Flatten Nests Function in Lisp - need help understanding

Question

I\'ve been trying to find a way to condense nested lists into numbers that go back in the original list, but I having some trouble.

I\'ve been looking at the flatte

Answer 1

In my day instead of (loop for a in l appending (g a)) we wrote (mapcan #'g l). Which is equivalent to (apply #'append (mapcar #'g l)), more or less:

(defun flatten (l) (if l (if (atom l) (list l) (mapcan #'flatten l))))

So what does it mean in this case? Imagine you call (flatten (list 1 2 3 4 5)). Each atom in your list gets enclosed in a list - becomes a singleton list, like (1) (2) etc. Then they all are appended together, giving us back ... the original list:

( 1 2 3 4 5 )

( (1) (2) (3) (4) (5) )

( 1 2 3 4 5 )

So flattening a list of atoms is an id operation (in Common LISP, that's #'identity). Now imagine flattening a list which has atoms in it, and also a list of atoms. Again, each element of the list gets transformed by flatten and then they are all appended together. A list of atoms stays as itself, as we just saw. Atoms get enclosed each in a list. So appending will give us back all the atoms that were on two levels in the nested list, now flattened:

( 11 12 (1 2 3 4) 13 )

( (11) (12) (1 2 3 4) (13) )

( 11 12 1 2 3 4 13 )

And so on and so forth, for more levels of nesting as well.

NILs as elements in lists pose a problem. NIL is an empty list, and empty list contains nothing, so should not contribute anything. But NIL is also an atom. So we make a special case for it, to not enclose it in a singleton list - leave it as it is, so when appended, it'll just disappear.