Access a specific row from an array

[Ferreira]

Is there any way I can access a specific row from an array, i.e., I have this array:

#2A((1 2) (3 4))

How can I get the first row as #A(1 2)?

Thanks in advance.

[gugamilare]

Unfortunately, I believe the only way to do this explicitly is to create a fresh vector and copy over the elements of that row. You can also do this implictly using closures:

Code: Select all

(defun nth-row (array index)
  (lambda (col)
    (aref array index col)))

And

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(let* ((array #2A((1 2) (3 4)))
       (row-0 (nth-row array 0)))
  (dotimes (i 2)
    (print (funcall row-0 i))))

And, if you want to be able to change the value

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(defun nth-row (array index)
  (lambda (col &optional new-val)
    (if new-val
      (setf (aref array index col) new-val)
      (aref array index col))))

[Ramarren]

See documentation on make-array, in particular :displaced-to and :displaced-index-offset, for example:

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CL-USER> (defparameter *a* (make-array (list 2 2) :initial-contents '((1 2)(3 4))))
*A*
CL-USER> (make-array 2 :displaced-to *a*)
#(1 2)
CL-USER> (make-array 2 :displaced-to *a* :displaced-index-offset 1)
#(2 3)
CL-USER> (make-array 2 :displaced-to *a* :displaced-index-offset 2)
#(3 4)


Note that due to how arrays are laid out in memory this can only be used to access rows or parts of rows, and not columns.

There is also affine indexing library, which might be used for similar purposes.

[Ferreira]

Thanks, make-array's keyworded parameters solved my problem.

[gugamilare]

See documentation on make-array, in particular :displaced-to and :displaced-index-offset, for example:

Code: Select all

CL-USER> (defparameter *a* (make-array (list 2 2) :initial-contents '((1 2)(3 4))))
*A*
CL-USER> (make-array 2 :displaced-to *a*)
#(1 2)
CL-USER> (make-array 2 :displaced-to *a* :displaced-index-offset 1)
#(2 3)
CL-USER> (make-array 2 :displaced-to *a* :displaced-index-offset 2)
#(3 4)


Note that due to how arrays are laid out in memory this can only be used to access rows or parts of rows, and not columns.

There is also affine indexing library, which might be used for similar purposes.

Hum, my bad, I didn't think about this. Oftenly I use displaced arrays to turn multidimentional arrays into one-dimensional, or to make a destructive version of subseq.

But this doesn't work with columns, and my version can be easily addapted