Assignment 4 (PostScript Interpreter – Part 1) Solution




An Interpreter for a Simple Postscript-like Language


Weight: The entire interpreter project (Part 1 and Part 2 together) will count for 12% of your course grade. This first part is worth 3% and second part is 9% – the intention is to make sure that you are on the right track and have a chance for mid-course correction before completing Part 2. However, note that the work and amount of code involved in Part 1 is a large fraction of the total project, so you need to get going on this part right away.


This assignment is to be your own work. Refer to the course academic integrity statement in the syllabus.

Turning in your assignment


All the problem solutions should be placed in a single file named When you are done and certain that everything is working correctly, turn in your file by uploading on the Assignment4(Interpreter-Part1) DROPBOX on Blackboard (under AssignmentSubmisions menu).


The file that you upload must be named . Be sure to include your name as a comment at the top of the file. Also in a comment, indicating whether your code is intended for Unix/Linux or Windows. You may turn in your assignment up to 3 times. Only the last one submitted will be graded.


Implement your code for Python3. The TA will run all assignments using Python3 interpreter. You will lose points if your code is incompatible with Python3.


The work you turn in is to be your own personal work. You may not copy another student’s code or work together on writing code. You may not copy code from the web, or anything else that lets you avoid solving the problems for yourself.




The assignment will be marked for good programming style (appropriate algorithms, good indentation and appropriate comments — refer to the Python style guide ) — as well as thoroughness of testing and clean and correct execution. You will lose points if you don’t (1) provide test functions / additional test cases, (2) explain your code with appropriate comments, and (3) follow a good programming style.


The Problem


In this assignment you will write an interpreter in Python for a simplified PostScript-like language, concentrating on key computational features of the abstract machine, omitting all PS features related to graphics, and using a somewhat-simplified syntax. The simplified language, SPS, has the following features of PS:


  • integer constants, e.g. 123: in Python3 there is no practical limit on the size of integers


  • string constants, e.g. (CptS355): string delimited in parenthesis (Make sure to keep the parenthesis delimiters when you store the string constants in the opstack and the dictstack.)


  • name constants, e.g. /fact: start with a / and letter followed by an arbitrary sequence of letters and numbers
  • names to be looked up in the dictionary stack, e.g. fact: as for name constants, without the /
  • code constants: code between matched curly braces { … }


  • built-in operators on numbers: add, sub, mul, div, mod, eq, lt, gt
  • built-in operators on string values: length, get, getinterval, put. See the lecture notes for more information on string functions.
  • built-in conditional operators: if, ifelse (you will implement if/ifelse operators in Part2)
  • built-in loop operator: for (you will implement for operator in Part 2).
  • stack operators: dup, copy, pop, clear, exch, roll
  • dictionary creation operator: dict; takes one operand from the operand stack, ignores it, and creates a new, empty dictionary on the operand stack (we will call this psDict)
  • dictionary stack manipulation operators: begin, end. begin requires one dictionary operand on the operand stack; end has no operands.
  • name definition operator: def.
  • defining (using def) and calling functions (we will call this psDef)
  • stack printing operator (prints contents of stack without changing it): stack


Part 1 – Requirements


In Part 1 you will build some essential pieces of the interpreter but not yet the full interpreter. The pieces you build will be driven by Python test code rather than actual Postscript programs. The pieces you are going to build first are:

  1. The operand stack
  2. The dictionary stack
  3. Defining variables (with def) and looking up names
  4. The operators that don’t involve code arrays: all of the operators except for loop, if/ifelse operators, and calling functions (You will complete these in Part 2)


  1. The Operand Stack – opstack


The operand stack should be implemented as a Python list. The list will contain Python integers, strings, and later in Part 2 code arrays. Python integers and lists on the stack represent Postscript integer constants and array constants. Python strings which start with a slash / on the stack represent names of Postscript variables. When using a list as a stack, assume that the top of the stack is he end of the list (i.e., the pushing and popping happens at the end of the list).


  1. The Dictionary Stack – dictstack



The dictionary stack is also implemented as a Python list. It will contain Python dictionaries which will be the implementation for Postscript dictionaries. The dictionary stack needs to support adding andb removing dictionaries at the top of the stack (i.e., end of the list), as well as defining and looking up names.



  1. Operators


Operators will be implemented as zero-argument Python functions that manipulate the operand and dictionary stacks. For example, the div operator could be implemented as the below Python function (with comments instead of actual implementations)


def div():


op1 = # pop the top value off the operand stack op2 = # pop the top value off the operand stack

# push (op1 / op2) onto the operand stack


The begin and end operators are a little different in that they manipulate the dictionary stack in addition to or instead of the operand stack. Remember that the psDict operator affects only the operand stack.


