CODEWITHSUNDEEP
listscheme
m_callens
m_callens

Reputation: 6360

Car and Cdr in Scheme

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))?

Upvotes: 0

Views: 891

Answers (2)

fearless_fool
fearless_fool

Reputation: 35189

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.

Upvotes: 4

Óscar López
Óscar López

Reputation: 236034

You can only put a few a's andd'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

Upvotes: 8

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