Question

I wrote a big program that use car and cdr, and do:

(map car (append (map caddr lists) (map cadr lists))

When lists is list of lists in the next format ((a (b) (c)) (d (e) (f))...(x (y) (z)))
When I did it I got one list (b c e f... y z)
(Note: b,c,…y,z is a list of numbers; a,d…x is a symbol)

But now, I found that b,c,…,y,z can be also empty list, and It gives the next error:

car: expects argument of type <pair>; given ()

What can I do?

Answer 1

Have you tried filtering away empty lists before your map? Something like this:

(map car (filter pair? (append (map caddr lists) (map cadr lists))))

The fundamental issue is that () is not a pair while car only acts on pairs. The simplest solution is just to get rid of everything that isn’t a pair before mapping car onto the list; this is what (filter pair? ...) does.