(Note about dict: Remember that the dict operator takes an integer operand from the operand stack and pushes an empty dictionary to the operand stack (affects only the operand stack). The initial size argument is ignored – Postscript requires it for backward compatibility of dict operator with the early Postscript versions).


The def operator takes two operands from the operand stack: a string (recall that strings that start with “/” in the operand stack represent names of postscript variables) and a value. It changes the dictionary at the top of the dictionary stack so that the string is mapped to the value by that dictionary. Notice that def does not change the number of dictionaries on the dictionary stack!


  1. define and lookup


You will write two helper functions, define and lookup, to define a variables and to lookup the value of a variable, respectively.


The define function adds the “name:value” pair to the top dictionary in the dictionary stack. Your psDef function ( i.e., your implementation of the Postscript def operator) should pop the name and value from operand stack and call the “define” function.


You should keep the ‘/’ in the name constant when you store it in the dictStack.


def define(name, value):




#add name:value pair to the top dictionary in the dictionary stack.


The lookup function should look-up the value of a given variable in the dictionary stack. In Part 2, when you interpret simple Postscript expressions, you will call this function for variable lookups and function calls.


def lookup(name):




  • return the value associated with name
  • What is your design decision about what to do when there is no definition for “name”? If “name” is not defined, your program should not break, but should give an appropriate error message. You may start your implementation using the below skeleton code. Please make sure to use the function names given below.


#————————- 10% ————————————-
  • The operand stack: define the operand stack and its operations opstack = [] #assuming top of the stack is the end of the list


  • Now define the helper functions to push and pop values on the opstack (i.e, add/remove elements to/from the end of the Python list)
  • Remember that there is a Postscript operator called “pop” so we choose different names for these functions.
  • Recall that `pass` in python is a no-op: replace it with your code.


def opPop():




  • opPop should return the popped value.


  • The pop() function should call opPop to pop the top value from the opstack, but it will ignore the popped value.


def opPush(value):



#————————– 20% ————————————-
  • The dictionary stack: define the dictionary stack and its operations dictstack = [] #assuming top of the stack is the end of the list
  • now define functions to push and pop dictionaries on the dictstack, to
  • define name, and to lookup a name


def dictPop():



# dictPop pops the top dictionary from the dictionary stack.


def dictPush(d):




#dictPush pushes the dictionary ‘d’ to the dictstack.


#Note that, your interpreter will call dictPush only when Postscript #“begin” operator is called. “begin” should pop the empty dictionary from #the opstack and push it onto the dictstack by calling dictPush.


def define(name, value):




#add name:value pair to the top dictionary in the dictionary stack.

#Keep the ‘/’ in the name constant.

#Your psDef function should pop the name and value from operand stack and #call the “define” function.


def lookup(name):




  • return the value associated with name
  • What is your design decision about what to do when there is no definition for “name”? If “name” is not defined, your program should not break, but should give an appropriate error message.



#————————— 10% ————————————- # Arithmetic operators: define all the arithmetic operators here

#Make sure to check the operand stack has the correct number of parameters


and types of the parameters are correct.

def add():



def sub():



def mul():




def div():




def mod():



def eq():



def lt():



def gt():




#————————— 15% ————————————-

# String operators: define the string operators length, get, getinterval, put

def length():



def get():




def getinterval():



def put():



#————————— 25% ————————————-

# Define the stack manipulation and print operators: dup, copy, pop, clear,

exch, roll, stack

def dup():




def copy():




def pop():



def clear():




def exch():




def roll():




def stack():




Important Note: For all operators you need to implement basic checks, i.e., check whether there are sufficient number of values in the operand stack and check whether those values have correct types. Examples:


def operator: the operands stack should have 2 values where the second value from top of the stack is a string starting with ‘/’

get operator : the operand stack should have 2 values; the top value on the stack should be an integer and the second value should be an string value.


Test your code:

def testAdd():





if opPop() != 3:

return False

return True


def testLookup():







if lookup(“n1”) != 3:

return False

return True


  • go on writing test code for ALL of your code here; think about edge cases,
  • and other points where you are likely to make a mistake.


Main Program


To run all the tests, your main may look like:


def main_part1():


testCases = [(‘define’,testDefine),(‘lookup’,testLookup),( ‘add’, testAdd), (‘sub’, testSub),(‘mul’, testMul),(‘div’, testDiv)]


  • add the rest of the test functions to this list along with suitable names failedTests =

[testName for (testName, testProc) in testCases if not testProc()]


if failedTests:

return (‘Some tests failed’, failedTests)


return (‘All part-1 tests OK’)


if __name__ == ‘__main__’:



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