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What is the difference between the following solutions (a setf function and a function)? Is one of them preferable, or are they only two ways to get the same result?

(defparameter *some-array* (make-array 10)) (defun (setf arr-index) (new-value index-string) (setf (aref *some-array* (parse-integer index-string)) new-value)) (defun arr-index-1 (index-string new-value ) (setf (aref *some-array* (parse-integer index-string)) new-value)) CL-USER> *some-array* #(0 0 0 0 0 0 0 0 0 0) CL-USER> (setf (arr-index "2") 7) 7 CL-USER> (arr-index-1 "3" 5) 5 CL-USER> *some-array* #(0 0 7 5 0 0 0 0 0 0)

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The setf function works as a place (aka. generalized reference). This means it can be used with modify macros such as INCF or ROTATEF. You do also have to write a corresponding getter function for them to work though.

(defparameter *some-array* (make-array 10)) (defun arr-index (index-string) (aref *some-array* (parse-integer index-string))) (defun (setf arr-index) (new-value index-string) (setf (aref *some-array* (parse-integer index-string)) new-value)) CL-USER> (setf (arr-index "3") 10) 10 CL-USER> (incf (arr-index "3")) 11 CL-USER> (incf (arr-index "3")) 12 CL-USER> (rotatef (arr-index "3") (arr-index "6")) NIL CL-USER> (incf (arr-index "3") 100) 100 CL-USER> *some-array* #(0 0 0 100 0 0 12 0 0 0)

A setf function is generally preferred unless you are, for some reason, making a stylistic choice not to use setf or modify macros.

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