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6.01 HW1: Calculator — Fall 2011 1
Homework 1: Calculator
1 Mechanics and due dates
This assignment consists of five parts, each of which can be entered into a problem on the tutor.
The first two parts are due one week from the start of the assignment; the other parts are due a
week after that. See the tutor for the due dates. Your code will be tested for correctness by the
tutor, but also graded for organization and style by a human. We will deduct points for repeated
and/or excessively complex code. You should include some comments that explain the key points
in your code. The last problem asks you to enter a text answer to a ’check yourself’ question; it
will be graded by a human.
Do your work in the file 6.01/hw1/hw1Work.py
You can discuss this problem, at a high level, with other students, but the programs and text you
submit must be your own work.
2 Symbolic Calculator
We will construct a simple symbolic calculator that reads, evaluates, and prints arithmetic expressions
that contain variables as well as numeric values. It is similar, in structure and operation, to
the Python interpreter.
To make the parsing simple, we assume that the expressions are fully parenthesized: so, instead of a
= 3 + 4, we will need to write (a = (3 + 4)). In other words, in any expression which involves
subexpressions that are not simple elements, those subexpressions will recursively be enclosed
within parentheses. Thus any complex expression contains an expression, an operator and another
expression, and each of these expressions, if not a number or a variable, is itself contained within
parentheses.
The following is a transcript of an interaction with our calculator, where the % character is the
prompt. After the prompt, the user types in an expression, the calculator evaluates the expression,
possibly changing the environment, and prints out both the value of the expression and the new
environment.
>>> calc()
% (a = 3)
None
env = {’a’: 3.0}
% (b = (a + 2))
None
env = {’a’: 3.0, ’b’: 5.0}
% b
5.0
env = {’a’: 3.0, ’b’: 5.0}
% (c = (a + (b * b)))
None
env = {’a’: 3.0, ’c’: 28.0, ’b’: 5.0}
3 Syntax Trees
The calculator operates in two phases. It
.
6.01 HW1: Calculator — Fall 2011 2
• Parses the input string of characters to generate a syntax tree; and then
• Evaluates the syntax tree to generate a value, if possible, and does any required assignments.
A syntax tree is a data structure that represents the structure of the expression. The nodes at the
bottom are called leaf nodes and represent actual primitive components (numbers and variables)
in the expression. Other nodes are called internal nodes. They represent an operation (such as
addition or subtraction), and contain instances of child nodes that represent the arguments of the
operation. The following tree represents the expression (1 + ((2 * 3) + (a / b))). Note the
use of parentheses to separate each subexpression:
We can represent syntax trees in Python using instances of the following collection of classes.
These definitions are incomplete: it will be your job to fill them in.
class BinaryOp:
def __init__(self, left, right):
self.left = left
self.right = right
def __str__(self):
return self.opStr + ’(’ + \
str(self.left) + ’, ’ +\
str(self.right) + ’)’
__repr__ = __str__
class Sum(BinaryOp):
opStr = ’Sum’
class Prod(BinaryOp):
opStr = ’Prod’
class Quot(BinaryOp):
opStr = ’Quot’
class Diff(BinaryOp):
opStr = ’Diff’
class Assign(BinaryOp):
opStr = ’Assign’
class Number:
def __init__(self, val):
self.value = val
def __str__(self):
return ’Num(’+str(self.value)+’)’
class Variable:
def __init__(self, name):
self.name = name
def __str__(self):
return ’Var(’+self.name+’)’
Leaf nodes are represented by instances of Number and Variable. Internal nodes are represented
by instances of Sum, Prod, Quot, and Diff. The superclass BinaryOp is meant to be a place
6.01 HW1: Calculator — Fall 2011 3
to put aspects of the binary operators that are the same for each operator, in order to minimize
repetition in coding.
We could create a Python representation of (1 + ((2 * 3) + (a / b))) with
Sum(Number(1.0), Sum(Prod(Number(2.0), Number(3.0)), Quot(Variable(’a’), Variable(’b’))))
Note that we will be converting all numbers to floating point to avoid problems with division later
on.
In addition to numerical expressions, the language of our calculator includes assignment ’statements’,
which we can represent as instances of an assignment class. They differ from the other
expressions in that they are not compositional: an assignment statement has a variable on the
left of the equality and an expression on the right, and it cannot itself be part of any further expressions.
