s c h e m a t i c s : c o o k b o o k

This Web

Other Webs

Schematics

RSS

Scheme Links

Traversing a Hash

Problem

You want to perform an action on each entry in a hash.

Solution

As with lists, see chapter ListRecipes, there are two basic ways to go over the elements of a hash table applying a procedure on each one. PLT provides hash-table-map, which accumulates the results into a list, and hash-table-for-each, which applies the procedure to the elements in the hash, but returns void.

(let ((t (make-immutable-hash-table '((1 . "one") (2 . "two")))))
  (values (hash-table-map t (lambda (key value)
                              (string-append "Foo:" value)))
          (hash-table-for-each t (lambda (key value)
                                   (display value)))))

Note that the procedure passed as the second argument receives two arguments, the first one is the key, and the second the value.

Comments about this recipe

It would be so much better to have the order of the arguments to hash-table-map, and hash-table-get reversed, to match the behavior of map, for-each, etc.

Contributors

-- KarlaRamirez - 19 May 2004