4, 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)
    (loop for s in students
         if (and (string= (second s) subject) (string= (fifth s) "retaking"))
          collect s
     )   
 )

(defun count-and-remove (students name)
    (let (count))
    (setf count 0)
    (let (new-students))
    (setf new-students (list))
    (loop for s in students
         if  (string= (first s) name)
             do (incf count)
          else
              do (setf new-students (nconc new-students (list s)))
     )
    (list new-students count)
    )

(defun sort-retakes (students subject)
    (setf students (slice-subject students subject))
    (let (name))
    (let (count))
    (let (new))
    (let (retakes))
    (setf retakes (list))
    (loop while (not (null students))
          do (setf name (first (first students)))
          do (setf new (count-and-remove students name))
          do (setf students (first new))
          do (setf retakes (nconc retakes (list (list (second new) name))))
     )
    (sort retakes #'< :key (lambda (el) (first el)))
    )

(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") 
                   ("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 "Algorithms")
(print "Предмет: ")
(print subject)
(print "__________________________________________")
(print "Упорядоченный по количеству пересдач список: ")
(print (sort-retakes students subject))

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