I ran over an example of a problem which should determine the list of all non-numeric atoms at any level in a non-linear list.
(Defun Lis(L)
(Cond
((Null L) Nil)
((Not (Numberp (Car L))) (Cons (Car L) (Lis (Cdr L))))
((Atom (Car L)) (Lis (Cdr L)))
(T (Append (Lis (Car L)) (Lis (Cdr L))))
))
I took an example, (Lis '(1 A ((B) 6) (2 (C 3)) D 4)) which should return (A B C D)
Now I don't understand how can the list be created when the 3rd element of the list is evaluated ((B) 6).It will enter on the 2nd branch and do the cons?But that isn't constructing the new list with ((B) 6)?When will it enter on the last branch? I'm a little confused of how this algorithm works,can somebody make it clear for me?
The code works fine if you "invert" the 2 middle tests:
(defun lis(L)
(cond
((null L) nil)
((numberp (car L)) (lis (cdr L)))
((atom (car L)) (cons (car L) (lis (cdr L))))
(t (append (lis (car L)) (lis (cdr L))))))
because (not (numberp (car L))) is also true for lists so in the initial version the code never recurses down into a sublist.
I would write it as:
(defun tree-keep-if (predicate tree)
"Returns the list of all non-numeric atoms at any level in a cons tree."
(mapcan (lambda (item)
(cond ((consp item) (tree-keep-if predicate item))
((funcall predicate item) (list item))
((atom item) nil)))
tree))
Using it:
CL-USER > (tree-keep-if (complement #'numberp) '(1 A ((B) 6) (2 (C 3)) D 4))
(A B C D)
A more sophisticated version might remove the recursion to not be limited by stack size.
Related
I've got a homework assignment that has stumped me! I have to create a function goo(A L) that will remove every A in L and it has to work on nested lists also.
Here's what I've got so far
(defun goo(A L)
(cond ((null L) nil) ; if list is null, return nil
(T ; else list is not null
(cond ((atom (car L))) ;if car L is an atom
((cond ((equal A (car L)) (goo A (cdr L))) ;if car L = A, call goo A cdr L
(T (cons (car L) (goo A (cdr L)))))) ;if car L != A,
(T (cons (goo A (car L)) (goo A (cdr L)))))) ;else car L is not atom, call goo on car L and call goo on cdr L
))
This function returns True no matter what I give it.
You parens are messed up. Move the last paren around (atom (car L)) to include the next cond expression. I suggest using an IDE which shows matching parens.
As for styling, if you didn't know, cond can accept multiple clauses. This way you don't need to have the t and then the cond again. You can also use 'if' if you are only testing a single predicate and making a decision based solely on that.
Note: this was originally posted as an edit to the question by the original asker in revision 2.
I tried another approach and it's working now.
(defun goo(A L)
(cond ((null L) nil)
((atom (car L)) (cond ((equal A (car L)) (goo A (cdr L)))
(T (cons (car L) (goo A (cdr L))))))
(T (cons (goo A (car L)) (goo A (cdr L))))
))
Note 2: this should conventionally be formatted like this to show the program structure:
(defun goo (a l)
(cond ((null l) nil)
((atom (car l))
(cond ((equal a (car l))
(goo a (cdr l)))
(t (cons (car l)
(goo a (cdr l))))))
(t (cons (goo a (car l))
(goo a (cdr l))))))
I think it might be easier to look at this a replacement into trees problem. It's easy to define a function that takes a tree and replaces subtrees in it that satisfy a test. There's a standard function subst-if that does that, but it replaces every matching subtree with the same thing. It will be more useful to us if we replace the element with a value computed from the subtree:
(defun %subst-if (new test tree)
"Replace subtrees of TREE that satisfy TEST with the result
of calling NEW with the subtree."
(cond
;; If tree satifies the test, return (new tree).
((funcall test tree)
(funcall new tree))
;; If tree is a cons, recurse.
((consp tree)
(cons (%subst-if new test (car tree))
(%subst-if new test (cdr tree))))
;; Otherwise, just return the leaf.
