Lisp grade to letter conversion - lisp

Question

Question asked:
Define a LISP function SCORE->GRADE which takes a single argument, s, and returns a symbol according to the following scheme:
s ≥ 90 A 73 ≤ s < 77 C+
87 ≤ s < 90 A– 70 ≤ s < 73 C
83 ≤ s < 87 B+ 60 ≤ s < 70 D
80 ≤ s < 83 B s < 60 F
77 ≤ s < 80 B–
If the argument s is not a number then the function should return NIL.
My answer is this:
(defun SCORE->GRADE (s)
(if (not (numberp s)) (return-from SCORE->GRADE “NIL”))
(progn
(if (>= s 90) (return-from SCORE->GRADE "A"))
(if (and (>= s 87) (< s 90)) (format nil “A-“))
(if (and (>= s 83) (< s 87)) (format nil “B+”))
(if (and (>= s 80) (< s 83)) (return-from SCORE->GRADE “B”))
(if (and (>= s 77) (< s 80)) (return-from SCORE->GRADE “B-“))
(if (and (>= s 73) (< s 77)) (return-from SCORE->GRADE “C+”))
(if (and (>= s 70) (< s 73)) (return-from SCORE->GRADE “C”))
(if (and (>= s 60) (< s 70)) (return-from SCORE->GRADE “D”)
(if (< s 60) (return-from SCORE->GRADE “F”))
)
)
)
It works for 90, returns A, then for anything else it just gives this error, with different variables in regards to what I enter
*** - RETURN-FROM: variable “B” has no value
*** - IF: variable “A-“ has no value
Can anyone explain why I can't get the same result for every line that is incredibly identical?
I've tried message, format t, cases, some work for up to the first 3 cases then stop. Haven't been able to figure anything out.

Answer 1

In addition to other answers, note that in your case you do not need to duplicate common boundaries, since you are trying to partition scores into grades.
(cond
((>= s 90) "A")
((>= s 87) "A-")
((>= s 83) "B+")
...
((>= s 70) "C")
((>= s 60) "D")
(t "F"))
It also reduces the number of invariants you have to keep in your code, which helps with maintenance.

Answer 2

when you fixed your problem with the double quotes, here are some additional hints:
you usually don't need to use RETURN-FROM in Lisp.
checking multiple conditions is best done with COND and not by multiple IF statements.
functions like >=, >, ... can take more than two arguments.
see for example:
(cond ((not (numberp s)) NIL)
((>= s 90) "A" )
((>= 90 s 87) "A-")
((>= 87 s 83) "B+")
((>= 83 s 80) "B" )
;...
((>= 60 s ) "F" ))

Answer 3

It seems that you are using the character “ instead of " to wrap the strings.

Answer 4

I had the same idea as #coredump.
(defun SCORE->GRADE (s)
(cond ((not (numberp s)) nil)
((>= s 90) "A")
((>= s 87) "A-")
((>= s 83) "B+")
((>= s 80) "B")
((>= s 77) "B-")
((>= s 73) "C+")
((>= s 70) "C")
((>= s 60) "D")
(t "F")))
Which again can be slightly more elegantly formulated as:
(defun SCORE->GRADE (s)
(when (numberp s)
(cond ((>= s 90) "A")
((>= s 87) "A-")
((>= s 83) "B+")
((>= s 80) "B")
((>= s 77) "B-")
((>= s 73) "C+")
((>= s 70) "C")
((>= s 60) "D")
(t "F"))))