Книга: Practical Common Lisp

Hash Table Iteration

Hash Table Iteration

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] 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.

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