Because assignments share the same initialization and string methods, we have made
Assign a subclass of BinaryOp, but they will require very different handling for evaluation.
4 Parsing
Parsing is the process of taking a string of characters and returning a syntax tree. We’ll assume
that we parse a single line, which corresponds to a single expression or assignment statement. The
processing happens in two phases: tokenization and then parsing a token sequence.
4.1 Tokenization
A tokenizer takes a sequence of characters as input and returns a sequence of tokens, which might
be words or numbers or special, meaningful characters. Forinstance, we might break up the string:
’((fred + george) / (voldemort + 666))’
into the list of tokens (each of which is, itself, a string):
[’(’, ’(’, ’fred’, ’+’, ’george’, ’)’, ’/’, ’(’, ’voldemort’, ’+’, ’666’, ’)’, ’)’]
We would like our tokenizer to work the same way, even if the spaces are deleted from the input:
’((fred+george)/(voldemort+666))’
Our special, single-character tokens will be:
seps = [’(’, ’)’, ’+’, ’-’, ’*’, ’/’, ’=’]
Step 1. Write a procedure tokenize(inputString) that takes a string of characters as input and returns
a list of tokens as output. The output of tokenize(’(fred + george)’) should be [’(’,
’fred’, ’+’, ’george’, ’)’]. There are other test cases in the hw1Work.py file.
Wk.2.4.1 After you have debugged your code in Idle, submit it via this tutor problem.
Include comments in your code.
4.2 Parsing a token sequence
The job of the parser is to take as input a list of tokens, produced by the tokenizer, and to return
a syntax tree as output. Parsing Python and other programming languages can be fairly difficult,
6.01 HW1: Calculator — Fall 2011 4
and parsing natural language is an open research problem. But parsing our simple language is
not too hard, because every expression is either:
• a number, or
• a variable name, or
• an expression of the form
( expression op expression )
where op is one of +, -, *, /, =.
This language can be parsed using a simple recursive descent parser. A good way to structure your
parser is as follows:
def parse(tokens):
def parseExp(index):

(parsedExp, nextIndex) = parseExp(0)
return parsedExp
The procedure parseExp is a recursive procedure that takes an integer index into the tokens list.
This procedure returns a pair of values:
• the expression found starting at location index. This is an instance of one of the syntax tree
classes: Number, Variable, Sum, etc.
• the index beyond where this expression ends. If the expression ends at the token with index
6, then the returned value would be 7.
In the definition of this procedure we make sure that we call it with the value index corresponding
to the start of an expression. So, we need to handle only three cases. Let token be the token at
location index. The cases are:
• If token represents a number, then make it into a Number instance and return that, paired with
index+1 . Note that the value attribute of a Number instance should be a Python floating point
number.
• If token represents a variable name, then make it into a Variable instance and return that,
paired with index+1. Note that the value attribute of a Variable instance should be a Python
string.
• Otherwise, the sequence of tokens starting at index must be of the form:
( expression op expression )
Therefore, token must be ’(’ . We need to:
− Parse an expression (using parseExp), getting a syntax tree that we’ll call leftTree and
the index for the token beyond the end of the expression.
− The token beyond leftTree should be a single-character operator token; call it op.
− Parse an expression (using parseExp) starting beyond op, getting a syntax tree that we’ll
call rightTree.
− Use op to determine what kind of internal syntax tree instance to make: construct it using
leftTree and rightTree and return it as the result of this procedure, paired with the
index of the token beyond the final right paren.
We will give you two useful procedures:
• numberTok takes a token as an argument and returns True if the token represents a number
and False otherwise.
6.01 HW1: Calculator — Fall 2011 5
• variableTok takes a token as an argument and returns True if the token represents a variable
name and False otherwise.
It is also useful to know that if token is a string representing a legal Python number, then
float(token) will convert it into a floating-point number.
We have implemented __str__ methods for the syntax-tree classes. The expressions print out
similarly to the Python expression that you would use to create the syntax tree:
>>> parse(tokenize(’(1 + ((2 * 3) + (a / b)))’))
Sum(Num(1.0), Sum(Prod(Num(2.0), Num(3.0)), Quot(Var(a), Var(b))))
It is very important to remember that this is simply the string representation of what is actually
an instance of the syntax tree class Sum.
