Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.

6.7.14.1 Hash Table Examples

For demonstration purposes, this section gives a few usage examples of some hash table procedures, together with some explanation what they do.

First we start by creating a new hash table with 31 slots, and populate it with two key/value pairs.

(define h (make-hash-table 31))

;; This is an opaque object
h
⇒
#<hash-table 0/31>

;; Inserting into a hash table can be done with hashq-set!
(hashq-set! h 'foo "bar")
⇒
"bar"

(hashq-set! h 'braz "zonk")
⇒
"zonk"

;; Or with hash-create-handle!
(hashq-create-handle! h 'frob #f)
⇒
(frob . #f)

You can get the value for a given key with the procedure hashq-ref, but the problem with this procedure is that you cannot reliably determine whether a key does exists in the table. The reason is that the procedure returns #f if the key is not in the table, but it will return the same value if the key is in the table and just happens to have the value #f, as you can see in the following examples.

(hashq-ref h 'foo)
⇒
"bar"

(hashq-ref h 'frob)
⇒
#f

(hashq-ref h 'not-there)
⇒
#f

It is often better is to use the procedure hashq-get-handle, which makes a distinction between the two cases. Just like assq, this procedure returns a key/value-pair on success, and #f if the key is not found.

(hashq-get-handle h 'foo)
⇒
(foo . "bar")

(hashq-get-handle h 'not-there)
⇒
#f

Interesting results can be computed by using hash-fold to work through each element. This example will count the total number of elements:

(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
⇒
3

The same thing can be done with the procedure hash-count, which can also count the number of elements matching a particular predicate. For example, count the number of elements with string values:

(hash-count (lambda (key value) (string? value)) h)
⇒
2

Counting all the elements is a simple task using const:

(hash-count (const #t) h)
⇒
3