Peter Siebel - Practical Common Lisp

(format nil "Value ~a because key not found." value))))

(setf (gethash 'bar *h*) nil) ; provide an explicit value of NIL

(show-value 'foo *h*) ==> "Value QUUX actually present."

(show-value 'bar *h*) ==> "Value NIL actually present."

(show-value 'baz *h*) ==> "Value NIL because key not found."

Since setting the value under a key to NIL leaves the key in the table, you'll need another function to completely remove a key/value pair. REMHASH takes the same arguments as GETHASH and removes the specified entry. You can also completely clear a hash table of all its key/value pairs with CLRHASH .

Common Lisp provides a couple ways to iterate over the entries in a hash table. The simplest of these is via the function MAPHASH . Analogous to the MAP function, MAPHASH takes a two-argument function and a hash table and invokes the function once for each key/value pair in the hash table. For instance, to print all the key/value pairs in a hash table, you could use MAPHASH like this:

(maphash #'(lambda (k v) (format t "~a => ~a~%" k v)) *h*)

The consequences of adding or removing elements from a hash table while iterating over it aren't specified (and are likely to be bad) with two exceptions: you can use SETF with GETHASH to change the value of the current entry, and you can use REMHASH to remove the current entry. For instance, to remove all the entries whose value is less than ten, you could write this:

(maphash #'(lambda (k v) (when (< v 10) (remhash k *h*))) *h*)

The other way to iterate over a hash table is with the extended LOOP macro, which I'll discuss in Chapter 22. [130] LOOP 's hash table iteration is typically implemented on top of a more primitive form, WITH-HASH-TABLE-ITERATOR , that you don't need to worry about; it was added to the language specifically to support implementing things such as LOOP and is of little use unless you need to write completely new control constructs for iterating over hash tables. The LOOP equivalent of the first MAPHASH expression would look like this:

(loop for k being the hash-keys in *h* using (hash-value v)

do (format t "~a => ~a~%" k v))

I could say a lot more about the nonlist collections supported by Common Lisp. For instance, I haven't discussed multidimensional arrays at all or the library of functions for manipulating bit arrays. However, what I've covered in this chapter should suffice for most of your general-purpose programming needs. Now it's finally time to look at Lisp's eponymous data structure: lists.

Lists play an important role in Lisp—for reasons both historical and practical. Historically, lists were Lisp's original composite data type, though it has been decades since they were its only such data type. These days, a Common Lisp programmer is as likely to use a vector, a hash table, or a user-defined class or structure as to use a list.

Practically speaking, lists remain in the language because they're an excellent solution to certain problems. One such problem—how to represent code as data in order to support code-transforming and code-generating macros—is particular to Lisp, which may explain why other languages don't feel the lack of Lisp-style lists. More generally, lists are an excellent data structure for representing any kind of heterogeneous and/or hierarchical data. They're also quite lightweight and support a functional style of programming that's another important part of Lisp's heritage.

Thus, you need to understand lists on their own terms; as you gain a better understanding of how lists work, you'll be in a better position to appreciate when you should and shouldn't use them.

Spoon Boy: Do not try and bend the list. That's impossible. Instead . . . only try to realize the truth.

Neo: What truth?

Spoon Boy: There is no list.

Neo: There is no list?

Spoon Boy: Then you'll see that it is not the list that bends; it is only yourself. [131] Adapted from The Matrix ( http://us.imdb.com/Quotes?0133093 )

The key to understanding lists is to understand that they're largely an illusion built on top of objects that are instances of a more primitive data type. Those simpler objects are pairs of values called cons cells , after the function CONS used to create them.

CONS takes two arguments and returns a new cons cell containing the two values. [132] CONS was originally short for the verb construct . These values can be references to any kind of object. Unless the second value is NIL or another cons cell, a cons is printed as the two values in parentheses separated by a dot, a so-called dotted pair.

(cons 1 2) ==> (1 . 2)

The two values in a cons cell are called the CAR and the CDR after the names of the functions used to access them. At the dawn of time, these names were mnemonic, at least to the folks implementing the first Lisp on an IBM 704. But even then they were just lifted from the assembly mnemonics used to implement the operations. However, it's not all bad that these names are somewhat meaningless—when considering individual cons cells, it's best to think of them simply as an arbitrary pair of values without any particular semantics. Thus:

(car (cons 1 2)) ==> 1

(cdr (cons 1 2)) ==> 2

Both CAR and CDR are also SETF able places—given an existing cons cell, it's possible to assign a new value to either of its values. [133] When the place given to SETF is a CAR or CDR , it expands into a call to the function RPLACA or RPLACD ; some old-school Lispers—the same ones who still use SETQ —will still use RPLACA and RPLACD directly, but modern style is to use SETF of CAR or CDR .

(defparameter *cons* (cons 1 2))

*cons* ==> (1 . 2)

(setf (car *cons*) 10) ==> 10

*cons* ==> (10 . 2)

(setf (cdr *cons*) 20) ==> 20

*cons* ==> (10 . 20)

Because the values in a cons cell can be references to any kind of object, you can build larger structures out of cons cells by linking them together. Lists are built by linking together cons cells in a chain. The elements of the list are held in the CAR s of the cons cells while the links to subsequent cons cells are held in the CDR s. The last cell in the chain has a CDR of NIL , which—as I mentioned in Chapter 4—represents the empty list as well as the boolean value false.

This arrangement is by no means unique to Lisp; it's called a singly linked list . However, few languages outside the Lisp family provide such extensive support for this humble data type.