# CSCI-603 PreTee Interpreter Lab 8 solution

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## Description

1 Introduction
An interpreter is a program that executes instructions for a programming language. The
python3 interpreter executes Python programs. This assignment involves writing an interpreter for a very simple language called PreTee.
The interpreter accepts prefix mathematical expressions as input, where a prefix expression
is one in which the mathematical operation is written at the beginning rather than in the
middle. Inside the interpreter is a parser, whose job is to convert prefix expressions, represented as a string, into a collection of tree structures known as parse trees. The PreTee
interpreter can evaluate mathematical expressions using the operators +, -, *, and //. The
supported operands are positive integer literals (e.g., 8) and variables (e.g., ’x’). A data
structure called a symbol table is used by the interpreter to associate a variable with its integer value. When given a prefix expression, PreTee displays the infix form of the expression
and evaluates the result.
Consider the following example using the prefix expression: ‘‘* 8 + x y’’
Variable Value
‘‘x’’ 10
‘‘y’’ 20
‘‘z’’ 30
tr :
×
8 +
x y
Infix expression: ‘‘(8 * (x + y))’’
Evaluation: 240
Let’s assume that the parse tree has already been constructed from the prefix expression.
The infix form of the expression can be obtained by doing an inorder (left, parent, right)
traversal of the tree from the root and constructing a string. Recall the private inorder
helper function from lecture that str called when getting a string representation of a
general purpose tree.
def __inorder(self, node):
if not node:
return ’ ’
else:
return self.__inorder(node.left) + \
str(node.val) + \
self.__inorder(node.right)
The result of the expression can be found by evaluating the root node in preorder fashion
(parent, left, right).
The implementation stage will cover details of the symbol table and evaluation of the parses
tree to generate the result of the expression.
1
2 Problem Solving
Work in a team of three to four students as determined by the instructor. Each team will
work together to complete a set of activities. Please complete the questions in order. Do not
1. Consider the following tree.
tr :

// x
y 2
Write the prefix expression for the tree, tr, above. Recall in a prefix traversal, the order
is parent, left then right.
2. Write the infix expression for the tree, tr, from problem 1. Use parentheses to specify
the order of operations.
3. The parse tree can be built by reading the individual tokens in the prefix expression
and constructing the appropriate node types. Given this prefix expression as the token
list:
[‘‘*’’, ‘‘//’’, ‘‘-’’, ‘‘x’’, ‘‘10’’, ‘‘y’’, ‘‘+’’, ‘‘4’’, ‘‘x’’]
(a) Draw the parse tree for the expression using the diagramming method above.
(b) Write the infix expression for the parse tree. Make sure you include parentheses
to preserve the operator precedence.
(c) What would be the result of evaluating the parse tree? Assume the symbol table
on the first page is in effect.
2
We will be representing the various types of nodes in the tree as Python classes.
Node class Slot name(s) Type(s) Constructor
LiteralNode val int init (self, val)
VariableNode id str init (self, id)
MathNode
left
right
token
object (a Node class)
object (a Node class)
str (“+”, “-”, “*”, or “//”)
init (self, left, right, token)
For example, the code below constructs a parse tree for the expression tree in the first
question:
>>> tr = MathNode( \
MathNode( \
VariableNode(’y’), \
LiteralNode(2), \
’\\’), \
VariableNode(’x’), \
’-’)
Assume the prefix expression has been converted into a list of string tokens:
>>> prefixExp = ’* 8 + x y’
>>> tokens = prefixExp.split()
>>> tokens
[’*’, ’8’, ’+’, ’x’, ’y’]
Now let’s build and return a parse tree using the token list and node classes. The
diagram below shows the node structure of the parse tree. For this problem, we will
call that tree tr. The root of the tree tr is a MathNode. Each node is indicated by
its type (one of the 3 possible node classes), and its internal slot values are also
shown. Arrows indicate that a node has a child node. Here is a picture of the parse
tree for the prefix expression, ‘‘* 8 + x y’’, which is stored in the token list as
[‘‘*’’, ‘‘8’’, ‘‘+’’, ‘‘x’’, ‘‘y’’]:
tr :
3
4. Write a function parse. It takes a list of tokens for a prefix expression and returns
a parse tree. The parse tree for the token list [‘‘*’’, ‘‘8’’, ‘‘+’’, ‘‘x’’, ‘‘y’’]
should produce the tree corresponding to the image above.
Note that a prefix expression has one of the following forms:
 A number that is non-negative in value (0 or greater).
 A variable.
 + e1 e2, where e1 and e2 are prefix expressions.
 − e1 e2, where e1 and e2 are prefix expressions.
 ∗ e1 e2, where e1 and e2 are prefix expressions.
 // e1 e2, where e1 and e2 are prefix expressions.
The string function isdigit is useful for testing whether or not a string looks like a
number. The string function isidentifier is useful for testing whether or not a string
looks like a variable.
>>> str1 = ’123’
>>> str1.isdigit()
True
>>> str2 = ’3x’
>>> str2.isdigit()
False
>>> str3 = ’x’
>>> str3.isidentifier()
True
>>> str4 = ’2x’
>>> str4.isidentifier()
False
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