Question

Why does this code not print out the content of the array –

(defun loopfn (state)
       (loop for x from 0 to 2 do
        (loop for y from 0 to 2 do
             (aref state x y))))

Here I am passing a 3×3 matrix which is built like this –

`(setq i (make-array '(3,3) :initial-contents '((0 1 3) (4 2 5) (7 8 6))))`

I am calling – (loopfn i)

Edit——–
@Greg
Thanks for pointing that out…
I had the following question..
Why does this print the output …

(defun loopfn ()
   (loop for x from 0 to 3 do
    (if (eq x 2)(return (list x)))))

Where as this prints a nil…

(defun loopfn ()
   (loop for x from 0 to 2 do
    (loop for y from 0 to 2 do
         (if (eq x 2)(return (list x y))))))

I am calling

(loopfn)

Answer 1

Your call to aref is getting the specified element, but you’re not doing anything with it. You could stick it on to a list which is then returned:

(defun loopfn (state)
  (let ((result '()))
    (loop for x from 0 to 2 do
     (loop for y from 0 to 2 do
          (setf result (cons (aref state x y) result))))
   result))

or you could just print it out:

(defun loopfn (state)
  (loop for x from 0 to 2 do
       (loop for y from 0 to 2 do
        (format t "~a~%" (aref state x y)))))

The former is far more useful ; you want to return things which can then be further processed, and anything that gets passed to the top level will be printed out for you.

As long as you are using LOOP you can easily gather up your values with COLLECT, APPEND, etc., which is the idiomatic way to do it.