SCXT 350

Exercise set #1 Amswers

1. Write a LISP expression which will calculate

(3 + 2) * (10 - 3)

     (* (+ 3 2) (- 10 3))

2. Write LISP expressions which will return the second item in a list lst. The third item in the list.

     (car (cdr lst))
     (car (cdr (cdr lst)))

3. Write a function which will return the largest of three numbers

   (defun max3 (a b c)
      (if (and (> a c) (> a b)) a
         (if (> b c) b c)))

4. Write a function which will calculate the average of two numbers

     (defun avg2 (a b) (/ (+ a b) 2.0))

5. Write a function which will take a list (with at least two elements) and return the list with the first two elements swapped. For example, (swapper '(a b c d e)) should return (b a c d e).

     (defun swapper (lst)
         (cons (car (cdr lst))
             (cons (car lst) (cdr (cdr lst)))))

6. Write a function to calculate the n'th Fibonacci number according to the following rule: If n is less than 3, then FIB(n) = 1. If n is bigger than or equal to 3, then FIB(n) = FIB(n-1) + FIB(n-2). Try using the trace function on this when you get it working.

     (defun fib (n)
        (if (< n 3) 1
          (+ (fib (- n 1)) (fib (- n 2)))))

7, (For students who have had at least a semester of computer science: Write a function CountAtoms which will count the number of atoms in a list. For example, (CountAtoms '(a b c)) = 3 and (CountAtoms '((a b) c (d (e f)))) = 6

 (defun countatoms (lst)
    (if (null lst) 0
      (if (atom (car lst))
         (+ 1 (countatoms (cdr lst)))
            (+ (countatoms (car lst)) 
                (countatoms (cdr lst))))))

In our lab on Monday, try your solutions out. On Friday, Sept. 15, turn in (in class) your written solutions to problems 1 - 6.