Common lisp exercise , assing a value to string

Question

I have been working on this problems, " make a function that given two months, say if the first month appears before the second in the calendar printing true " . I can only use , cons, append, list, equal, defun, car, cdr, cond , T , NIL

I though the best way to do this is , assing a value to each of the months , and make separated functions , one funtions for the months and another one that verify the month.

Here my code: I wont paste all of it just cause im still testing: I did it this way , not sure if I assigned the correct value to the month , cause im not sure how to assing a value to a string.

 (defun months(month)
             (cond ((null month)0)
                   ((= month (format t "january") 1))
                    ((= month (format t "february") 2))
                             nil ))

    the other function:

    (defun check-month(month1 month2)
        (if (< (months month1) (months month2))
            T
            NIL
        )   
    )

and when I called the function check-month it says:

 (check-month "january" "february")

*** - IF: variable months has no value


The actual result should be like this: 
  (check-month “january” ”february”)
      T

Nowhere in the problem statement is there a reference to strings, are you sure you aren't making this more difficult than necessary? — coredump
your code makes a bunch of basic errors. This book might help to learn the basics of Lisp: cs.cmu.edu/~dst/LispBook You can download a PDF from there. — Rainer Joswig

Unknown Unknown · Accepted Answer · 2019-10-12T13:27:53

This isn't an answer in the form of code, since I don't want to do your homework for you.

First of all: the error you mention clearly comes from some code other than the code in your question: it is always a good idea to actually check the code you think you are checking.

Secondly, what purpose does an expression like (if (f ...) t nil) serve? The only useful purpose is if you want to canonicalize the return value of (f ...) to t or nil. But what does (equal a b) return?

Secondly: if you are really only allowed to use cons, append, list, equal, defun, car, cdr, cond t and nil then you are not allowed to use <, or =, or a number of other functions and other operators that you have used. So you have an interesting problem to solve.

An approach to solving it is to write a function called first-in-list: this takes three arguments, and here's its specification:

(first-in-list a b l) returns whichever of a or b is first in l, comparing with equal, or nil if neither of them is (and it's up to you what it returns if a and b are the same).

You can write this function with cond, car, cdr, equal, nil, t and an auxiliary function called first-in-list which you will have to write.

Now you can write a function month-before-p which takes two arguments ma and mb and:

checks if ma is equal to mb and returns nil if so (this avoids the ambiguity in first-in-list's specification;
checks if (first-in-list ma mb (list "january" ....)) is equal to ma or mb;
for added value could check if the result of the call to first-in-list was nil, which is an error.

This function can also be written with cond, equal, t, and nil, as well as the auxilliary function first-in-list you have already written.

Common lisp exercise , assing a value to string

2 Answers