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Asked: 2026-05-13T05:32:46+00:00

I’m trying to learn lisp and as i’m making my first steps i got

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I’m trying to learn lisp and as i’m making my first steps i got stuck.
How can i get c element form following list: (a b (c.d))

I’ve tried: (caar (last '(a b (c.d)))) but it returns c.d and not only c
This however works if there are spaces between c, . , d ie: (caar (last '(a b (c . d))))

The problem i’m trying to resolves has the list specified without spaces. Can that be done or it’s a typo in the exercise?

Thanks.

LE: Uisng GNU Clisp http://clisp.cons.org/
Is it possible that the issue it’s caused by the implementation?

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1 Answer

  1. Editorial Team

    Your code is right, it’s a typo (or maybe a really bad font?) in the exercise.

    In Lisp (Common Lisp and Scheme are the two I tested just now, I don’t know about Clojure), [nearly] the only divisions between symbols are spaces and parentheses. Even though . is used as literal syntax for cons, if you type '(c.d), you get one symbol in a list, not two symbols in a cons cell.

    For example,

    'c.d     ; is one symbol
'(c . d) ; is two symbols in a single cons cell
'((c)d)  ; is two symbols, the first in a nested list

    Edit: Since you are using Common Lisp, here is the list of relevant characters and the reader algorithm. To summarise, ( and ) are terminating characters, while . is not. CLisp is performing according to the Common Lisp spec.

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