(guile.info.gz) Adding or Setting Alist Entries

Info Catalog

(guile.info.gz) Alist Key Equality

(guile.info.gz) Association Lists

(guile.info.gz) Retrieving Alist Entries

 
 22.7.2.2 Adding or Setting Alist Entries
 ........................................
 
 `acons' adds a new entry to an association list and returns the
 combined association list.  The combined alist is formed by consing the
 new entry onto the head of the alist specified in the `acons' procedure
 call.  So the specified alist is not modified, but its contents become
 shared with the tail of the combined alist that `acons' returns.
 
    In the most common usage of `acons', a variable holding the original
 association list is updated with the combined alist:
 
      (set! address-list (acons name address address-list))
 
    In such cases, it doesn't matter that the old and new values of
 `address-list' share some of their contents, since the old value is
 usually no longer independently accessible.
 
    Note that `acons' adds the specified new entry regardless of whether
 the alist may already contain entries with keys that are, in some
 sense, the same as that of the new entry.  Thus `acons' is ideal for
 building alists where there is no concept of key uniqueness.
 
      (set! task-list (acons 3 "pay gas bill" '()))
      task-list
      =>
      ((3 . "pay gas bill"))
 
      (set! task-list (acons 3 "tidy bedroom" task-list))
      task-list
      =>
      ((3 . "tidy bedroom") (3 . "pay gas bill"))
 
    `assq-set!', `assv-set!' and `assoc-set!' are used to add or replace
 an entry in an association list where there _is_ a concept of key
 uniqueness.  If the specified association list already contains an
 entry whose key is the same as that specified in the procedure call,
 the existing entry is replaced by the new one.  Otherwise, the new
 entry is consed onto the head of the old association list to create the
 combined alist.  In all cases, these procedures return the combined
 alist.
 
    `assq-set!' and friends _may_ destructively modify the structure of
 the old association list in such a way that an existing variable is
 correctly updated without having to `set!' it to the value returned:
 
      address-list
      =>
      (("mary" . "34 Elm Road") ("james" . "16 Bow Street"))
 
      (assoc-set! address-list "james" "1a London Road")
      =>
      (("mary" . "34 Elm Road") ("james" . "1a London Road"))
 
      address-list
      =>
      (("mary" . "34 Elm Road") ("james" . "1a London Road"))
 
    Or they may not:
 
      (assoc-set! address-list "bob" "11 Newington Avenue")
      =>
      (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
       ("james" . "1a London Road"))
 
      address-list
      =>
      (("mary" . "34 Elm Road") ("james" . "1a London Road"))
 
    The only safe way to update an association list variable when adding
 or replacing an entry like this is to `set!' the variable to the
 returned value:
 
      (set! address-list
            (assoc-set! address-list "bob" "11 Newington Avenue"))
      address-list
      =>
      (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
       ("james" . "1a London Road"))
 
    Because of this slight inconvenience, you may find it more
 convenient to use hash tables to store dictionary data.  If your
 application will not be modifying the contents of an alist very often,
 this may not make much difference to you.
 
    If you need to keep the old value of an association list in a form
 independent from the list that results from modification by `acons',
 `assq-set!', `assv-set!' or `assoc-set!', use `list-copy' to copy the
 old association list before modifying it.
 
  -- Scheme Procedure: acons key value alist
  -- C Function: scm_acons (key, value, alist)
      Add a new key-value pair to ALIST.  A new pair is created whose
      car is KEY and whose cdr is VALUE, and the pair is consed onto
      ALIST, and the new list is returned.  This function is _not_
      destructive; ALIST is not modified.
 
  -- Scheme Procedure: assq-set! alist key val
  -- Scheme Procedure: assv-set! alist key value
  -- Scheme Procedure: assoc-set! alist key value
  -- C Function: scm_assq_set_x (alist, key, val)
  -- C Function: scm_assv_set_x (alist, key, val)
  -- C Function: scm_assoc_set_x (alist, key, val)
      Reassociate KEY in ALIST with VALUE: find any existing ALIST entry
      for KEY and associate it with the new VALUE.  If ALIST does not
      contain an entry for KEY, add a new one.  Return the (possibly
      new) alist.
 
      These functions do not attempt to verify the structure of ALIST,
      and so may cause unusual results if passed an object that is not an
      association list.
 

Info Catalog

(guile.info.gz) Alist Key Equality

(guile.info.gz) Association Lists

(guile.info.gz) Retrieving Alist Entries