Lisp common elements in a list - runtime-error

We've recently started learning Common Lisp in our class. I am trying to implement a function that takes two list and outputs their common elements. We are restricted to using basic functional forms.
(defun myCommon(L1 L2)
(cond
((null L1) nil) ;;check if the first list is empty
((null L2) nil) ;;check if the second list is empty
((eq (car L1) (car L2)) ((car L1) (myCommon (cdr L1) L2)))
(t (myCommon (cdr L1) L2)) ;;if they are not the same, recurse myCommon with L1 as (cdr L1)
)
)
My problem is that I cannot understand why it results as a type error (offending datum: (CAR L1)). It seems to be expecting a function type as far as I could make sense of it.
Error: TYPE-ERROR :DATUM (CAR L1) :EXPECTED-TYPE FUNCTION
Fast links are on: do (si::use-fast-links nil) for debugging
Signalled by COND.
TYPE-ERROR :DATUM (CAR L1) :EXPECTED-TYPE FUNCTION
Broken at COND.

Your problem is due to the fact that in the second branch of the cond, the expression which is evaluated when the condition about the equality of the two cars is true is: ((car L1) (myCommon (cdr L1) L2))).
Here you have a list with two elements ((car L1) (myCommon...)) in a position where a form is required (that is list with a structure like (function argument1 argument2 ...)). So the system gives an error since (car l1) is not a function. What I suppose you want to do is to produce a new list with those two arguments, which could be obtained for instance by “consing” them, like in (cons (car L1) (myCommon...)), so this is your function rewritten:
(defun myCommon(L1 L2)
(cond ((null L1) nil) ;;check if the first list is empty
((null L2) nil) ;;check if the second list is empty
((eq (car L1) (car L2)) (cons (car L1) (myCommon (cdr L1) L2)))
(t (myCommon (cdr L1) L2)))) ;;if they are not the same, recurse myCommon with L1 as (cdr L1)
Note that if you try this function for instance on: (mycommon '(1 2 3) '(1 2 3 4) you will find that the answer is (1) and not (1 2 3). This is because you are “consuming” only the first argument (by recurring on it with (cdr L1)). So, you need to correct the function “consuming” also the second argument when necessary.

