Question

I’m in the process of writing a matrix transpose function in lisp. My approach can be seen from the following code:

(defun matrix-T (matrix)
  (cond ((null matrix) matrix)
        (t (list
            (do ((i 0 (+ i 1)))
                ((> i (length matrix)))
              (format t "(mapcar #'(lambda (x)(nth ~A x)) matrix)  %" i))))))

As you can see, I’m trying to get the output from the do loop to pass as an
argument for the list function. However, I only get the do loop output returned from matrix-T. Is there anyway I can rectify this?

Answer 1

A dead-simple straight forward way to transpose a matrix:

(defun transpose-matrix (matrix)
  (let ((result
         (make-array (reverse (array-dimensions matrix))
                     :element-type (array-element-type matrix))))
    (dotimes (i (array-dimension result 0) result)
      (dotimes (j (array-dimension result 1))
        (setf (aref result i j) (aref matrix j i))))))

(print
 (transpose-matrix
  #2A((1 2 3)
      (4 5 6))))
;; #2A((1 4) (2 5) (3 6))