LispForum

Hi everyone

I have a little task I'm wondering about. Because I'm suppose to figure out, how deep a list is.
Lets say f.ex a function like:

( ( ) , ( ( ) ( ) ) , ( ( ( ( ) ) ) ) )

And I figure out this list should be like "4" or something.

I've set the function like this:

(defun ld (x)
(if (listp x)
(if (null x)
else (if (ld (car x))
(if (ld (cdr x))
)
)
)

But I don't get this function: ( ( ) , ( ( ) ( ) ) , ( ( ( ( ) ) ) ) ) to work. Yes, I'm a newb in CL.
But hopefully some of you could explain something for me.

Thanks Ceenth

Firstly, commas don't get included in lists in Lisp - instead of "( ( ) , ( ( ) ( ) ) , ( ( ( ( ) ) ) ) )" it's "( ( ) ( ( ) ( ) ) ( ( ( ( ) ) ) ) )". Secondly, that is not a function, it's a list. Also, you use "else" in an "if" and use closing parentheses as if you're trying to make Lisp look like C. You probably need to do some reading up on Lisp - the first few chapters of Practical Common Lisp should do you well.

If you're determined to plough on, what I would make a function do is:

• If the input is an atom, treat it as a "list" of depth 0.
• Otherwise, the input is a list. Find the depth of the deepest list it contains, and add one to it.

First of all, thanks for an answer, I'm trying really to figure this out.
I have to deliver this to my teacher at monday.

Ok, so far I've done this:



And then when I try to f.ex type a list like: (ld '( ( ) ( ( ) ( ) ) ( ( ( ( ) ) ) ) )

I get an error message: Stack overflow on value stack. [Condition of type CCL::STACK-OVERFLOW-CONDITION]

So with simple words, what does that mean? Am I near an solution or will there more hours with "head knocking, to the wall"?

Thanks again

Don't include code as a screenshot, use

I think I need to learn how to do this more simply, but I took the above advice from Ramarren, except I didn't seem to need REDUCE. As a base case, I know the depth of an atom (which I checked, includes nil, the empty list) is 0. For every other case, just add one to the maximum depth of all the elements in the list.

But getting that meaning out the the line of code is hard for a newcomer to lisp (at least it was for me). The first problem is one of terminology: I thought to myself that I need to recursively apply depth to each element in tree. But to do that, you can't use apply. I had to realize that mapcar runs a function on each element of a list and returns one list with all the depths.

It also took me the longest time to understand the difference between funcall and apply - and to finally realize that (max (mapcar #'depth tree)) is wrong because (max '(2 3 1 2 3 4)) is wrong. I need to write (max 2 3 1 2 3 4). How, I asked myself, do I flatten a list as a return value?! I still don't think I fully understand in general (though I have found a flatten function in the book "Let over Lambda" - a fantastic read).

But (apply #'max '(2 3 1 2 3 4)) would work, so (apply #'max (mapcar #'depth tree)) seems to do the trick. Though if you are worried that you might be passed in a cyclic structure, this will bomb out with a blown stack. I think.

The last newcomer hurdle is why you have to use #' instead of nothing. In scheme you don't, but in CL you do. I think of it as it Perl: $var @var %var and &var are all different namespaces. In CL, there is only $var and &var because any variable can have a list. That's not the whole story because any variable can have a function too. But defun puts the function definition into a different spot than setf, so you need either #'max or the (function max).