Related

Palindrome check with recursion in Lisp

I have developed code to check through input to see if it is a palindrome or not but I am having difficulty figuring out how to print the output. I want the output to return "t" if the input is a palindrome and "nil" if not. Also a challenge that I wanted to give myself was to not use the reverse function so thats why my code is not as simple as it could be. Thanks in advance.
(defun palindromep(l)
(cond ((null l) nil (write nil))
(t (append (list (car l)) (palindromep (cdr l)) (list (car l) )))))
(palindromep '(a b b a))
(terpri)
(palindromep '(a b c b a))
(terpri)
(palindromep '(a b c))
(terpri)
(palindromep '(a (d e) (d e) a))
(terpri)
(palindromep '(a (d e) (e d) a))
Firstly, an empty list is a palindrome! If we reverse it, we get the same empty list.
Secondly, Lisp functions don't print their result values; they return these values.
In an interactive session, it is the listener which prints the resulting value(s) that emerge from the expression being evaluated. That expression itself doesn't have to print anything.
Therefore, we begin like this:
(defun palindromep (l)
(cond
((null l) t) ;; the empty list is a palindrome: yield true.
Note, by the way, that if we write this:
((null l) nil t) ;; the empty list is a palindrome: yield true.
that doesn't do anything. The extra nil expression is evaluated, producing nil, which is thrown away. The Lisp compiler will completely eliminate that.
What if the list is not a list at all, but an atom other than nil? Let's just go with that being a palindrome. A clarification of requirements is needed, though:
((atom l) t)
Now we know we are dealing with a non-empty list. If it has exactly one item, then it is a palindrome:
((null (cdr l)) t)
Now we know we are dealing with a list of two or more items. That is a palindrome if the first and last items are the same, and if the items in between them form a palindrome.
(t (let* ((first (car l))
(rest (cdr l))
(tail (last l))
(interior (ldiff rest tail)))
(and (eql first (car tail)) (palindromep interior))))))
The whole thing:
(defun palindromep (l)
(cond
((null l) t)
((atom l) t)
((null (cdr l)) t)
(t (let* ((first (car l))
(rest (cdr l))
(tail (last l))
(interior (ldiff rest tail)))
(and (eql first (car tail)) (palindromep interior))))))
Code golfing: in the cond construct described by ANSI CL, a clause is permitted to have just one form. If that forms yields a true value, then that value is returned. Thus we can remove the t's:
(defun palindromep (l)
(cond
((null l)) ;; t removed
((atom l)) ;; likewise
((null (cdr l))) ;; likewise
(t (let* ((first (car l))
(rest (cdr l))
(tail (last l))
(interior (ldiff rest tail)))
(and (eql first (car tail)) (palindromep interior))))))
Documentation about the functions ldiff and last can be found here.
Further golfing: if we have this pattern (cond (A) (B) ... (t Z)) we can just replace it by (or A B ... Z):
(defun palindromep (l)
(or (null l)
(atom l)
(let* ((first (car l))
(rest (cdr l))
(tail (last l))
(interior (ldiff rest tail)))
(and (eql first (car tail)) (palindromep interior)))))
cond is like a generalization of or that can specify an alternative result value for the each terminating true case.
To go on with code-golfing, since t or nil is expected, you can use only or and nil to express conditionals (and using short-circuitry of or and nil expressions).
Also it is good to be able to determine a :test keyword - since you want to control the crucial testing behavior.
To be able to use also inner lists, one could e.g. use equalp or even a custom comparison function.
(defun palindromep (l &key (test #'equalp))
(or (null l) (and (funcall test (car l) (car (last l)))
(palindromep (butlast (cdr l)) :test test))))
This evaluates
(palindromep '(a (d e) (d e) a))
as t but
(palindromep '(a (d e) (e d) a))
as nil.
Well, it is maybe a philosophical question, whether the latter should be t and the former nil.
To revert that behavior, we could write a custom testing function.
Like this:
(defun reverse* (l &optional (acc '()))
(cond ((null l) acc)
((atom (car l)) (reverse* (cdr l) (cons (car l) acc)))
(t (reverse* (cdr l) (cons (reverse* (car l) '()) acc)))))
(defun to-each-other-symmetric-p (a b)
(cond ((and (atom a) (atom b)) (equalp a b))
(t (equalp a (reverse* b)))))
Well, I use here some kind of a reverse*.
Then if one does:
(palindromep '(a (d e) (d e) a) :test #'to-each-other-symmetric-p) ;; NIL
;; and
(palindromep '(a (d e) (e d) a) :test #'to-each-other-symmetric-p) ;; T
Just to complete the other answers, I would like to point out that not using reverse will not only complicate your code enormously, but also make it far more inefficient. Just compare the above answers with the classic one:
(defun palindromep (l)
(equal l (reverse l)))
reverse is o(l), i.e. it takes time proportional to the length of the list l, and so does equal. So this function will run in o(l). You can't get faster than this.

Weird syntax in common lisp

I found this lisp function while I was googling
(defun filter (lst items-to-filter)
(cond ((null lst) nil)
((member (car lst) items-to-filter) #1=(filter (cdr lst) items-to-filter))
(t (cons (car lst) #1#))))
It's just set difference, but this is the first time i see #1= and #1#, syntax. I think I understand what it means just by looking at the code, but I am not too sure. I think the #1= is used to label an expression so as not to retype it later when needed, one can just refer to it by #index#, in this case index=1. I was wondering if someone could shed some light on this. What are these constructs called, if there's a reference for them, and if they are widely used in modern lisp code. Thanks
To see it in written source code is very very unusual. Most of the time you see it in data. It is used to create or print shared data items in s-expressions. This way you can also read or print circular s-expressions.
You could use it for easier creation of repeated code, but usually one writes functions or macros for that. Functions have the advantage that they save code space - unless they are inlined.
CL-USER 3 > (pprint '(defun filter (lst items-to-filter)
(cond ((null lst) nil)
((member (car lst) items-to-filter)
#1=(filter (cdr lst) items-to-filter))
(t (cons (car lst) #1#)))))
(DEFUN FILTER (LST ITEMS-TO-FILTER)
(COND ((NULL LST) NIL)
((MEMBER (CAR LST) ITEMS-TO-FILTER)
(FILTER (CDR LST) ITEMS-TO-FILTER))
(T
(CONS (CAR LST) (FILTER (CDR LST) ITEMS-TO-FILTER)))))
As you see above the printer does not print it that way. Why is that?
There is a global variable *print-circle* which controls it. For above example it was set to NIL. Let's change that:
CL-USER 4 > (setf *print-circle* t)
T
CL-USER 5 > (pprint '(defun filter (lst items-to-filter)
(cond ((null lst) nil)
((member (car lst) items-to-filter)
#1=(filter (cdr lst) items-to-filter))
(t (cons (car lst) #1#)))))
(DEFUN FILTER (LST ITEMS-TO-FILTER)
(COND ((NULL LST) NIL)
((MEMBER (CAR LST) ITEMS-TO-FILTER)
#1=(FILTER (CDR LST) ITEMS-TO-FILTER))
(T
(CONS (CAR LST) #1#))))
So this shows that one can read and print such s-expressions in Common Lisp
Sharing some source code data structures is more common in computed code:
CL-USER 22 > (defmacro add-1-2-3 (n) `(,n 1 2 3))
ADD-1-2-3
CL-USER 23 > (walker:walk-form '(+ (add-1-2-3 4) (add-1-2-3 5)))
(+ (4 . #1=(1 2 3)) (5 . #1#))

