Chapter 6. Lists
So far, we've seen lists in a few contexts, but we haven't really explored how they work and why they're useful. Since lists are central to Lisp, this chapter provides a thorough look at this data structure.
The Simple View of Lists
As we've already seen, a list in Lisp is a sequence of zero or more Lisp expressions enclosed in parentheses. Lists may be nested; that is, the enclosed subexpressions may include one or more lists. Here are a few examples:
(a b c) ;list of three symbols (7 "foo") ;list of number and string ((4.12 31178)) ;list of one element: a sublist of two numbers
The empty list () is synonymous with the symbol nil.
The functions car and cdr [23] are used to access parts of a list: car yields the first element in a list, and cdr yields the remainder of the list (everything but the first element).
(car '(a b c)) â a (cdr '(a b c)) â (b c) (car (cdr '(a b c))) â b
(Recall that quoting an expressionâeven a complete listâmeans to use that expression literally. So '(a b c) means the list containing a, b, and c, not the result of calling function a on arguments b and c.)
The cdr of a one-element list is nil:
(cdr '(x)) â nil
The car and cdr of the empty list are both nil:
(car '()) â nil (cdr '()) â nil
Note that this is also true of the list containing nil:
(car '(nil)) â nil (cdr ...
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