How to pass a variable and a list in a lisp function

Question

Apologies if this is a beginner question. Harsh comments are welcome. I am learning LISP and got a snippet like the below. It checks a constant value in a list and returns only the elements greater than it.

(defun greaterthanX (l)
  (if l
    (if (> (car l) 5)
      (cons (car l) (greaterthanX (cdr l)))
      (greaterthanX (cdr l))
    )
  )
)

(print(greaterthanx '(1 2 3 4 5 6 7 8 3)))

Output : (6 7 8)

My question is how do I pass a variable inside the recursive function and modify it instead of passing a constant value (ie. 5 in the above case)?

I am looking for something like this :

(defun greaterthanX (x l)
  (if l
    (if (> (car l) x)
      (cons (car l) (greaterthanX (cdr l)))
      (greaterthanX (cdr l))
    )
  )
)

(print(greaterthanx '5 '(1 2 3 4 5 6 7 8 3)))

Rainer Joswig · Accepted Answer · 2022-10-05 17:09:16Z

4

you would additionally pass the x value in the two recursive calls to greaterthanX:

CL-USER 5 > (defun greater-than-x (x l)
              (if (consp l)
                  (if (> (first l) x)
                      (cons (first l)
                            (greater-than-x x (rest l)))
                    (greater-than-x x (rest l)))))
GREATER-THAN-X

CL-USER 6 > (print (greater-than-x 5 '(1 2 3 4 5 6 7 8 3)))

(6 7 8)   ; printed
(6 7 8)   ; repl output

answered Oct 5, 2022 at 17:09

Rainer Joswig

140k11 gold badges230 silver badges357 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Kaz · Accepted Answer · 2022-10-05 21:39:56Z

3

If you don't want the recursion to have to pass down the value, then you use an a local function:

(defun keep-greater-than (x l)
  (labels ((rec (l)
             (if l
                (if (> (car l) x)
                  (cons (car l) (rec (cdr l)))
                  (rec (cdr l))))))
    (rec l)))

The local function rec accesses the outer function's parameter x, which is visible to it.

answered Oct 5, 2022 at 21:39

Kaz

59.1k10 gold badges113 silver badges167 bronze badges

1 Comment

DrPepper Over a year ago

This is a very common pattern. I do this for almost every recursive function that I write. +1

Gwang-Jin Kim · Accepted Answer · 2022-10-06 07:00:50Z

0

Also greater-than-x is nothing else than filter:

(defun greater-than-x (x l)
  (filter (lambda (el) (< x el)) l))

Or a typical reduce function. filter you can express als reduce:

(defun my-filter (fun l)
  (nreverse 
    (reduce (lambda (result el) 
              (if (funcall fun el)
                  (cons el result)
                  result))
            l :initial-value '())))

or recursion:

(defun my-filter (fun l &optional (acc '()))
  (cond ((null l) (nreverse acc))
        ((funcall fun (car l)) (my-filter fun (cdr l) (cons (car l) acc)))
        (t (my-filter fun (cdr l) acc))))

answered Oct 6, 2022 at 7:00

Gwang-Jin Kim

11.1k20 silver badges39 bronze badges

Collectives™ on Stack Overflow

How to pass a variable and a list in a lisp function

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related