Return value in Lisp

So i started learning Lisp yesterday and started doing some problems.
Something I'm having a hard time doing is inserting/deleting atoms in a list while keeping the list the same ex: (delete 'b '(g a (b) l)) will give me (g a () l).
Also something I'm having trouble with is this problem.
I'm suppose to check if anywhere in the list the atom exist.
I traced through it and it says it returns T at one point, but then gets overriden by a nil.
Can you guys help :)?
I'm using (appear-anywhere 'a '((b c) g ((a))))
at the 4th function call it returns T but then becomes nil.
(defun appear-anywhere (a l)
(cond
((null l) nil)
((atom (car l))
(cond
((equal (car l) a) T)
(T (appear-anywhere a (cdr l)))))
(T (appear-anywhere a (car l))(appear-anywhere a (cdr l)))))
Let's look at one obvious problem:
(defun appear-anywhere (a l)
(cond
((null l) nil)
((atom (car l))
(cond
((equal (car l) a) T)
(T (appear-anywhere a (cdr l)))))
(T (appear-anywhere a (car l))(appear-anywhere a (cdr l)))))
Think about the last line of above.
Let's format it slightly differently.
(defun appear-anywhere (a l)
(cond
((null l) nil)
((atom (car l))
(cond
((equal (car l) a) T)
(T (appear-anywhere a (cdr l)))))
(T
(appear-anywhere a (car l))
(appear-anywhere a (cdr l)))))
The last three lines: So as a default (that's why the T is there) the last two forms will be computed. First the first one and then the second one. The value of the first form is never used or returned.
That's probably not what you want.
Currently your code just returns something when the value of a appears anywhere in the rest of the list. The first form is never really used.
Hint: What is the right logical connector?

need to combine results of two functions in Scheme with a driver

I have a Scheme assignment in which a user is to input a list of numbers, and the output should be the max value and min value from the list. The assignment says we can have two separate functions, and combine the result with a driver, but I don't know how to do this. Here is what I have so far:
(define (findmin l) (if (null? (cdr l)) (car l)
(if (< (car l) (findmin (cdr l)))(car l)
(findmin (cdr l)))))
(define (findmax l) (if (null? (cdr l)) (car l)
(if (> (car l) (findmax (cdr l)))(car l)
(findmax (cdr l)))))
I can't seem to get around having to input a list for findmin, and another list for findmax. The user should only have to input one list.
driver:
(define (min-and-max l) (list (findmin l) (findmax l)))

Scheme: sequential execution

I need to do something basically like this:
(define test
(λ (ls1 ls2)
(cond
((empty? ls2) null)
(else
(append ls1 (car ls2)) (test ls1 (cdr ls2))) (displayln ls1))))
The issue is the else-clause and the function that follows it. I need both clauses of the else-clause to execute and then I need the last function to execute but I can't get the syntax right.
I need (test '(1 2 3) '(4 5 6)) to result in displaying '(1 2 3 4 5 6) and it has to use the recursive call.
Any advice is appreciated.
Thanks.
There is several problem there. First you make an append on a list and an atom (not a list)... at least if (car l2) is not a list. Second you probably think that (append l1 (list (car l2)) modifies l1. But this is not the case. The result is a new list.
To sequence your operation you can do as larsmans have said. But you can also write the following
(define (test l1 l2)
(if (null? l2)
(displayln l1)
(let ((l1-follow-by-car-l2 (append l1 (list (car l2)))))
(test l1-follow-by-car-l2 (cdr l2)) ))
This has exactly the same behavior.
If you desperately want to solve this recursively, and you don't care about the return value, use
(define (test l1 l2)
(if (null? l2)
(displayln l1)
(test (append l1 (list (car l2))) (cdr l2))))
(This is very inefficient btw: O(n × m) memory allocations.)

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