Caml
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Problem Set 3: Map and Caml-Mathica

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Objectives

In this assignment, you will be given the opportunity to practice functional decomposition of complex problems into smaller, simpler ones. More specifically, you will look at functional decomposition of list processing problems in to subproblems that can be implemented by map, reduce, fold and filter functions. Functional programmers do this all the time in day-to-day programming tasks, but it is also the basis for programming parallel applications in frameworks like Hadoop or Google's Map-Reduce (it's no coincidence the names are the same!)

In addition, you will write your own language for symbolic differentiation using OCaml. This will illustrate just how easy it is to develop your own mini-language inside a functional language like OCaml using recursive data types and pattern matching. A key idea here is that programs can be represented very effectively using OCaml datatypes. Once we have represented programs using datatypes, we can analyze them and transform them in various ways. This is what compilers of all kinds do (like the OCaml compiler itself). In this case our "programs" are small arithmetic expressions, but the idea generalizes to arbitrarily complex programming languages.

Getting Started

Download the code here. Unzip and untar it: 

$ tar xfz a3.tgz
Inside, you will see several files. The main ones you need to concern yourself with are:

  • mapreduce.ml which is a self-contained series of exercises involving higher order functions.
  • expression.ml in which you will develop functions to perform symbolic differentiation of algebraic expressions.
  • Support code can be found in ast.ml and expressionLibrary.ml
  • A .merlin file for Merlin to find your source code and additional files after you have compiled.

A few important things to remember before you start:

  • This assignment must be done individually.
  • Make sure that the functions you are asked to write have the correct names and the number and type of arguments specified in the assignment. Programs that do not compile will be subject to penalties on top of regular deductions.
  • Style is naturally subjective, but the COS 326 style guide provides some good rules of thumb.
  • As always, think first; write second; test and revise third. Aim for elegance.
  • Compilation:
    • make mapreduce compiles only Part 1.
    • make expression compiles only Part 2.
    • make and make all compile everything. (If you don't know why these two commands do the same thing, ask your preceptor.)

Part 1: Higher Order Functions (mapreduce.ml)

Map, filter and fold are functions that capture extremely common recursion patterns over lists. A good functional programmer uses these functions to construct solutions to interesting problems using very little code. In this part, you will get practice with higher-order functions by using map and fold to write a number of functions.

  • map is implemented in OCaml by the function List.map.
  • filter is implemented in OCaml by the function List.filter.
  • fold is implemented in OCaml by the function List.fold_right. However, the standard OCaml library function has its arguments in an inconvenient order. Thus, we have provided you with the function "reduce" which computes identically to fold_right but takes arguments in a different order (discuss with your preceptor why this ordering is more useful).

All instructions for Part 1 can be found in mapreduce.ml, but as a reminder, the rec keyword and other explicit recursive solutions are prohibited for this part. You should be using higher-order functions to complete your tasks.

Part 2: A Language for Symbolic Differentiation (expression.ml)

In the Summer of 1958, John McCarthy (recipient of the Turing Award in 1971) made a major contribution to the field of programming languages. With the objective of writing a program that performed symbolic differentiation of algebraic expressions in a effective way, he noticed that some features that would have helped him to accomplish this task were absent in the programming languages of that time. This led him to the invention of LISP (published in Communications of the ACM in 1960) and other ideas, such as list processing (the name Lisp derives from "List Processing"), recursion, and garbage collection, which are essential to modern programming languages. Nowadays, symbolic differentiation of algebraic expressions is a task that can be conveniently accomplished on modern mathematical packages, such as Mathematica and Maple.

The objective of this part is to build a language that can differentiate and evaluate symbolically represented mathematical expressions that are functions of a single variable. Symbolic expressions consist of numbers, variables, and standard math functions (plus, minus, times, divide, sin, cos, etc).

Conceptual Overview

To get you started, we have provided the datatype that defines the abstract syntax tree for such expressions in ast.ml. 

