# CMSC 471

### HOMEWORK ONE due 9/17/07

*Don't forget to put the statement of sources at the top of your homework! You don't need to cite the McCarthy or Graham articles.

## 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)  5pts. Write a function (nextint n) to return the the number that is one more than its integer argument n. For example, (nextint 2) should return 3; (nextint 5) should return 6.

(b) 5pts. 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?) Even though there are more efficient methods, you should use recursion to write this function.

### 2. Operating on lists (25  pts.)

(a) 5 pts. Write the function (my-third l) which returns the third element of the list l. Do not use the built-in function (third). You can define this function either recursively or iteratively.

(b) 5 pts. Write the function (scale-by-two l) which takes a list of integers l and returns a list of integers where each element is double the corresponding element in l. Your solution should use a lambda expression. For example, (scale-by-two '(1 2 3 4)) should return (2 4 6 8) and (scale-by-two '(-1 0 1)) should return (-2 0 2).

(c) 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.

1. Implement the posint function using mapcar.
2. Implement the posint function using the loop macro.
3. Implement the posint function recursively.

### 3. Conditionals and strings (15 pts.)

Write a function (case? s) that returns the symbol 'upper if the characters in the string s are all upper case, 'lower if the characters are all lower case, and 'mixed if there are some lower case and some upper case letters (or if the string contains any non-letter characters). Hint: first write two subroutines, upper? that tests to see if a string is upper case and lower? that tests to see if a string is lower case. Then use cond with these subroutines to test for which case to return. Useful built-in functions for solving this problem include char, upper-case-p, and lower-case-p.

### 4. Flattening a nested list (20 pts.)

Write a function (flatten-tree l) that takes an arbitrarily deeply nested list of atoms, 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).