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2 points by jsgrahamus 812 days ago | link | parent

Shouldn't be redefining, right?

This seems better:

  arc> (def make-list (alist size val)
         (if (is size 1)
           (cons val alist)
           (make-list (cons val alist) (- size 1) val)))
  #<procedure: make-list>
  arc> (make-list nil 5 -1)
  (-1 -1 -1 -1 -1)
  arc> (make-list nil 11 -1)
  (-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1)
  arc> (make-list nil 111 -1)
  (-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1   -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1   -1 -1 -1 -1 -1
   -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1   -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1   -1 -1 -1 -1 -1
   -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1   -1 -1 -1 -1)
  arc>
But I shouldn't have to invoke make-list with a nil list to begin with.


2 points by zck 812 days ago | link

> But I shouldn't have to invoke make-list with a nil list to begin with.

You can use a helper function:

    (def make-list (size val)
         (make-list-helper nil size val))
    
    (def make-list-helper (alist size val)
         (if (is size 1)
             (cons val alist)
           (make-list-helper (cons val alist) (- size 1) val)))
But this clutters up the namespace. We can use a local helper function to move make-list-helper inside the body of make-list, and wrap it in afn to make it able to recurse.

    (def make-list (size val)
         (let helper (afn (alist size val)
                          (if (is size 1)
                              (cons val alist)
                            (self (cons val alist) (- size 1) val)))
              (helper nil size val)))

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2 points by akkartik 812 days ago | link

Yes you don't need the nil argument. Since make-list returns a list, do the conses on the way out of recursive calls rather than on the way in:

  (def make-list (size val)
    (if (> size 0)
      (cons val
            (make-list (- size 1) val))))
This is identical to your version, except I dropped the now-unnecessary base case (if now generates the initial list at the bottom-most recursive call) and moved the cons outside the call to make-list.

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2 points by jsgrahamus 812 days ago | link

Great. Still wrapping my head around recursion.

Thanks.

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1 point by akkartik 812 days ago | link

I'm surprised by the redefinition warnings. They don't happen for me. Perhaps you have some of your own code being loaded? Or maybe you have an old version? (Though I don't remember make-list ever being a thing.)

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2 points by jsgrahamus 812 days ago | link

I'm caught. Still using the old version of arc 3.1 on Windows 7 with racket.

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