I've a question, how to return a list without the nth element of a given list? E.g., given list: (1 2 3 2 4 6), and given n = 4, in this case the return list should be (1 2 3 4 6).
A simple recursive solution:
(defun remove-nth (n list)
(declare
(type (integer 0) n)
(type list list))
(if (or (zerop n) (null list))
(cdr list)
(cons (car list) (remove-nth (1- n) (cdr list)))))
This will share the common tail, except in the case where the list has n or more elements, in which case it returns a new list with the same elements as the provided one.
Using remove-if:
(defun foo (n list)
(remove-if (constantly t) list :start (1- n) :count 1))
butlast/nthcdr solution (corrected):
(defun foo (n list)
(append (butlast list (1+ (- (length list) n))) (nthcdr n list)))
Or, maybe more readable:
(defun foo (n list)
(append (subseq list 0 (1- n)) (nthcdr n list)))
Using loop:
(defun foo (n list)
(loop for elt in list
for i from 1
unless (= i n) collect elt))
Here's an interesting approach. It replaces the nth element of a list with a new symbol and then removes that symbol from the list. I haven't considered how (in)efficient it is though!
(defun remove-nth (n list)
(remove (setf (nth n list) (gensym)) list))
(loop :for i :in '(1 2 3 2 4 6) ; the list
:for idx :from 0
:unless (= 3 idx) :collect i) ; except idx=3
;; => (1 2 3 4 6)
loop macro can be very useful and effective in terms of generated code by lisp compiler and macro expander.
Test run and apply macroexpand above code snippet.
A slightly more general function:
(defun remove-by-position (pred lst)
(labels ((walk-list (pred lst idx)
(if (null lst)
lst
(if (funcall pred idx)
(walk-list pred (cdr lst) (1+ idx))
(cons (car lst) (walk-list pred (cdr lst) (1+ idx)))))))
(walk-list pred lst 1)))
Which we use to implement desired remove-nth:
(defun remove-nth (n list)
(remove-by-position (lambda (i) (= i n)) list))
And the invocation:
(remove-nth 4 '(1 2 3 2 4 6))
Edit: Applied remarks from Samuel's comment.
A destructive version, the original list will be modified (except when n < 1),
(defun remove-nth (n lst)
(if (< n 1) (cdr lst)
(let* ((p (nthcdr (1- n) lst))
(right (cddr p)))
(when (consp p)
(setcdr p nil))
(nconc lst right))))
That's elisp but I think those are standard lispy functions.
For all you haskellers out there, there is no need to twist your brains :)
(defun take (n l)
(subseq l 0 (min n (length l))))
(defun drop (n l)
(subseq l n))
(defun remove-nth (n l)
(append (take (- n 1) l)
(drop n l)))
My horrible elisp solution:
(defun without-nth (list n)
(defun accum-if (list accum n)
(if (not list)
accum
(accum-if (cdr list) (if (eq n 0) accum (cons (car list) accum))
(- n 1))))
(reverse (accum-if list '() n)))
(without-nth '(1 2 3) 1)
Should be easily portable to Common Lisp.
A much simpler solution will be as follows.
(defun remove-nth (n lst)
(append (subseq lst 0 (- n 1)) (subseq lst n (length lst)))
)
Related
This code in Scheme is used to output a list in which the element of the input lists are repeated n times. I do not understand what is happening in the code.
(define (echo-lots lst n)
(define (helper lst1 n1)
(if (= n1 0)
'()
(cons lst1 (helper lst1 (- n1 1)))))
(if (null? lst)
'()
(append (helper (car lst) n) (echo-lots (cdr lst) n)))
First, note that helper does not use any of the parameters of echo-lots.
That means that you can move the definition outside and figure it out in isolation:
(define (helper lst1 n1)
(if (= n1 0)
'()
(cons lst1 (helper lst1 (- n1 1)))))
> (helper 1 3)
'(1 1 1)
> (helper #f 4)
'(#f #f #f #f)
> (helper '(a b c) 2)
'((a b c) (a b c))
It makes a list whose elements are the first parameter repeated the number of times given by the second parameter.
(It's not difficult to prove this formally, if you're in the mood for that.)
Note also that the first parameter's name is misleading - it does not have to be a list.
Let's improve the naming:
(define (repeat item n)
(if (= n 0)
'()
(cons item (repeat item (- n 1)))))
Now echo-lots is clearer:
(define (echo-lots lst n)
(if (null? lst)
'()
(append (repeat (car lst) n)
(echo-lots (cdr lst) n))))
and it's easier to see that it makes a list of n copies of the first element of lst - (repeat (car lst) n) - and appends that to a list consisting of n copies of each of the remaining elements - (echo-lots (cdr lst) n).
