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6.7.12.4 Removing Alist Entries

To remove the element from an association list whose key matches a specified key, use assq-remove!, assv-remove! or assoc-remove! (depending, as usual, on the level of equality required between the key that you specify and the keys in the association list).

As with assq-set! and friends, the specified alist may or may not be modified destructively, and the only safe way to update a variable containing the alist is to set! it to the value that assq-remove! and friends return.

address-list
⇒
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))

Note that, when assq/v/oc-remove! is used to modify an association list that has been constructed only using the corresponding assq/v/oc-set!, there can be at most one matching entry in the alist, so the question of multiple entries being removed in one go does not arise. If assq/v/oc-remove! is applied to an association list that has been constructed using acons, or an assq/v/oc-set! with a different level of equality, or any mixture of these, it removes only the first matching entry from the alist, even if the alist might contain further matching entries. For example:

(define address-list '())
(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
address-list
⇒
(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("mary" . "11 Elm Street"))

In this example, the two instances of the string "mary" are not the same when compared using eq?, so the two assq-set! calls add two distinct entries to address-list. When compared using equal?, both "mary"s in address-list are the same as the "mary" in the assoc-remove! call, but assoc-remove! stops after removing the first matching entry that it finds, and so one of the "mary" entries is left in place.

Scheme Procedure: assq-remove! alist key
Scheme Procedure: assv-remove! alist key
Scheme Procedure: assoc-remove! alist key
C Function: scm_assq_remove_x (alist, key)
C Function: scm_assv_remove_x (alist, key)
C Function: scm_assoc_remove_x (alist, key)

Delete the first entry in alist associated with key, and return the resulting alist.


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