CMSC 631, Fall 2011

Program Analysis and Understanding

Project 1

Due September 16, 2011


  • Deadline pushed to September 16 (from September 14)

Part 1: List functions

Write your solutions to the following problems in a file

Implement the following functions in OCaml. Do not use any functions in the List module, i.e., write every function from scratch using just list notation [...], ::, and pattern matching.

  1. Write a function prod l : int list -> int that returns the product of the elements of l. The function prod should return 1 if the list is empty.
  2. Write a function max l : int list -> int that returns the largest integer in l. You may assume that l is not empty.
  3. Write a function addTail l e : 'a list -> 'a -> 'a list that takes a list l and a single element e and returns a new list where e is appended to the back of l. For example, addTail [1;2] 3 = [1;2;3].
  4. Write a function index l e : 'a list -> 'a -> int that takes a list and a single element and returns the position of the first occurrence of that element in the list (indexed by zero). You should return -1 if the element is not in the list.
  5. Write a function rindex l e : 'a list -> 'a -> int that takes a list and a single element and returns the position of the last occurrence of that element in the list (indexed by zero). You should return -1 if the element is not in the list.
  6. Write a function fill n x : int -> 'a -> 'a list that returns a list containing n occurrences of x. For example, fill 3 "foo" = ["foo"; "foo"; "foo"]. You may assume that n is non-negative.
  7. Write a function unzip l : ('a*'b) list -> ('a list)*('b list) that given a list of pairs, returns a pair of lists with the elements in the same order. For example, unzip [(1, 2); (3, 4)] = ([1; 3], [2;4]).
  8. Write a function app_int f m n : (int->'a)->int->int->'a list that returns the list [f m; f (m+1); ...; f n]. It should return the empty list if n < m.

Part 2: Boolean Formulae

In this part of the project, you can use the List library. Write your solutions to this part of the project in a file

You will use the following data type, representing boolean formulae:

type formula =
  | True
  | Var of char
  | And of formula * formula
  | Or of formula * formula
  | Not of formula
  | Forall of char * formula
  | Exists of char * formula

Here False and True represent the obvious values. Var c represents the boolean variable with name c (notice that variable names can only be characters). The constructors And, Or, and Not represent boolean conjunction, disjunction, and negation, respectively. For example, the mathematical formula (a or b) and c would be represented by the OCaml value And(Or(Var 'a', Var 'b'), Var 'c'). (We'll explain Forall and Exists in a moment.)

We will use associative lists, which are just lists of pairs, to represent assignments of truth values to variables:

  type assignment = (char * bool) list

Here if an assignment contains the pair (c,b), then that assignment gives the variable represented by the character c the value b. When working with the type assignment, you will find the functions List.assoc, List.mem_assoc, and related functions helpful. See the OCaml library documentation for more details.

You may assume for purposes of this project that whenever you work with an assignment, all listed variables are distinct (i.e., you don't need to worry about names in one part of the list "shadowing" names elsewhere in the list).

The last two kinds of formula, Forall and Exists, represent the similarly named quantifiers. The boolean formula Forall(x, f) is true if f is true under all assignments to x, i.e., if f is true when x=true and when x=false. The boolean formula Exists(x, f) is true if f is true either for x=true or x=false. For example, the formula Forall('x', Or(Var 'x', Var 'y')) is true under the assignment [('y', true)] and false under the assignment [('y', false)].

Write the following functions that work with boolean formulae:

  • nnf : formula -> bool. This function returns true if the formula is in negation normal form (NNF). A formula is in NNF if negation appears only "in front" of variables. For example, Or(Not (Var 'x'), True) is in NNF, but Not(And(Var 'x', False)) is not.

  • to_nnf : formula -> formula. Given an arbitrary formula, this function returns an equivalent formula in NNF. To write this function, you can use the following equivalences:
    Not True=False
    Not False=True
    Not(Not f)=f
    Not(And(f1, f2))=Or(Not f1, Not f2)
    Not(Or(f1, f2))=And(Not f1, Not f2)
    Not(Forall(x, f))=Exists(x, Not f)
    Not(Exists(x,f))=Forall(x, Not f)

  • eval : formula -> assignment -> bool. This function evaluates the boolean formula on the given variable assignment and returns the result as an OCaml bool. For example, given the OCaml value And(Or(Var 'x', Var 'y'), Var 'z') and the assignment [('x', true); ('y', false); ('z', true)], the eval function should return true. Your function can assume that all of the formula's free variables are specified in the assignment.

  • vars_of : formula -> char list. This function takes a formula and returns a list of the names of the free variables of the formula. The free variables are those that are not bound by a quantifier. For example, for Exists('x', Or(Var 'x', Var 'y')), you should return a list containing only 'y'. The list should contain no duplicates. There is no requirement as to the order that the variable names must appear in the list.

  • sat : formula -> assignment option. This function returns Some a, where a is a satisfying assignment, if the formula is satisfiable, or None otherwise. For example, sat (Or(Var 'x', Var 'y')) could return three things: Some ([('x', true); ('y', false)]), Some ([('x', false); ('y', true)]), or Some ([('x', true); ('y', true)]), whereas sat (And(Var 'x', Not (Var 'x'))) would return None. If more than one assignment is possible, you may return any assignment. You might find the vars_of function handy here. Do not worry about the efficiency of this function; it's fine if it takes exponential time. (For a better algorithm, read up on DPLL.)

What to Submit

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