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# CSCI 316: Lisp Assignment 3 solved

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## Description

1. Define a LISP function MIN-2 with the following properties. MIN-2 takes two arguments,
A and B. If the arguments A and B are numbers such that A ≤ B, then MIN-2 returns A. If A
and B are numbers such that A > B, then MIN-2 returns B. If A or B is not a number, then
MIN-2 returns the symbol ERROR. Examples: (MIN-2 21.3 7/2) => 7/2 (MIN-2 17.5 29) => 17.5
(MIN-2 5 ‘APPLE) => ERROR (MIN-2 ‘(31) ‘(54)) => ERROR

LISP Assignment 3: Page 2 of 3
2.[Exercise 4 on p. 72 of Wilensky] Write a LISP function SAFE-AVG that takes 2 arguments
and returns the average of those arguments if they are numbers. If one or both of the arguments
is not a number, the function should return NIL.
Examples: (SAFE-AVG 23 47.4) => 35.2 (SAFE-AVG 3 8) => 11/2
(SAFE-AVG ‘(23.1) 47.3) => NIL (SAFE-AVG ‘ORANGE ‘PLUM) => NIL
3.[Exercise 2 on p. 72 of Wilensky] Write a LISP predicate ODD-GT-MILLION that takes one
argument, and which returns T if its argument is an odd integer greater than a million, but returns
NIL otherwise. Hint: Make use of the predicate INTEGERP.

## Examples:

(ODD-GT-MILLION 92010231) => T (ODD-GT-MILLION 17) => NIL (ODD-GT-MILLION 92010232) => NIL
(ODD-GT-MILLION 21/5) => NIL (ODD-GT-MILLION 1718671.24) => NIL
(ODD-GT-MILLION ‘(2010231)) => NIL (ODD-GT-MILLION ‘APPLE) => NIL
4.[Exercise 3 on p. 72 of Wilensky] Re-read the discussion of MEMBER in sec. 6.6 of Touretzky
or on p. 51 of Winston & Horn. Then write a LISP predicate MULTIPLE-MEMBER that takes
two arguments and behaves as follows: If the first argument is a symbol or number and the
second is a list, then MULTIPLE-MEMBER returns a true value if the first argument occurs at
least twice in the second argument, and returns NIL otherwise.
Examples: (MULTIPLE-MEMBER ‘A ‘(B A B B A C A D)) => (A C A D)
(MULTIPLE-MEMBER ‘A ‘(B A B B C C A D)) => (A D)
(MULTIPLE-MEMBER ‘A ‘(B A B B C D)) => NIL

[Notice that the behavior of MULTIPLE-MEMBER is unspecified in cases where the first
argument is not a symbol or number, and in cases where the second argument is not a list. In
other words, your definition may return any value or produce an evaluation error in such cases.]
5. Define a LISP function MONTH->INTEGER which takes as argument a symbol that should be the
name of a month, and which returns the number of the month. For example:
(MONTH->INTEGER ‘MARCH) => 3 (MONTH->INTEGER ‘JUNE) => 6
If the argument is not a symbol that is the name of a month, the function should return the symbol

## ERROR. For example:

(MONTH->INTEGER ‘C) => ERROR (MONTH->INTEGER 7) => ERROR
(MONTH->INTEGER ‘QUOTE) =>ERROR (MONTH->INTEGER ‘(MAY)) =>ERROR
6. Define a LISP function SCORE->GRADE which takes a single argument, s, and returns a symbol
according to the following scheme:
s ≥ 90 A 73 ≤ s < 77 C+
87 ≤ s < 90 A– 70 ≤ s < 73 C
83 ≤ s < 87 B+ 60 ≤ s < 70 D
80 ≤ s < 83 B s < 60 F
77 ≤ s < 80 B– If the argument s is not a number then the function should return NIL. Examples: (SCORE->GRADE 86.3) => B+ (SCORE->GRADE 106) => A (SCORE->GRADE –10.1) => F

