LISP Programming using Allegro CL

[ProximaC]

I seriously need help with this LISP Programming. There are 8 questions but I've done the first 4 questions. Right now, I'm totally clueless with the rest of the remaining questions.

Please help.

1. Give iterative definition of a function (symbol-count-iter) that takes a list and a symbol a and returns the number of times the symbol a occurs in it.

(defconstant sym 'a)

(defun symbol-count-iter (lst)

Sample output::

(symbol-count-iter '(a c d e a 2 3))
1
(symbol-count-iter '(a c d e a 2 3))
2

1. Your friend is trying to write a function that returns the sum of all the non-nil elements in a list. He has written two versions of this function, and neither of them works. Explain what’s wrong with each and give a correct version.
   (a)
   (defun summit (lst)
   (remove nil lst)
   (apply #'+ lst))

(b)
(defun summit (lst)
(let ((x (car lst)))
(if (null x)
(summit (cdr lst))
(+ x (summit (cdr lst))))))

1. Define a function (count-numbers?) which returns a number which counts only numbers in a list.
   (defun count-numbers? (lst)

Sample output:

(count-numbers? '(a b c d 3 4 5 6 nil))
4

1. Write a function filter which takes a list and a predicate, and returns the list of the elements from the original list for which the predicate returns true. (There are actually LISP built-ins to do this called remove-if and remove-if-not. You should not use them for this problem.)
   >(defun even(num) (= (mod num 2) 0))
   >(filter '(6 4 3 5 2) #'even)
   (6 4 2)
   (defun filter (lst pred)

Sample output:

(filter '(2 4 5 6 7 8 10 11) #'oddp)
(5 7 11)
(filter '(2 4 5 6 7 8 10 11) #'oddp)
(5 7 11)