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copied!<p>Like every tool <code>format</code> has its limitations and it's not suited for such problems very well. Probably the best you can get with plain format without resorting to black magic tricks with <code>~?</code> or <code>~/</code>, that you or anyone else probably won't understand in the future, is this code:</p> <pre><code>CL-USER> (format t "~{~{~A ~}~%~}" '((X X X) (X X X X X X) (X X X X X X X X X))) X X X X X X X X X X X X X X X X X X </code></pre> <p>If you want to get your sophisticated output structure, try to do some pre-processing. Like, if the format of the list is hard-coded, you can use this:</p> <pre><code>(format t "~{~{~6A~} ~%~}" (mapcar (lambda (l) (loop :for i :from 0 :to (1- (length l)) :by (/ (length l) 3) :collect (format nil "~{~A ~}" (subseq l i (+ i (/ (length l) 3)))))) '((X X X) (X X X X X X) (X X X X X X X X X)))) </code></pre> <p>Here we first collect the items of a list into same number of groups for each list, print them and this way get 3 lists with the same number of elements, which can then be processed by <code>format</code>.</p> <p>You can find out more about <code>format</code> in the appropriate chapter of Peter Seibel's excelent Lisp book: <a href="http://gigamonkeys.com/book/a-few-format-recipes.html" rel="nofollow">http://gigamonkeys.com/book/a-few-format-recipes.html</a></p> <p><strong>EDIT</strong></p> <p>If you have a variable number of lists, with each one being twice bigger than the previous one, you'll also need to prepare the format string beforehand:</p> <pre><code>CL-USER> (defun format-custom-list (list) (format t (format nil "~~{~~{~~~DA~~} ~~%~~}" (* 2 (length list))) (mapcar (lambda (l) (let* ((len (length l)) (len/3 (/ len 3))) (loop :for i :from 0 :to (1- len) :by len/3 :collect (format nil "~{~A ~}" (subseq l i (+ i len/3)))))) list))) CL-USER> (format-custom-list '((X X X) (X X X X X X) (X X X X X X X X X) (X X X X X X X X X X X X))) X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X NIL </code></pre> <p>(The trailing <code>nil</code> is the output of <code>format</code>, which isn't printed to the output stream <code>t</code>. If you want to get a string out of this function use <code>nil</code> as <code>format</code>'s output stream.)</p>

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