CODEWITHSUNDEEP
recursionlispcommon-lisplet
Programmer
Programmer

Reputation: 59

Let being called multiple times in recursion

I am trying to declare a local variable for use within a recursive function, but my let seems to be being called each time the function recurses. I want the let function to be called once to declare a variable and give it the value nil, I'd then like to update the value of this local variable within my recursive function without it being wiped with each recurse.

Here is a simplified framework of my code.

(defun recursive-function (l1 l2)
    (let ((?x nil))
    (cond (...
           ...
           ... ;trying to update value of ?x here with (setf ?x 5)
           (recursive-funtion (rest l1)(restl2)) ;recursive call made
           ))))

Upvotes: 1

Views: 732

Answers (1)

Inaimathi
Inaimathi

Reputation: 14065

What you've written is exactly what you've said you don't want; a local variable inside of the recursive function that should be updating it with each pass. If I've understood you correctly, you'll need to do something like

(defun recursive-function (l1 l2)
  (let ((?x nil))
    (labels ((actual-recursion (a b)
               (cond (...
                      ...
                      ...
                      (actual-recursion (rest a) (rest b))))))
      (actual-recursion l1 l2))))

so that each recursive call doesn't create a new binding of ?x.

Upvotes: 8

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