(* abstract syntax tree *)

(* Binary operators. *)
type binop = Add | Sub | Mul 

type expression =
  | Num of float
  | Var
  | Binop of binop * expression * expression

Var represents an occurrence of the single variable "x". Binop(Add, Var, Num 3.0) represents the expression that adds 3.0 to x. The operators "Sub" and "Mul" refer to subtraction and multiplication, respectively. More complicated mathematical expressions involving addition, subtraction, multiplication, constants and the variable x can be constructed using combinations of the constructors in the above datatype definition. For example, "2.0*x + (x*x - 3.0)" can be represented as: 

 Binop(Add, Binop(Mul, Num(2.0), Var), Binop(Sub, Binop(Mul,Var,Var), Num(3.0))))

Each such expression represents a tree where nodes are the constructors and the children of each node are the specific operator to use and the arguments of that constructor. Such a tree is called an abstract syntax tree (or AST for short).

How to Compile and Test Part 2

The code you will be editing is in expression.ml. We have used modules to keep this file clean and easy to navigate. There are several ways to compile and test your code, but this is the easiest:

  1. Edit your code until you are ready to test some part
  2. Type make expression in your shell, which will identify any compilation errors.
  3. Run the compiled code with ./expression.d.byte, which will invoke any tests you have constructed.
  4. Evaluate the results of your tests and repeat.

Provided Infrastructure

We have provided some functions to create and manipulate expression values. checkexp is contained in expression.ml. The others are contained in expressionLibrary.ml.

  • parse : translates a string in infix form (such as "x*x - 3.0*x + 2.5") into an expression (treating "x" as the variable). The parse function parses according to the standard order of operations - so "5+x*8" will be read as "5+(x*8)".
  • to_string : prints expressions in a readable form, using infix notation. This function adds parentheses around every binary operation so that the output is completely unambiguous.
  • make_exp : takes in a length l and returns a randomly generated expression of length at most 2l.
  • rand_exp_str : takes in a length l and returns a string representation of length at most 2l.
  • checkexp : takes in a string expression and an x value and prints the results of calling every function to be tested except find_zero.

Problem Instructions

For this portion of the assignment, you should fill in function definitions in expression.ml.

Problem 2.1: Expression Evaluation

Your first take is to write a function that will evaluate (i.e., interpret) an expression. Given an expression e and a floating point value v for x, produce the floating point result of evaluating expression e when x is v. For example, 

evaluate (parse "x*x + 3") 2.0 = 7.0
Problem 2.2: Derivatives

Next, we want to develop a function that takes an expression e as its argument and returns an expression e' representing the derivative of the expression with respect to x. This process is referred to as symbolic differentiation.

If you don't remember your calculus, here's the necessary crib sheet, some formulae for computing derivatives that you will use: 

derivative(f + g)(x) = derivative(f)(x) + derivative(g)(x)
derivative(f - g)(x) = derivative(f)(x) - derivative(g)(x)
derivative(f * g)(x) = derivative(f)(x) * g(x) + f(x) * derivative(g)(x)
In addition, the derivative of any constant value is 0 and the derivative of a variable x is 1.

Your task is to implement the derivative function. The type of this function is expression -> expression. The result of your function must be correct, but need not be expressed in the simplest form. Take advantage of this in order to keep the code in this part as short as possible. You can implement this function in as little as 20–30 lines of code.

To help you, we provide a function, checkexp, which checks parts 2.1-2.3 for a given input. The portions of the function that require your attention read failwith "Not implemented". Do not attempt to run the function until you have replaced all of the failwith expressions with valid code.

Problem 2.3: Zero Finding

One application of the derivative of a function is to find zeros of a function. One way to do so is Newton's method. The function should take an expression, a guess for the zero, a precision requirement, and a limit to the number of iterations. If a guess g (to include the starting guess) is sufficient to find a zero within the required precision, it should immediately return Some g. Otherwise, so long as the limit has not been reached, it should produce a new guess and try again. It should return None if no zero was found within the desired precision by the time the limit was reached.

Your task is to implement the find_zero:expression -> float -> float -> int -> float option function.

Note: If the expression that find_zero is operating on is 'f(x)' and the precision is epsilon, we are asking you to find a value x such that |f(x)| < epsilon. That is, the value that the expression evaluates to at x is "within epsilon" of 0.

We are not requiring you to find an x such that |x - x0| < epsilon for some x0 for which f(x0) = 0.

Problem 2.4: Symbolic Zero-Finding

The function you wrote above allows you to find the zero (or a zero) of most functions that can be represented with our AST. We could also extend our little language with other operations like sin, cos, exponentiation, logarithms, etc., and this method would continue to work. This makes it quite powerful. However, in addition to numeric solving like this, Mathematica and many similar programs can perform symbolic algebra. These programs can solve equations using techniques similar to those you learned in middle and high school (as well as more advanced techniques for more complex equations) to get exact, rather than approximate answers.

