How can I get the sum of items in a multislot - clips

Question

I have a template like the one shown bellow. How can I get the sum of items in the multislot grades?
(deftemplate student
(multislot name)
(multislot grades)
)

Answer 1

Here's one way to do it. In the reg6 rule, the + function is given two arguments of 0 in addition to the grades to insure that the + function allows has at least 2 arguments; otherwise, if there were zero or one grades for the student you'd get an error.
CLIPS (6.31 2/3/18)
CLIPS>
(deftemplate student
(multislot name)
(multislot grades))
CLIPS>
(deftemplate sum
(multislot name)
(slot grade))
CLIPS>
(defrule reg6
(student (name $?name)
(grades $?grades))
=>
(assert (sum (name ?name)
(grade (+ 0 0 (expand$ ?grades))))))
CLIPS>
(assert (student (name David Green) (grades))
(student (name Sue Brown) (grades 90))
(student (name Frank Black) (grades 85 75)))
<Fact-3>
CLIPS> (run)
CLIPS> (facts)
f-0 (initial-fact)
f-1 (student (name David Green) (grades))
f-2 (student (name Sue Brown) (grades 90))
f-3 (student (name Frank Black) (grades 85 75))
f-4 (sum (name Frank Black) (grade 160))
f-5 (sum (name Sue Brown) (grade 90))
f-6 (sum (name David Green) (grade 0))
For a total of 7 facts.
CLIPS>

Answer 2

You can use the expand$ function. Check in the Basic Programming Guide the Multifield Expansion Function chapter to know more.
(deftemplate student
(multislot name)
(multislot grades))
(defrule grades-sum
(student (grades $?grades))
=>
(printout t "Student grades sum is " (+ (expand$ ?grades))))
(assert (student (grades (create$ 1 2 3 4 5))))
(student (name) (grades 1 2 3 4 5))
(run)
Student grades sum is 15