I'm trying to get a list with the average of the following n elements. I'm reading a csv file that has 7 columns im just using the 6th one that has number values in order to get the average.
This is the code
;Function that returns a list containing the values of the desired column
(define (get-column col)
(let loop
([file (cdr(all-rows csv-path read-csv))]
[result empty])
(if (empty? file)
result
(loop (cdr file)
(cond
[(equal? col 1) (append result (list (caar file)))]
[(equal? col 2) (append result (list (string->number(cadar file))))]
[(equal? col 3) (append result (list (string->number(caddar file))))]
[(equal? col 4) (append result (list (string->number(car (cdddar file)))))]
[(equal? col 5) (append result (list (string->number(cadr (cdddar file)))))]
[(equal? col 6) (append result (list (string->number(caddr (cdddar file)))))]
[(equal? col 7) (append result (list (string->number(car (cdddr (cdddar file))))))]
)))))
(define (suma-SMA col n)
(let loop
([n n]
[res 0]
[col col])
(if (zero? n)
res
(loop (sub1 n) (+ res (car col)) (cdr col)))))
(define (get-SMA days)
(let loop
([col (get-column 6)]
[result empty])
(if (empty? col)
result
(loop (cdr col)(append result (list (suma-SMA col days)))))))
Here's a function that does what you asked for in the comments, e.g. given (1 2 3 4) it produces ((1+2)/2 (2+3)/2 (3+4)/2).
(define (sum list)
(cond
((null? list)
'()) ;; error?
((null? (cdr list))
'())
(else
(cons (/ (+ (car list) (cadr list)) 2) (sum (cdr list))))))
I'm still a bit confused because even the combination of get-SMA and suma-SMA does nothing like this. It's completely unclear what the days variable is doing, as you can see I didn't need it in my code above.
So I may have misunderstood what you are trying to do, but the function above does what you actually asked for so hopefully it will be helpful.
Here is the answer that I found useful for my problem.
(define (sum list n)
(cond
((null? list)
'()) ;; error?
((null? (cdr list))
'())
(else
(cons (suma-SMA list n) (sum (cdr list) n)))))
I'm begginer at LISP, and I have a question need your help.
Write a function COUNT-NUMBERS that counts the number of numbers in a list,and return " NO NUMBER" if there is no number in the list
For example, for a list: (A 2.3 B C 4 5), it returns 3.
I've tried with the following code, but it doesn't work . Could you help me to figure out? Moreover, I don't know how to return "NO NUMBER" if there is no number in the list.
(defun count-numbers (x)
(cond ((null x) 0)
((numberp x) 1)
(t (+(count-numbers (car x))(count-numbers (cdr x))))))
Thanks in advance,
You could to define a inner helper function to do the counting, and check the result to decide what to return in the main function:
(defun number-counter (lst)
(labels ((do-count (l)
(cond ((null l) 0)
((numberp (car l)) (+ 1 (do-count (cdr l))))
(t (do-count (cdr l))))))
(let ((r (do-count lst)))
(if (= r 0) 'NO-NUMBER r))))
This would be a tail-recursive version. Somehow you have to check what to return.
(defun count-numbers (list &optional (n 'no-number))
(cond ((null list) n)
((numberp (first list))
(count-numbers (rest list)
(if (eq n 'no-number)
1
(1+ n))))
(t (count-numbers (rest list) n))))
With a LOOP you can write that this way:
(defun count-numbers (list)
(loop for element in list
count (numberp element) into n
finally (return (if (zerop n) 'no-number n))))
(defun suma (L)
(setq var 0)
(do
((i 0 (+ i 1)))
((= i (length L)))
(+ var (nth i L)))
var)
Why does it always returns 0?
Shouldn't it return sum of list L?
+ does not modify its arguments, so, since you never modify var, its initial value of 0 is returned.
You need to replace (+ var (nth i L)) with (incf var (nth i L)), of, equivalently, (setq var (+ var (nth i L))).
See incf.
Note that you should bind var with let instead of making it global with setq.
Most importantly, note that your algorithm is quadratic in the length of the list argument (because nth scans your list every time from the start).
Here are some better implementations:
(defun sum-1 (l)
(reduce #'+ l))
(defun sum-2 (l)
(loop for x in l sum x))
(defun sum-3 (l)
(let ((sum 0))
(dolist (x l sum)
(incf sum x))))
Here is a bad implementation:
(defun sum-4 (l)
(apply #'+ l))
The problem with sum-4 is that it will fail if the length of the supplied list is larger than call-arguments-limit.
I thought this would be a comment for the full learning experience, but I was not able to put code in the comment.
