Common Lisp: how to access a row of a certain multi-dimension array?

Question

Let's say I wrote

(setf s (make-array (list 9 9) :element-type 'bit))

so s is a 9x9 matrix of bits.

and I want to get the 1st row of s. How do I get that?

I could have done the following:

(setf s (make-array 9 
          :element-type 'array 
          :initial-element 
          (make-array 9 :element-type 'bit)))

and access the first row by (svref s 0).
But I want to know if there is a built-in way.
(And the 2 dim array seems to allocate less bytes).

Answer 1

(defun array-slice (arr row)
    (make-array (array-dimension arr 1) 
      :displaced-to arr 
       :displaced-index-offset (* row (array-dimension arr 1))))

This only works for row slices and doesn't, IIRC, copy the array. Writing to the slice will modify the original array.