Question

how can I find a list from given database??
Database:

(10 math phys)

(11 math chem)

(13 bio chem)

(15 geo phys)

I want to implement time which shows the first and second exams in a list.

>(time ’10)

(list ‘math ‘phys)

>(time ’19)

empty

And I want to implement secondExams which returns the time of second exams.

>(secondExams ‘phys)

(list ‘10 ’15)

>(secondExams ‘chem)

(list ’11 ’13)

thank you

Answer 1

First, let’s clarify your database format. Make it a list of lists:

(define database
  '((10 math phys)
    (11 math chem)
    (13 bio chem)
    (15 geo phys)))

Then time just needs to step through your list recursively, and compare the first element of the sub-list with your target value. If we find no matching value, we’ll return the empty list. We’ll do this in a helper procedure to match the functionality you’re precisely seeking.

(define time-ish
  (lambda (target lst)
    (cond ((null? lst) lst)
          ((eq? target (caar lst)) (cdar lst))
          (else
           (time-ish target (cdr lst))))))

(define (time lookin-for)
  (time-ish lookin-for database))

Then we can do something very similar for secondExams. Except we’ll recursively build a list of matches this time.

(define exam-helper
  (lambda (target lst)
    (cond ((null? lst) lst)
          ((eq? target (third (car lst))) (cons (first (car lst))
                                                (exam-helper target (cdr lst))))
          (else
           (exam-helper target (cdr lst))))))

(define (secondExams lookin-for)
  (exam-helper lookin-for database))

I haven’t tested this code, but I’m pretty sure it’ll work for you.