4rec, Common Lisp - rextester

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;gnu clisp  2.49.60
#|Про каждого студента известная следующая информация: 
    какой предмет сдавал, каковы были результаты (оценки от 2 до 5), когда происходил экзамен или пересдача. 
Написать функцию, которая по заданным предметам упорядочивает список студентов по возрастанию количества пересдач.
|#

(defun slice-subject (students subject)
    (cond
        ((null students) nil)
        ((and (string= (second(first students)) subject) ( string= (fifth (first students)) "retaking")) 
             (cons (first students) (slice-subject (rest students) subject)) )
        (t (slice-subject (rest students) subject))
     )
)

(defun students-retakes (students)
    (cond
        ((null students) nil)
        (t (cons (list (first (first students)) (count-if #'(lambda (x) (string= x (first (first students)))) students :key #'first) ) 
                       (students-retakes (remove-if #'(lambda (x) (string= x (first (first students)))) students :key #'first))))
        
        )
    )

(defun find-max (students st)
    (cond
        
        ((null students) st)
        ((< (second st) (second (first students))) (find-max (rest students) (first students)))
        (t (find-max (rest students) st))
    )
    )

(defun sort-retakes (students) 
    (cond 
        ((null students) nil)
        (t (cons (find-max (rest students) (first students)) (sort-retakes (remove (find-max (rest students) (first students)) students))))
        
        )
    
    )

(let (students))
(setf students '(("Magalish" "Algorithms" 5 (20 5 2020) "exam") ("Magalish" "Algorithms" 2 (20 5 2020) "retaking") ("Magalish" "Algorithms" 53 (20 10 2020) "retaking")
                  ("Skopina" "Algorithms" 2 (20 5 2020) "retaking")
                    ("Magalish" "Modeling" 2 (20 6 2020) "exam") ("Magalish" "Modeling" 4 (20 6 2020) "retaking") ("Magalish" "Modeling" 4 (21 6 2020) "retaking")  ("Magalish" "Modeling" 4 (22 6 2020) "retaking") 
                   ("Skopina" "Modeling" 2 (20 6 2020) "exam") ("Skopina" "Modeling" 4 (20 6 2020) "retaking") ("Skopina" "Modeling" 4 (23 6 2020) "retaking") ))
(print "Студенты: ")
(print students)
(print "__________________________________________")
(let (subject)) (setf subject "Modeling")
(print "Предмет: ")
(print subject)
(print "__________________________________________")
;(print "Упорядоченный по количеству пересдач список: ")
(print (sort-retakes (students-retakes (slice-subject students subject))))
(print "__________________________________________")
;(let (name)) (setf name "Skopina")
;(print (remove-if #'(lambda (x) (string= x name)) students :key #'first))
;(print (count-if #'(lambda (x) (string= x name)) students :key #'first))

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