# CMSC 471

## PART I.  What is AI? (20 pts)

ASSIGNMENT:  Does McCarthy see the primary goal of AI as modeling human intelligence?  Does he think that this goal is achievable?  Why or why not?  Summarize some of the key challenges in achieving human-level intelligence.

## PART II. Why Lisp? (10 pts.)

ASSIGNMENT:  Describe three of the key features of Lisp that, according to Graham, make it a good language for developing applications.

## PART III.  Lisp Programming (70 pts.)

ASSIGNMENT: These problems are intended to help you become familiar with the basic programming concepts in the Lisp language. Documentation and error checking are essential in this class, so although these problems are simple, your code must be documented, and error cases must be handled.  (For example, in problem #2, what happens if the argument isn't a list? What if it is a list, but is less than three elements long?)

### 1. Writing simple functions (10 pts.)

(a) 5 pts.   Write a function (lesstwo n) to return the the number that is two less than its integer argument n. For example, (twoless 2) should return 0; (lesstwo 5) should return 3.

(b) 5 pts. Write a function (fact n) to return the factorial of the argument n.  (The factorial of an integer is the product of all integers from 1 to that integer.) For example, (fact 3) should return 6; (fact 10) should return 3628800. (What do you think (fact 'hello) should return?) You should use recursion to write this function.

### 2. Operating on lists (45  pts.)

(a) 5 pts. Write the function (my-third l) which returns the third element of the list l. Use only car and cdr (i.e., you may not use built-in functions like third or caddr).

(b) 15 pts. There are often many different ways to solve the same problem in Lisp. In this problem, you will need to use your creativity and knowledge of Lisp functions to write the same function in several different ways. The function (posint l) should take a list l and return a list containing only the positive integers in the list. For example, (posint '(a 2.3 -1 5 hello 3 (1 2)))) should return (5 3 1 2). You can use the built-in function integerp in your solutions. All three implementations should be recursive.

1. Implement posint1, a version of the posint function using mapcar.
2. Implement posint2, a version of the posint function using the loop macro.
3. Implement posint3, a version of the posint function that operates recursively but does not use mapcar or loop..

For extra credit (5 pts), you may also implement posint4, a version of the posint that is not recursive. This is actually much harder because of the arbitrary nesting. My solution uses the loop macro and a local variable to build up the answer. I found it rather tricky to write.

### 3. Flattening a nested list (15 pts.)

Write a function (flatten-list l) that takes an arbitrarily deeply nested list of atoms (possibly including dotted pairs), and returns a flattened list of these atoms (in the same order they appear in the original list). For example, (flatten-tree '((((1) 2) ((3 . 4) 5)) 6)) should return (1 2 3 4 5 6).