Ordering a list of mixed data types in lisp

Question

Given a list of any length and data type calculate:

• Lower item
• Higher item
• Symbols
• Average
• Reversed list (implementing the function)

I've been trying to learn lisp by myself, since my class is kinda useless (read my other question =_=;), and I've done this:

(defun higher(l)
    (let (x)
        (setf x (first (sort l #'>)))))

Then it didn't order symbols, of course... so I tried this:

(defun higher(l)
    (let ((x 0))
        (dolist (a l)
            (setf a (coerce a 'integer))
            (if (> a x)
                (setf x a)))
        x))

but coerce is not helping me to make my symbols integers... thanks in advance for your help, and also, I would like to know if I'm formatting well (I wasn't taught to use "let").

Answer 1

Function #'Char> won't work with symbols, because symbol is not type character.

Before you compare symbols you can use symbol-name:

(symbol-name 'a)
CL-USER> "A"

So now you can use #'string>

To write function which compare> any data type you can use typecase. Small example:

(defun compare> (x y)
  (when (subtypep (type-of x) (type-of y))
    (typecase (and x y)
      (integer (> x y))
      (character (char> x y)))))

As Terje said, you shouldn't use sort, reduce is much more better :)