(tree)))
With this, its easy to define the kind of function we need. When an element X appears somewhere in a nested list structure, it means that there is a cons cell whose car is X. We want to replace that cons cell with its cdr, but to also recurse on the cdr of the cell. This isn't hard:
(defun replace* (x list &key (test 'eql))
"Remove occurrences of X in LIST and its sublists."
(%subst-if
(lambda (cons)
"Replace elements of the form (X . more) with
(replace* x more :test test)."
(replace* x (cdr cons) :test test))
(lambda (subtree)
"Detect subtrees of the form (X . more)."
(and (consp subtree)
(funcall test x (car subtree))))
list))
(replace* 'a '(1 a (2 a 3) a 4 a 5))
;=> (1 (2 3) 4 5)
I'm new to lisp, and am writing code for quicksort. I am almost done, although the output is giving me some trouble. This is currently what I have:
(defun fil(P L)
(if (null L) nil
(if (funcall P (first L)) (cons (first L) (fil P (rest L)))
(fil P (rest L)))))
(defun qs(L)
(if (null L) nil
(let ((x (first L))
(gt (fil (lambda (x) (<= (first L) x))(rest L) ))
(lt (fil (lambda (x) (> (first L) x))(rest L))))
(cons (cons (qs lt) (first L)) (qs gt)))))
(write (qs '(4 2 3 1 7 3 5 3 6)))
This works, but the output looks like this:
((((((NIL . 1)) . 2) (NIL . 3) (NIL . 3) (NIL . 3)) . 4)
(((NIL . 5) (NIL . 6)) . 7))
I am not sure where the extra nils and periods and parentheses are coming from or how to fix it. Any advice is appreciated.
Look at
(cons '(a b c d) 'e)
Above code does not append E to the list.
CL-USER 4 > (cons '(a b c d) 'e)
((A B C D) . E)
It creates a new cons cell (a two element container) with the first arg and the second arg with its elements.
What you need, is to APPEND lists into a result list.
Adding to what #RainerJoswig said:
(defun %filter (pred l)
(cond ((null l) nil)
((funcall pred (car l)) (cons (car l) (%filter pred (cdr l))))
(t (%filter pred (cdr l)))))
(defun quicksort (l)
(cond ((null l) nil)
(t (let ((greater-than (%filter (lambda (x) (<= (car l) x)) (cdr l)))
(less-than (%filter (lambda (x) (> (car l) x)) (cdr l))))
(append (quicksort less-than) (list (car l)) (quicksort greater-than))))))
(quicksort '(4 2 3 1 7 3 5 3 6))
;; (1 2 3 3 3 4 5 6 7)
Alternatively also:
(defun %filter (pred l)
(mapcan (lambda (x) (if (funcall pred x) (list x) nil)) l))
(defun quicksort (l)
(cond ((null l) nil)
(t (append (quicksort (%filter (lambda (x) (< x (car l))) (cdr l)))
(list (car l))
(quicksort (%filter (lambda (x) (<= (car l) x)) (cdr l)))))))
This is my function that's supposed to implement infix evaluation for * and + operations.
(defun calculate(l)
(cond
((eql (cadr l) '+) (+ (car l) (cddr l)))
((eql (cadr l) '*) (- (car l) (cddr l)))
)
)
When I run this with the list '(3 + 4) it gives me an error saying "(4) is not a number". Any ideas what the problem might be?
Symbols can be called as functions. Thus your code is just this:
(defun calculate (l)
(funcall (second l) (first l) (third l)))
or
(defun calculate (l)
(destructuring-bind (arg1 op arg2)
l
(funcall op arg1 arg2)))
Example:
CL-USER 77 > (calculate '(20 + 30))
50
The part with (cddr l) should be (caddr l). You have to access the first element of the list, not the list. The code should be then:
(defun calculate(l)
(cond
((eql (cadr l) '+) (+ (car l) (caddr l)))
((eql (cadr l) '*) (- (car l) (caddr l)))
)
)
I've got a homework assignment that has stumped me! I have to create a function goo(A L) that will remove every A in L and it has to work on nested lists also.