There is a way to do sums without modifying any argument, and that is by doing it recursively:
(defun recsum (list)
(if list
(+ (first list) (recsum (rest list)))
0))
This version can be tail call optimized by the compiler, and as fast as a loop:
(defun recsum2 (list &optional (accumulator 0))
(if list
(recsum2 (rest list) (+ accumulator (first list)))
accumulator))
What you are trying to do could be done with do like this:
(defun suma (l)
(do
((var 0)
(i 0 (+ i 1)))
((= i (length l)) var)
(incf var (nth i l))))
But we don't usually do anything in dos body, so it's like this then:
(defun suma (l)
(do
((i 0 (+ i 1))
(var 0 (+ var (nth i l))))
((= i (length l)) var)))
But nth and length are slow, so better do it this way:
(defun suma (l)
(do*
((var (first l) (+ var (first list)))
(list (rest l) (rest list)))
((null list) var)))
This one is without the * in do, and returns 0 on empty list:
(defun suma (l)
(do
((acc 0 (+ acc (first list)))
(list l (rest list)))
((null list) acc)))
But my favorite is the reduce version from #sds which also can return 0 on empty list with :initial-value 0
EDIT: recsum2 did not return anything, so it needed a fix.
First, I should make it clear that this is required for an academic project. I am trying to find the maximum number of child nodes for any node in a tree, using Common Lisp.
My current code is shown below - I'm not 100% on the logic of it, but I feel it should work, however it isn't giving me the required result.
(defun breadth (list y)
(setf l y)
(mapcar #'(lambda (element)
(when (listp element)
(when (> (breadth element (length element)) l)
(setf l (breadth element (length element)))
))) list)
l)
(defun max-breadth(list)
(breadth list (length list))
)
As an example, running
(max-breadth '(a ( (b (c d)) e) (f g (h i) j)))
should return 4.
Edit:
Trace results and actual return values, forgot these:
CG-USER(13): (max-breadth '(a ( (b (c d)) e) (f g (h i) j)))
0[6]: (BREADTH (A ((B (C D)) E) (F G (H I) J)) 3)
1[6]: (BREADTH ((B (C D)) E) 2)
2[6]: (BREADTH (B (C D)) 2)
3[6]: (BREADTH (C D) 2)
3[6]: returned 2
2[6]: returned 2
1[6]: returned 2
1[6]: (BREADTH (F G (H I) J) 4)
2[6]: (BREADTH (H I) 2)
2[6]: returned 2
1[6]: returned 2
0[6]: returned 2
2
Does anyone have any ideas where I'm going wrong? I suspect it's related to the second conditional, but I'm not sure.
First, standard formatting:
(defun breadth (list y)
(setf l y)
(mapcar #'(lambda (element)
(when (listp element)
(when (> (breadth element (length element)) l)
(setf l (breadth element (length element))))))
list)
l)
(defun max-breadth (list)
(breadth list (length list)))
Your problem is the (setf l y), which should give you a warning about l being undefined. Setf should not be used on unbound variables. Use let to make a lexical scope:
(defun breadth (list y)
(let ((l y))
(mapcar #'(lambda (element)
(when (listp element)
(when (> (breadth element (length element)) l)
(setf l (breadth element (length element))))))
list)
l))
Then, instead of two nested when, use a single one and and:
(when (and (listp element)
(> (breadth element (length element)) 1))
(setf l (breadth element (length element))))
I find dolist more concise here:
(dolist (element list)
(when (and (listp element)
(> (breadth element (length element)) l))
(setf l (breadth element (length element)))))
The parameter y is always the length of the parameter list, so this call can be simplified. You also do not need to alias y:
(defun breadth (list &aux (y (length list)))
(dolist (element list)
(when (and (listp element)
(> (breadth element) y))
(setf y (breadth element))))
y)
You could eliminate the double recursive call through a let, but we can use max here:
(defun breadth (list &aux (y (length list)))
(dolist (element list)
(when (listp element)
(setf y (max y (breadth element)))))
y)
You could also use reduce for this:
(defun breadth (l)
(if (listp l)
(reduce #'max l
:key #'breadth
:initial-value (length l))
0))
L is not a local variable, so the function will return the last value assigned to it (ie, the breadth of the last subtree).
Use LET to declare a local variable:
(LET ((l y))
...
)
Isn't the correct answer 6? Since e and j in your example are also technically child nodes? If that's how you're defining your problem, the following solution should get you there:
(defun max-breadth (lst)
(cond
((atom lst) 0)
((every #'atom lst) (length lst))
(t (+ (max-breadth (car lst)) (max-breadth (cdr lst))))))
version 2:
(defun max-breadth (lst)
(cond
((atom lst) 0)
((every #'atom lst) (length lst))
(t (+
(max-breadth (car lst))
(max-breadth (remove-if-not #'consp (cdr lst)))))))