Unused Lexical Variable

Question

Just started learning lisp. I have no idea why I am getting these errors or even what they mean. I am simply trying to code an approximation of pi using the Gregory-Leibniz series, here is the code.

(defun gl (n)
           (defparameter x 0)                        ;init variable to hold our runnning sum
           (loop for y from 0 to n                   ;number of iterations, starting from 0 to desired n
              (if (= y 0)                                      ;if n is 0 then we just want 4
                 (defparameter w 4))
              (if (> y 0)                                      ;else, 4*(-1^y)/((2 * y)+1)
                 (defparameter w (* 4 (/ (exp -1 y) (+ (* 2 y) 1)))))
              (+ x w))                                         ;add to our running sum
           (write x))                                        ;once loop is over, print x.

I have tried using setq, defvar, let etc. instead of defparameter but I still get "Undeclared free variable X".

I also get the error "Unused lexical variable N" even though I am using it for my loop, which is weird also.

How can I fix this and why is it happening? Thanks!

Don’t use defparameter inside defun. You should check the Hyperspec or some introduction book (see the info section for recommendations) for the idiomatic way to define local variables using let or inside loop. — Martin Buchmann

coredump coredump · Accepted Answer · 2019-03-29T16:22:39

Here is the code after Emacs auto-indented it:

(defun gl (n)
  (defparameter x 0)
  (loop for y from 0 to n
       (if (= y 0)
           (defparameter w 4))
       (if (> y 0)
           (defparameter w (* 4 (/ (exp -1 y) (+ (* 2 y) 1)))))
       (+ x w))
  (write x))

Compiling the following code with SBCL gives one error and two warnings.

One warning says that x is undefined. You should not call defparameter from inside your function, since defvar and defparameter are used to declare dynamic variables and to set their value in the global scope. Prefer to have let bindings, or, since you already are using a loop, a with clause. When you want to modify a binding, use setf.

The errors comes from the macroexpansion of LOOP, which is malformed. For SBCL, that means that the code is treated as dead-code for the rest of the function compilation; that explains why n appears not to be used, which is what the second warning is about.

There are various fixes remaining to be done:

Use function EXPT, not EXP.
Calling (+ x w) only computes a value but does not modify x, the result is useless.
Prefer using if as expression, like a ternary operator in other languages, in your case the code can be simplified
Adding one can be done with function 1+ (that's the name of the function, not a special syntax for adding constants)
The write operation is rarely needed, especially if you are computing a mathematical formula; just return the value, and the REPL will print it automatically.

Small corrections that make your code works:

(defun gl (n)
  (let ((x 0))
    (loop
       for y from 0 to n
       for w = (if (= y 0)
                   4
                   (* 4 (/ (expt -1 y) (+ (* 2 y) 1))))
       do (setf x (+ x w)))
    (write x)))

I would personally get rid of x and w, and use a SUM loop clause.

(defun gl (n)
  (loop
     for y from 0 to n
     sum (if (zerop y)
             4
             (* 4 (/ (expt -1 y)
                     (1+ (* 2 y)))))))

Unused Lexical Variable

1 Answers