + yj + zk, so they have no real part w. This is just another way of representing threedimensional space using standard co-ordinate axes. In effect, the three basic quaternion units i, j, k
are unit vectors along the x, y, z-axes, respectively, and correspond to points (1, 0, 0),(0, 1, 0),(0, 0, 1)
A rotation is specified by giving its axis as a unit vector, i.e., a pure quaternion of length 1, and
its angle as a number. If the unit-length pure quaternion ˆq gives the direction of the axis of rotation,
and θ is the angle of rotation around that axis (clockwise, as viewed from the origin looking in the
direction in which q points), then the quaternion Rotation(θ, qˆ) that represents the rotation is given
Rotation(θ, qˆ) = cos(θ/2) + sin(θ/2) · q. ˆ
More generally, if q is any nonzero quaternion, then Rotation(θ, q) scales q to give the unit-length
quaternion ˆq that points in the same direction, and gives
Rotation(θ, q) = cos(θ/2) + sin(θ/2) · q. ˆ
It is this quaternion that is returned by the function rotation (provided in plus-times-power.y)
and by the Rotation operation in quaternion expressions.
In order to apply the rotation to a point p, we first represent p as a pure quaternion, p =
xi + yj + zk, and then form the expression
Rotation(θ, q) ∗ p/Rotation(θ, q).
So, our earlier calculation that Rotation(120,2i+2j+2k) * i / Rotation(120,2i+2j+2k) evaluates to j expresses the fact that, if your axis is the line with direction i + j + k, then rotating the
point (1, 0, 0) by 120◦
clockwise around this axis gives the point (0, 1, 0).
In this assignment, we restrict to angles θ ≥ 0. We lose no generality by doing this, since any
rotation is equivalent to rotation about the same axis by some angle θ in the range 0◦ ≤ θ < 360◦
[1] Nick Bobic, Rotating objects using quaternions, Game Developer, Feb. 1998. Available at:
[2] Daniel Chan, Quaternions are turning tomb raiders on their heads, Parabola 40 (no. 2) (2004).
Available at:
2004/issue-2/vol40_no2_2.pdf or
6The last symbol listed in this set is a comma.
7BUT the multiplication we use for quaternions is not the same as the dot product or cross product of vectors,
although it is closely related to both. In fact, historically, quaternions gave rise to these products of vectors.
Imagine you are standing at the origin on an infinite x, y-plane. A walk is a seque