Performing symbolic manipulation on complex expressions is quite difficult, and we do not expect you to do it. However, there is one type of expression for which this is not so difficult. These are polynomial expressions that can be simplified to the form ax + b. You likely learned how to solve equations of the form ax + b=0 years ago, and can apply the same skills in writing a program to solve these.

More specifically, for the purposes of this question, a degree-one expression is one that can be simplified to the form ax + b.

Your task is to write the function find_zero_exact, which will exactly find the zero of those degree-one expressions that do have zeros. More specifically, for degree-one expressions that do have zeros your function should return Some of an expression that

  • contains no variables
  • evaluates to the zero of the given expression and
  • is exact.
If the expression is not degree one or has no zero, return None.

Note: degree one expressions need not be as simple as ax + b. Something like 5x - 3 + 2(x - 8) is also a degree-one expression since it can be turned into ax + b by distributing and simplifying. Likewise x*x - x*x + x can be simplified to x since the quadratic expression cancels.

You may also assume idealized floating point arithmetic (+., -., *. never overflow and result in exact values). If you are given an expression that is equivalent to ax + b where a is equal to 0.0, you should return None.

You will need to think about how to handle various forms of degree one expressions. You will also need to think about how to determine whether an expression is degree one in the first place. You should aim for a very general solution --- trying to solve this problem with a list of special cases for particular kinds of expressions (like x*x - x*x) will not lead to a general solution. We suggest converting each expression into some kind of standard form, and then manipulating those standard forms. In particular, consider how you might represent and program with polynomials. Unlike previous questions, you will do best here if you build yourself a collection of useful helper functions.

How you handle these operations must be, at the core, symbolic manipulation instead of a series of evaluation steps. An example of the latter would be first confirming degree-one by evaluating the second derivative and comparing with 0, and then evaluating the function at 0 and 1 to determine the two coefficients of the closed-form solution x = -a0/a1. Do not do this, as computational solutions like this will receive no credit.

Optional Problem 2.5: The amazing effectiveness of Newton's method.

Newton's method often converges really fast! Consider using it to calculate sqrt(N):
let f N x = x^2 - N
To find sqrt(N), just find a zero of the function (f N).

If you want K bits of precision, how many iterations of Newton's method do you have to run? Let newton(f N,N,i) be the result of running i iterations of Newton's method on that function (f N) with starting point N. Now calculate E(i) = -lg(abs(newton(f N, N, i) - sqrt(N))), where lg is the binary logarithm. That tells you how many bits of accuracy you have after i iterations. How does E grow as a function of i? How many iterations do you need to calculate to 56 bits of accuracy, which the length of the mantissa of double-precision floating point? That's how your computer's sqrt function actually works, by the way.

How many points is this "optional" thing worth?

Perhaps none at all. Some things you do because they are intellectually interesting. If it's not interesting, or if you're pressed for time, leave it out.

Handin Instructions and Grading Information

This problem set is to be done individually.

You must hand in these files to dropbox (see link on assignment page):

  1. mapreduce.ml
  2. expression.ml

Final Warning: Before you submit, be sure to compile your code one last time with make and then to test that none of your tests fail by running the mapreduce and expression executables produced by using make. You should also make sure that the "check all submitted files" button on dropbox does not report any compilation errors nor abort with an exception.

Assignments that do not compile will be treated harshly. It is much better to turn in an assignment that compiles but is incomplete than to turn in an assignment that does not compile. If you do not have a compiling version of a function, please comment out all your code for that function rather than reverting to the failwith "Not implemented" line; this way your files will still compile and will not disrupt the automatic grader. Document in a comment anything you tried or any problems you had with a particular portion of the assignment.

There are 30 total points in this assignment, broken down as 8 for problem 1.1, 9 for problem 1.2, and 13 for problem 2. Further subdivisions are as follows:

  • Each subproblem 1.1.a-1.1.d: 2
  • Each subproblem 1.2.a-1.2.c: 3
  • Problem 2.1: 2
  • Problem 2.2: 3
  • Problem 2.3: 3
  • Problem 2.4: 5

Princeton University, Computer Science Department
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