LISP car of the last element?

Question

LISP stumps me yet again... Why can't I get the value of the last element in a list? I have a list set up similar to this:

(setq bar '(((1 2) 3 4 5)((6 7) 8 9 10)))

Now I get a return of 4 for:

(caddar bar)

There is a return of (5) for:

(cdddar bar)

But I can't get a 5 for:

(cadddar bar)

Why is this--and how do I get the value of the 5?

Error:

; Warning: This function is undefined:
;   CADDDAR

Error in KERNEL:%COERCE-TO-FUNCTION:  the function CADDDAR is undefined.
[Condition of type UNDEFINED-FUNCTION]

Answer 1

The functions with 5 or more a's and d's are not defined. Only 4 and fewer. There are too many possible functions of that length for it be be practical.

You have to just spell it out: (car (cdr (cdr (cdr (cdr (car x))))))

Answer 2

Well, per the error message, there is no cadddar function. Keep in mind that car and cdr are the primitive list-reading functions. Others like caddar are convenience functions that are built from a combination of one or more car and cdr. That is, you could perform list manipulation just fine with only car and cdr if caddar etc. didn't exist, the extended functions just make your life a bit easier.

So, the way to approach this is to synthesize your own cadddar using car and cdr. If it isn't immediately apparent how to do this, start simplier (with, say, cadr or cdar) and build up to cadddar.

Answer 3

(first (last (first '(((1 2) 3 4 5) ((6 7) 8 9 10)))))

-> 5

Answer 4

The functions with more than 4 as and ds aren't defined by the standard, maybe because there are 32 of them [and it gets exponentially messier from then on].

A sure-fire way to get the last element of a list: last returns the last cons cell, so

(car (last list))

gives you the last list element. Of course, list could be something else like (first list).