计划中的汽车和Cdr

Question

So I've been learn Scheme for school, and have run into a situation using car and cdr series that doesn't quite make sense to me.

So given a list: (define x '(1 2 3 4 5))

How come (caddddr x) spits an error at me, while (cddddr x) returns (5) and (car (cddddr x)) returns 5 .

Isn't (caddddr x) the same as (car (cddddr x)) ?

Answer 1

You can only put a few a 's and d 's in there :-) check the documentation , between the initial c and the final r there can be between 1 and 4 characters in any combination of a 's and d 's. If you need to access a specific element beyond that, consider using list-ref , which returns an element given its zero-based index on the list, for example:

(define x '(1 2 3 4 5))
(list-ref x 4)
=> 5

Answer 2

Because the scheme definition goes up to (cddddr pair)‌‌ but not beyond. In the words of the specification for car and cdr and friends: "Arbitrary compositions, up to four deep, are provided." See (for example):

http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-14.html#node_idx_620

And as has been noted elsewhere, list-ref is probably what you want in this case.