CODEWITHSUNDEEP
scheme
Adam Sh
Adam Sh

Reputation: 8577

Concatenation in scheme

I have the following code:

(define rest '(((di (a) (b c)) (sh (b) (e d))) ((al (a) (b)))))
(define first '((di (a) (5)) (sh (b) (3))))

I want to get the following list:

(((di (a) (5)) (sh (b) (3))) ((di (a) (b c)) (sh (b) (e d))) ((al (a) (b)))))

meaning, add the list first, to be the first element in rest.

When I do append, it gives me:

((di (a) (5)) (sh (b) (3)) ((di (a) (b c)) (sh (b) (e d))) ((al (a) (b))))

And any other library function, or function that I try to do, didn't help.

Thank you.

Upvotes: 6

Views: 10630

Answers (2)

Óscar López
Óscar López

Reputation: 236004

It's not a good idea to use first and rest as names for your lists, since they're predefined procedures in some versions of scheme (Racket, for instance) and you'd be redefining them.

A simple cons will do the trick, as you can see below:

(define fst '((di (a) (5)) (sh (b) (3))))
(define rst '(((di (a) (b c)) (sh (b) (e d))) ((al (a) (b)))))

(cons fst rst)
> (((di (a) (5)) (sh (b) (3))) ((di (a) (b c)) (sh (b) (e d))) ((al (a) (b))))

It works because the list you want to obtain is simply rst but with a new element, fst (a list) at the beginning, whereas you'd typically use append when you want to add the elements of one list at the end of another list.

Upvotes: 2

Tikhon Jelvis
Tikhon Jelvis

Reputation: 68152

Append takes two lists and puts them together. Given that you have a first and a rest, you probably want cons. Cons takes an element and prepends it to a list. In this case, the element is first and the list is rest. So you want something like 

(cons first rest)

Upvotes: 7

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