Here's what I've got so far
(defun goo(A L)
(cond ((null L) nil) ; if list is null, return nil
(T ; else list is not null
(cond ((atom (car L))) ;if car L is an atom
((cond ((equal A (car L)) (goo A (cdr L))) ;if car L = A, call goo A cdr L
(T (cons (car L) (goo A (cdr L)))))) ;if car L != A,
(T (cons (goo A (car L)) (goo A (cdr L)))))) ;else car L is not atom, call goo on car L and call goo on cdr L
))
This function returns True no matter what I give it.
You parens are messed up. Move the last paren around (atom (car L)) to include the next cond expression. I suggest using an IDE which shows matching parens.
As for styling, if you didn't know, cond can accept multiple clauses. This way you don't need to have the t and then the cond again. You can also use 'if' if you are only testing a single predicate and making a decision based solely on that.
Note: this was originally posted as an edit to the question by the original asker in revision 2.
I tried another approach and it's working now.
(defun goo(A L)
(cond ((null L) nil)
((atom (car L)) (cond ((equal A (car L)) (goo A (cdr L)))
(T (cons (car L) (goo A (cdr L))))))
(T (cons (goo A (car L)) (goo A (cdr L))))
))
Note 2: this should conventionally be formatted like this to show the program structure:
(defun goo (a l)
(cond ((null l) nil)
((atom (car l))
(cond ((equal a (car l))
(goo a (cdr l)))
(t (cons (car l)
(goo a (cdr l))))))
(t (cons (goo a (car l))
(goo a (cdr l))))))
I think it might be easier to look at this a replacement into trees problem. It's easy to define a function that takes a tree and replaces subtrees in it that satisfy a test. There's a standard function subst-if that does that, but it replaces every matching subtree with the same thing. It will be more useful to us if we replace the element with a value computed from the subtree:
(defun %subst-if (new test tree)
"Replace subtrees of TREE that satisfy TEST with the result
of calling NEW with the subtree."
(cond
;; If tree satifies the test, return (new tree).
((funcall test tree)
(funcall new tree))
;; If tree is a cons, recurse.
((consp tree)
(cons (%subst-if new test (car tree))
(%subst-if new test (cdr tree))))
;; Otherwise, just return the leaf.
(tree)))
With this, its easy to define the kind of function we need. When an element X appears somewhere in a nested list structure, it means that there is a cons cell whose car is X. We want to replace that cons cell with its cdr, but to also recurse on the cdr of the cell. This isn't hard:
(defun replace* (x list &key (test 'eql))
"Remove occurrences of X in LIST and its sublists."
(%subst-if
(lambda (cons)
"Replace elements of the form (X . more) with
(replace* x more :test test)."
(replace* x (cdr cons) :test test))
(lambda (subtree)
"Detect subtrees of the form (X . more)."
(and (consp subtree)
(funcall test x (car subtree))))
list))
(replace* 'a '(1 a (2 a 3) a 4 a 5))
;=> (1 (2 3) 4 5)
(defun rep(list)
(format t"~a~%" list)
(cond
((null list) nil)
((atom (car list)) (cons (car list) (rep (cdr list))))
((listp (car list)) (cons (car (reverse (car list))) (cdr list)))
(t (rep list))
)
)
Write a function to replace each sublist of a list with its last element.
A sublist is an element from the first level, which is a list.
Example:
(a (b c) (d (e (f)))) ==> (a c (e (f))) ==> (a c (f)) ==> (a c f)
(a (b c) (d ((e) f))) ==> (a c ((e) f)) ==> (a c f)
I have the above problem to solve. Got it till one point but I'm stuck.
Apparently it doesn't go to the next elements in the list and I don't know why. Any ideas?
I would break it down like this:
(defun last-element (lst)
(if (listp lst)
(last-element (car (last lst)))
lst))
(defun rep (lst)
(when lst
(cons (last-element (car lst)) (rep (cdr lst)))))
then
(rep '(a (b c) (d (e (f)))))
=> '(A C F)
Did it without using map functions
(defun rep(list)
(cond
((null list) nil)
((listp (car list)) (rep (cons (car (reverse (car list))) (rep (cdr list)))))
(t (cons (car list) (rep (cdr list))))
)
)