LISP Assignment 3: Page 3 of 3
Solve problems 7, 8, and 9 below without using COND, IF, WHEN, UNLESS, or CASE.
7. Define a LISP function GT with the following properties. GT takes two arguments. It returns T if the
arguments are numbers and the first argument is greater than the second; otherwise it returns NIL. Examples: (GT 0 –1) => T (GT –3 –7) => T (GT 40 40) => NIL (GT ‘B ‘A) => NIL

8. Define a LISP function SAME-PARITY with the following properties. SAME-PARITY takes
two arguments. It returns T if both arguments are even integers or if both arguments are odd
integers. In all other cases SAME-PARITY returns NIL.
Examples: (SAME-PARITY 0 –1) => NIL (SAME-PARITY –3 –9) => T (SAME-PARITY 30 90) => T
(SAME-PARITY ‘A ‘A) => NIL (SAME-PARITY 4.1 3.7) => NIL
9. Define a LISP function SAFE-DIV with the following properties. SAFE-DIV takes two arguments.
If both arguments are numbers and the second does not satisfy ZEROP, then the function returns the
result of dividing the first argument by the second. In all other cases it returns NIL.
Examples: (SAFE-DIV 6 4) => 3/2 (SAFE-DIV 6.0 4) => 1.5 (SAFE-DIV 6 0) => NIL
(SAFE-DIV 6 0.0) => NIL (SAFE-DIV ‘(6) 4) => NIL (SAFE-DIV 6 T) => NIL

## How to Submit Your Solutions

To submit, leave a copy of the final version of your file your last name in lowercase-3.lsp
in your home directory on euclid no later than midnight on the due date. [Note: If euclid unexpectedly goes
down after 6 p.m. on the due date, there will be no extension. Try to submit no later than noon that day, and sooner if possible.]
If you are working on venus, a PC, or a Mac, use an scp or sftp client to copy your file to
you can enter the following command––including the colon at the end––on venus, or in a powershell
window* on a PC, or in a terminal window on a Mac, to copy the file winston-3.lsp from your
working directory** to your home directory on euclid:
scp winston-3.lsp xxxx_yyyy316@euclid.cs.qc.cuny.edu:
*You can open a powershell window on a PC as follows: Type Win-r (i.e., hold down the Windows key
and type r), then type powershell into the “Open:” textbox, and then pressE.

**If the file you wish to submit is in another directory (e.g., in c:/316lisp on your PC), use the cd command
(e.g., cd c:/316lisp) to change your working directory to that directory before you enter the scp command!
IMPORTANT: Try doing such a file transfer at least a week before the submission deadline, so that
if you have problems then you can see me well before the submission deadline. Problems with
transferring files to euclid will not be considered as legitimate reasons for late submission! Indeed, it
is partly because students occasionally have such problems (e.g., because of forgotten passwords) that
you are allowed as many as 3 late submissions this semester without penalty.

After leaving the file your last name in lowercase-3.lsp on euclid as explained above, login
to your euclid account and test your Lisp functions on euclid as follows: Start Clisp by entering cl at
at Clisp’s > prompt, and then call each of your functions with test arguments. (If the above call of load
produces a Break … > prompt, your assignment has NOT been successfully submitted!)
Do NOT open your submitted file in any editor on euclid after the due date, unless you are
(re)submitting a corrected version of your solutions as a late submission! Also do not execute mv,
chmod, or touch with your submitted file as an argument after the due date. (However, it is OK to
view a submitted file using the less file viewer after the due date.)

As mentioned on page 3 of the first-day handout, you are required to keep a backup copy of your
submitted file on venus, and another copy elsewhere. You can enter the following two commands on
euclid to email a copy of your submitted file to yourself and to put a copy of the file on venus: echo . | mailx -s “copy of submission” -a your last name in lowercase-3.lsp \$USER