Here are some examples:
>>> parse([’888’])
Num(888.0)
>>> print parse([’(’, ’fred’, ’+’, ’george’, ’)’])
Sum(Var(fred), Var(george))
>>> print parse([’(’, ’(’, ’a’, ’*’, ’b’, ’)’, ’/’, ’(’, ’cee’, ’-’, ’doh’, ’)’ ,’)’])
Quot(Prod(Var(a), Var(b)), Diff(Var(cee), Var(doh)))
>>> print parse(tokenize(’((a * b) / (cee - doh))’))
Quot(Prod(Var(a), Var(b)), Diff(Var(cee), Var(doh)))
Step 2. Implement parse and test it on the examples in the work file, or other strings of tokens you make
up, or on the output of the tokenizer. Start by making sure it handles single numbers and variable
names correctly, then work up to more complex nested expressions.
Wk.2.4.2 After you have debugged your code in Idle, submit it via this tutor problem.
You should include only the code you wrote for parse. Include comments
in your code.
5 Evaluation
Once we have an expression represented as a syntax tree, we can evaluate it. We will start by
considering the case in which every expression can be evaluated fully to get a number; then we’ll
extend it to the case where expressions may remain symbolic, if the variables have not yet been
defined.
For our calculator, just as for Python, expressions are evaluated with respect to an environment.
We will represent environments using Python dictionaries (which you should read about in the
Python documentation at
http://docs.python.org/tutorial/datastructures.html#dictionaries), where the
keys are variable names and the values are the values of those variables.
5.1 Eager evaluation
Here are the operation rules of the basic calculator, which tries to completely evaluate every expression
it sees. The value of every expression is a number. The evaluation of expr in env works as
follows:
6.01 HW1: Calculator — Fall 2011 6
• If expr is a Number, then return its value.
• If expr is a Variable, then return the value associated with the name of the variable in env.
• If expr is an arithmetic operation, then return the value resulting from applying the operation
to the result of evaluating the left-hand tree and the result of evaluating the right-hand tree.
• If expr is an assignment, then evaluate the expression in the right-hand tree and find the name
of the variable on the left-hand side of the expression; change the dictionary env so that the
variable name is associated with the value of the expression from the right-hand side. Note
that all the values in the environment should be floating point numbers.
Optional: You can make your program more beautiful and compact, using functional programming
style, by storing the procedures associated with each operator in the subclass. The Python
module operator provides definitions of the procedures for the arithmetic operators. Here is an
example of using operators.
import operator
>>> myOp = operator.add
>>> myOp(3, 4)
7
Step 3. Write an eval method for each of the expression classes that might be returned by the parser. It
should take the environment as an argument and return a number. In real life, we would worry a
lot about error checking; for now, just assume that you are only ever given perfect expressions to
evaluate.
Test your program incrementally, using expressions like:
>>> env = {}
>>> Number(6.0).eval(env)
6.0
>>> env[’a’] = 5.0
>>> Variable(’a’).eval(env)
5.0
>>> Assign(Variable(’c’), Number(10.0)).eval(env)
>>> env
{’a’: 5.0, ’c’: 10.0}
>>> Variable(’c’).eval(env)
10.0
You may find the testEval procedure useful for testing your code.
5.2 Putting it all together
Now, it’s time to put all your pieces together and test your calculator. The work file defines calc,
a procedure that will prompt the user with a ’%’ character, then read in the next line of input that
the user types into a string called inp. On the following line, you should make whatever calls are
necessary to tokenize, parse, and evaluate that input. The procedure will print the result of the
evaluation, as well as the state of the environment after that evaluation.
For debugging, it can be easier to type in all the expressions at once. The calcTest procedure in
the work file takes a list of strings as input, and processes them one by one (much the way Idle
works when you ask it to ’run’ a Python file). You can use testExprs in the work file, as input to
this procedure for testing. And feel free to make up test cases of your own.
Step 4. Fill in the calcTest procedure, so that it calls your code, and make sure it works on the examples.
