Lisp II. How EQUAL could be defined (defun equal (x y) ; this is how equal could be defined (cond ((numberp x) (= x y)) ((atom x) (eq x y)) ((atom y)

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Lisp II

How EQUAL could be defined (defun equal (x y) ; this is how equal could be defined (cond ((numberp x) (= x y)) ((atom x) (eq x y)) ((atom y) nil) ((equal (car x) (car y)) (equal (cdr x) (cdr y)))))

Some simple list processing examples (defun member (x theList) (cond ((atom theList) nil) ((equal x (car theList)) (cdr theList)) (T (member x (cdr theList)))) (defun append (l1 l2) (if (null l1) l2 (cons (car l1) (append (cdr l1) l2))))

Variations on reverse ;; either of these does O(n^2) cons operations (defun reverse (theList) (if (null theList) nil (append (reverse (cdr theList)) (list (car theList))))) (defun reverse (theList) (and theList (append (reverse (cdr theList)) (list (car theList)))))

Variations on reverse ;; this tail recursive operation does O(n) conses (defun reverse (theList) (reverse1 theList nil)) (defun reverse1 (theList acc) (if (null theList) acc (reverse (cdr theList) (cons (car theList) acc))))

Flatten I (defun flatten (theList) (cond ((null theList) nil) ; empty list do nothing ((atom (car theList)) ; cons an atom onto flattend cdr (cons (car theList) (flatten (cdr theList)))) ; otherwise flatten head & tail and append results (t (append (flatten (car theList)) (flatten (cdr theList))))))

Flatten II ;; this version avoids append, which is expensive. (defun flatten (theList) (flatten1 theList nil)) (defun flatten1 (theList acc) (cond ((null theList) acc) ; all done ((atom theList) ; stick atom on the front of acc (cons theList acc)) ; of the accumulator (t (flatten1 (car theList) (flatten1 (cdr theList) acc)))

Higher order functions (defun mapcar (f theList) (if (null theList) nil (cons (apply f (list (car theList))) (mapcar f (cdr theList)))))

Mapcar II (defun mapcar1 (f theList) (if (null theList) nil (cons (apply f (list (car theList))) (mapcar f (cdr theList))))) (defun mapcar (f &rest args) "Apply f to successive cars of all ARGS. Return the list of results." ;; If no list is exhausted, (if (not (memq 'nil args)) ; apply function to CARs. (cons (apply f (mapcar1 'car args)) (mapcar* f (mapcar1 'cdr args)))))

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