First let's put the data into a list called data
. Pay attention that the list needs to be quoted by '
, and that the .
dot signs need to be surrounded by spaces because they have a signification of their own:
(defparameter *data* '((course (john . math) (jim . english) (carl . biology))
(year (john . 2) (jim . 1) (carl . 3))
(age (john . 22) (jim . 20) (carl . 27))))
Now let's try using procedure assoc
:
? *data*
((COURSE (JOHN . MATH) (JIM . ENGLISH) (CARL . BIOLOGY)) (YEAR (JOHN . 2) (JIM . 1) (CARL . 3)) (AGE (JOHN . 22) (JIM . 20) (CARL . 27)))
? (assoc 'course *data*)
(COURSE (JOHN . MATH) (JIM . ENGLISH) (CARL . BIOLOGY))
? (cdr (assoc 'course *data*))
((JOHN . MATH) (JIM . ENGLISH) (CARL . BIOLOGY))
? (assoc 'john (cdr (assoc 'course *data*)))
(JOHN . MATH)
? (cdr (assoc 'john (cdr (assoc 'course *data*))))
MATH
so the function becomes
(defun lookup (name attr data)
(cdr (assoc name (cdr (assoc attr data)))))
(lookup 'john 'course *data*)
=> MATH
(lookup 'carl 'age *data*)
=> 27