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7.1 car and cdr

The car of a list is, quite simply, the first item in the list. Thus the car of the list (rose violet daisy buttercup) is rose.

If you are reading this in Info in GNU Emacs, you can see this by evaluating the following:

     (car '(rose violet daisy buttercup))

After evaluating the expression, rose will appear in the echo area.

Clearly, a more reasonable name for the car function would be first and this is often suggested.

car does not remove the first item from the list; it only reports what it is. After car has been applied to a list, the list is still the same as it was. In the jargon, car is `non-destructive'. This feature turns out to be important.

The cdr of a list is the rest of the list, that is, the cdr function returns the part of the list that follows the first item. Thus, while the car of the list '(rose violet daisy buttercup) is rose, the rest of the list, the value returned by the cdr function, is (violet daisy buttercup).

You can see this by evaluating the following in the usual way:

     (cdr '(rose violet daisy buttercup))

When you evaluate this, (violet daisy buttercup) will appear in the echo area.

Like car, cdr does not remove any elements from the list—it just returns a report of what the second and subsequent elements are.

Incidentally, in the example, the list of flowers is quoted. If it were not, the Lisp interpreter would try to evaluate the list by calling rose as a function. In this example, we do not want to do that.

Clearly, a more reasonable name for cdr would be rest.

(There is a lesson here: when you name new functions, consider very carefully what you are doing, since you may be stuck with the names for far longer than you expect. The reason this document perpetuates these names is that the Emacs Lisp source code uses them, and if I did not use them, you would have a hard time reading the code; but do, please, try to avoid using these terms yourself. The people who come after you will be grateful to you.)

When car and cdr are applied to a list made up of symbols, such as the list (pine fir oak maple), the element of the list returned by the function car is the symbol pine without any parentheses around it. pine is the first element in the list. However, the cdr of the list is a list itself, (fir oak maple), as you can see by evaluating the following expressions in the usual way:

     (car '(pine fir oak maple))
     
     (cdr '(pine fir oak maple))

On the other hand, in a list of lists, the first element is itself a list. car returns this first element as a list. For example, the following list contains three sub-lists, a list of carnivores, a list of herbivores and a list of sea mammals:

     (car '((lion tiger cheetah)
            (gazelle antelope zebra)
            (whale dolphin seal)))

In this example, the first element or car of the list is the list of carnivores, (lion tiger cheetah), and the rest of the list is ((gazelle antelope zebra) (whale dolphin seal)).

     (cdr '((lion tiger cheetah)
            (gazelle antelope zebra)
            (whale dolphin seal)))

It is worth saying again that car and cdr are non-destructive—that is, they do not modify or change lists to which they are applied. This is very important for how they are used.

Also, in the first chapter, in the discussion about atoms, I said that in Lisp, “certain kinds of atom, such as an array, can be separated into parts; but the mechanism for doing this is different from the mechanism for splitting a list. As far as Lisp is concerned, the atoms of a list are unsplittable.” (See Lisp Atoms.) The car and cdr functions are used for splitting lists and are considered fundamental to Lisp. Since they cannot split or gain access to the parts of an array, an array is considered an atom. Conversely, the other fundamental function, cons, can put together or construct a list, but not an array. (Arrays are handled by array-specific functions. See Arrays.)