Here is the desired behavior of the evaluator on testExprs:
6.01 HW1: Calculator — Fall 2011 7
>>> calcTest(testExprs)
% (2 + 5)
7.0
env = {}
% (z = 6)
None
env = {’z’: 6.0}
% z
6.0
env = {’z’: 6.0}
% (w = (z + 1))
None
env = {’z’: 6.0, ’w’: 7.0}
% w
7.0
env = {’z’: 6.0, ’w’: 7.0}
Wk.2.4.3 Note that this is due at a later date than the first two problems.
After you have debugged your code in Idle, submit it via this tutor problem.
You should include the class definitions for Sum, Prod, Quot, Diff,
Assign, Number, Variable and any other class or procedure definitions
that they depend on. Include comments in your code.
6 Extensions
You should do one of the extensions to the calculator described below: tokenizing by state machines
or lazy partial evaluation. Submit your program (in Wk.2.4.4) and your answer to the
Check Yourself question (in Wk.2.4.5).
6.1 Tokenizing by State Machine
Step 5. Write a state machine class, called Tokenizer, whose input on each time step is a single character
and whose output on each time step is a either a token (a string of 1 or more characters) or the
empty string, ”, if no token is ready. Tokenizer should be a subclass of sm.SM. Remember that
the state of a state machine can be a string.
Here are some examples. Note that there must be a space at the end of the string.
Tokenizer().transduce(’fred ’)
[’’, ’’, ’’, ’’, ’fred’]
Tokenizer().transduce(’777 ’)
[’’, ’’, ’’, ’777’]
Tokenizer().transduce(’777 hi 33 ’)
[’’, ’’, ’’, ’777’, ’’, ’’, ’hi’, ’’, ’’, ’33’]
Tokenizer().transduce(’**-)( ’)
[’’, ’*’, ’*’, ’-’, ’)’, ’(’]
Tokenizer().transduce(’(hi*ho) ’)
[’’, ’(’, ’’, ’hi’, ’*’, ’’, ’ho’, ’)’]
Tokenizer().transduce(’(fred + george) ’)
[’’, ’(’, ’’, ’’, ’’, ’fred’, ’’, ’+’, ’’, ’’, ’’, ’’, ’’, ’’, ’george’, ’)’]
Step 6. Now, write a procedure tokenize(inputString) that takes a string of characters as input and
returns a list of tokens as output. The output of tokenize(’(fred + george) ’) should be
[’(’, ’fred’, ’+’, ’george’, ’)’]. To do this, your procedure should:
corresponding
6.01 HW1: Calculator — Fall 2011 8
• Make an instance of your Tokenizer state machine.
• Call its transduce method on the input string, with a space character appended to the end of
it. An important thing to understand about Python is that almost any construct that iterates
over lists or tuples will also iterate over strings. So, even though transduce was designed to
operate on lists, it also operates on strings: if we feed a string into the transduce method of a
state machine, it will call the step method with each individual character in the string.
• Remove the empty strings to return a list of good tokens.
Wk.2.4.4 Note that this is due at a later date than the first two problems.
After you have debugged your code in Idle, submit it via this tutor problem.
Enter the Tokenizer state machine class and the tokenize procedure, as
specified above. Also enter any helper procedures that you might have
written; do not enter any procedures or classes that we gave you. Include
comments in your code.
Check Yourself 1.
• Explain precisely why you need a space character appended to the end
of the input to the Tokenizer input. • Compare and contrast your two tokenizer implementations.
Wk.2.4.5 Note that this is due at a later date than the first two problems.
Enter your answer to Check Yourself 1 question into this Tutor problem.
6.2 Lazy partial evaluation
Or, you can do this extension. You should do one of these extensions to the calculator.
To make the calculator flexible, we will allow you to define an expression, like (d = (b + c)),
even before b and c are defined. Later, if b and c are defined to have numeric values, then evaluating
d will result in a number.
Step 7. Change your eval methods, so that they are lazy, and can handle symbolic expressions for which
we do not have values of all the symbols.
• If the expression is a Variable, test to see if it is in the dictionary. If it is in the dictionary,
return the result or evaluate the value for the variable in the environment, otherwise, simply
return the variable. (The Python expression ’a’ in d returns True if the string ’a’ is a key
in dictionary d).
• When you evaluate an assignment do not evaluate the right hand side; simply assign the value
of the variable in the environment to be the unevaluated syntax tree. Notice this means that the
values in the environment will always be syntax trees and not numbers as in eager evaluation.
