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copied!<p>The iterative version proposed by Rainer is very nice, it's compact and more efficient since you traverse the sequence only one time; <strike>in contrast to the recursive version which calls <code>position</code> at every iteration and thus traverse the sub-sequence every time.</strike> (<strong>Edit:</strong> I'm sorry, I was completely wrong about this last sentence, see Rainer's comment)</p> <p>If a recursive version is needed, another approach is to advance the <code>start</code> until it meets the <code>end</code>, collecting the result along its way.</p> <pre><code>(defun precede (obj vec &key (start 0) (end (length vec)) (test #'eql)) (if (or (null vec) (< end 2)) nil (%precede-recur obj vec start end test '()))) (defun %precede-recur (obj vec start end test result) (let ((next (1+ start))) (if (= next end) (nreverse result) (let ((newresult (if (funcall test obj (aref vec next)) (adjoin (aref vec start) result) result))) (%precede-recur obj vec next end test newresult))))) </code></pre> <p>Of course this is just another way of expressing the <code>loop</code> version.</p> <p>test:</p> <pre><code>[49]> (precede #\a "abracadabra") (#\r #\c #\d) [50]> (precede #\a "this is a long sentence that contains more characters") (#\Space #\h #\t #\r) [51]> (precede #\s "this is a long sentence that contains more characters") (#\i #\Space #\n #\r) </code></pre> <p>Also, I'm interested Robert, did your teacher say why he doesn't like using <code>adjoin</code> or <code>pushnew</code> in a recursive algorithm?</p>

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