Need advice on how to print a matrix in lisp

I have a matrix defined so if I do this

(format t "~a" (get-real-2d 0 0))

it prints out the element in the first row first column

and if I do this

(format t "~a" (get-real-2d a 0 1))

it prints out the element in first row second column

and if I do this

(format t "~a" (get-real-2d a 1 0))

it prints out the element in second row first column.

The matrix a looks like this

a =
((0 1 2)
(3 4 5)
(6 7 8))

and I was hoping you can show me exactly how to write a dotimes loop or other loop that would in as few lines as possible would print out the matrix using the get-real-2d function so the output looks like this:

0 1 2 
3 4 5
6 7 8

I'm just hoping you can show me a slick loop that would be real small that I can use to print matrices that I can use in my lisp library something real professional looking, like one that would use only variables. Something like:

(format t "~a" (get-real-2d i j))

instead of a bunch of:

(format t "~a" (get-real-2d 0 0))
(format t "~a" (get-real-2d 0 1))
(format t "~a" (get-real-2d 0 2))

;;;;LATEST EDIT;;; to make this simple I call

(defparameter a (create-mat 3 3 +32fc1+))

to create a 3x3 matrix - create-mat is a wrapper for opencv's cvCreateMat

the output from that command at repl is

(defparameter a (create-mat 3 3 +32fc1+))
A
CL-OPENCV> a
#.(SB-SYS:INT-SAP #X7FFFD8000E00)

i/e the variable a is a pointer to the 3x3 matrix

then I run

(defparameter data (cffi:foreign-alloc :float :initial-contents 
          '(0.0f0 1.0f0 2.0f0 3.0f0 4.0f0 5.0f0 6.0f0 7.0f0 8.0f0)))

to create the data for the matrix - which I next will allocate to the matrix

the output from that command at repl is

CL-OPENCV> (defparameter data (cffi:foreign-alloc :float :initial-contents 
          '(0.0f0 1.0f0 2.0f0 3.0f0 4.0f0 5.0f0 6.0f0 7.0f0 8.0f0)))
DATA
CL-OPENCV> data
#.(SB-SYS:INT-SAP #X7FFFD8000E40)

i/e the variable a is data pointer to the data ill add to the matrix

then I call..

(set-data a data 12) to add the data to the matrix - set-data is a wrapper for opencv's cvSetData

so now when I run - (get-real-2d is a wrapper for opencv's cvGetReal2d)

(get-real-2d a 0 0)  it gets the element of matrix a at row 0 col 0 which is 0.0d0

the output from that command at repl is

CL-OPENCV> (get-real-2d a 0 0)
0.0d0

and now when I run

(get-real-2d a 0 1)  it gets the element of matrix a at row 0 col 1 which is is 0.0d0

the output from that command at repl is

CL-OPENCV> (get-real-2d a 0 1)
1.0d0

and when I run this loop

 (dotimes (i 3)
  (dotimes (j 3)
(format t "~a~%" (get-real-2d a i j))))

the output from that command at repl is

CL-OPENCV> (dotimes (i 3)
  (dotimes (j 3)
(format t "~a~%" (get-real-2d a i j))))
0.0d0
1.0d0
2.0d0
3.0d0
4.0d0
5.0d0
6.0d0
7.0d0
8.0d0
NIL

but when I try your method @Svante

(dotimes (i 3)
  (dotimes (j 3)
(format t "~{~{~a~^ ~}~%~}" (get-real-2d a i j))))

I get error:

The value 0.0d0 is not of type LIST.
    [Condition of type TYPE-ERROR]

because the output of 1 run of get-real-2d is just a 1 number float i/e

CL-OPENCV> (get-real-2d a 0 0)
0.0d0

with that info can you help me print the matrix so it looks like this

0.0d0 1.0d0 2.0d0 
3.0d0 4.0d0 5.0d0
6.0d0 7.0d0 8.0d0

Solution

Your question can be understood in two ways, and that is why it has two solutions:

Define method for printing object of type matrix (in this case it may use the knowledge about the internal structure of matrix):
```
(defmethod print-object ((matrix matrix) stream)
    (format stream "~{~{~a~^ ~}~%~}" matrix))
```
Using format as is shown in the answers.

Define client function that can use the only method of your object - get-real-2d:

(defun print-matrix (matrix dimension-x dimension-y)
    (dotimes (x dimension-x)
        (dotimes (y dimension-y)
            (princ (get-real-2d matrix x y))
            (princ #\Space))
        (princ #\Newline)))

Just using dotimes.