This is called lazy evaluation, because we don’t evaluate expressions until we need their values.
• If your expression is an arithmetic operation, evaluate both the left and right subtrees. If they
are both actual numbers, then return a new number computed using the appropriate operator,
as before. If not, then make a new instance of the operator class, whose left and right children
are the results of having evaluated the left and right children of the original expression (because
6.01 HW1: Calculator — Fall 2011 9
the evaluation process may have simplified one or the other of the arguments) and return the
operator node. This is called partial evaluation because we only evaluate the expression to the
degree allowed by the variable bindings.
• When you look a variable up in the environment, evaluate the result before returning it, because
it might be a symbolic expression.
If you want to check whether something is an actual number (float or int), you can use the isNum
procedure defined in the work file.
Note that, if you are writing your eval method in the BinaryOp class, you will need to be able to
make a new instance of the subclass that self belongs to (e.g. Sum). Python provides a __class__
method for all objects, so that self.__class__ can be called to create a new instance of that same
class.
Here are some ideas for testing eval by itself:
>>> env = {}
>>> Assign(Variable(’a’), Sum(Variable(’b’), Variable(’c’))).eval(env)
>>> Variable(’a’).eval(env)
Sum(Var(b), Var(c)]
>>> env[’b’] = Number(2.0)
>>> Variable(’a’).eval(env)
Sum(2.0, Var(c))
>>> env[’c’] = Number(4.0)
>>> Variable(’a’).eval(env)
6.0
>>> calcTest(lazyTestExprs)
% (a = (b + c))
None
env = {’a’: Sum(Var(b), Var(c))}
% (b = ((d * e) / 2))
None
env = {’a’: Sum(Var(b), Var(c)), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0))}
% a
Sum(Quot(Prod(Var(d), Var(e)), 2.0), Var(c))
env = {’a’: Sum(Var(b), Var(c)), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0))}
% (d = 6)
None
env = {’a’: Sum(Var(b), Var(c)), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’d’: Num(6.0)}
% (e = 5)
None
env = {’a’: Sum(Var(b), Var(c)), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’: Num(6.0)}
% a
Sum(15.0, Var(c))
env = {’a’: Sum(Var(b), Var(c)), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’: Num(6.0)}
% (c = 9)
None
env = {’a’: Sum(Var(b), Var(c)), ’c’: Num(9.0), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’:
Num(6.0)}
% a
24.0
env = {’a’: Sum(Var(b), Var(c)), ’c’: Num(9.0), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’:
Num(6.0)}
% (d = 2)
None
env = {’a’: Sum(Var(b), Var(c)), ’c’: Num(9.0), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’:
Num(2.0)}
% a
14.0
env = {’a’: Sum(Var(b), Var(c)), ’c’: Num(9.0), ’b’: Quot(Prod(Var(d), Var(e)), Num(2.0)), ’e’: Num(5.0), ’d’:
Num(2.0)}
>>> calcTest(partialTestExprs)
% (z = (y + w))
None
6.01 HW1: Calculator — Fall 2011 10
env = {’z’: Sum(Var(y), Var(w))}
% z
Sum(Var(y), Var(w))
env = {’z’: Sum(Var(y), Var(w))}
% (y = 2)
None
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w))}
% z
Sum(2.0, Var(w))
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w))}
% (w = 4)
None
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w)), ’w’: Num(4.0)}
% z
6.0
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w)), ’w’: Num(4.0)}
% (w = 100)
None
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w)), ’w’: Num(100.0)}
% z
102.0
env = {’y’: Num(2.0), ’z’: Sum(Var(y), Var(w)), ’w’: Num(100.0)}
Wk.2.4.4 Note that this is due at a later date than the first two problems.
After you have debugged your code in Idle, submit it via this tutor problem.
You should include the class definitions for BinaryOp, Sum, Prod,
Quot, Diff, Assign, Number, Variable and any other class or procedure
definitions that they depend on. Include comments in your code.
Check Yourself 2. What happens if you evaluate
(a = 5)
(a = (a + 1))
a
• Using eager evaluation? • Using lazy evaluation?
Explain why.
Wk.2.4.5 Note that this is due at a later date than the first two problems.
Enter your answer to Check Yourself 2 into this Tutor problem.
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6.01SC Introduction to Electrical Engineering and Computer Science
Spring 2011
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