Here are some functions that rearrange lists “destructively” by modifying the CDRs of their component cons cells. We call these functions “destructive” because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.
delq, in Sets And Lists, for another function
that modifies cons cells.
This function returns a list containing all the elements of lists.
append (see Building Lists), the lists are
not copied. Instead, the last CDR of each of the
lists is changed to refer to the following list. The last of the
lists is not altered. For example:
(setq x '(1 2 3)) ⇒ (1 2 3)
(nconc x '(4 5)) ⇒ (1 2 3 4 5)
x ⇒ (1 2 3 4 5)
Since the last argument of
nconc is not itself modified, it is
reasonable to use a constant list, such as
'(4 5), as in the
above example. For the same reason, the last argument need not be a
(setq x '(1 2 3)) ⇒ (1 2 3)
(nconc x 'z) ⇒ (1 2 3 . z)
x ⇒ (1 2 3 . z)
However, the other arguments (all but the last) must be lists.
A common pitfall is to use a quoted constant list as a non-last
nconc. If you do this, your program will change
each time you run it! Here is what happens:
(defun add-foo (x) ; We want this function to add (nconc '(foo) x)) ;
footo the front of its arg.
(symbol-function 'add-foo) ⇒ (lambda (x) (nconc (quote (foo)) x))
(setq xx (add-foo '(1 2))) ; It seems to work. ⇒ (foo 1 2)
(setq xy (add-foo '(3 4))) ; What happened? ⇒ (foo 1 2 3 4)
(eq xx xy) ⇒ t
(symbol-function 'add-foo) ⇒ (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
This function reverses the order of the elements of list.
nreverse alters its argument by reversing
the CDRs in the cons cells forming the list. The cons cell that
used to be the last one in list becomes the first cons cell of the
(setq x '(a b c)) ⇒ (a b c)
x ⇒ (a b c) (nreverse x) ⇒ (c b a)
;; The cons cell that was first is now last. x ⇒ (a)
To avoid confusion, we usually store the result of
back in the same variable which held the original list:
(setq x (nreverse x))
Here is the
nreverse of our favorite example,
(a b c),
Original list head: Reversed list: ------------- ------------- ------------ | car | cdr | | car | cdr | | car | cdr | | a | nil |<-- | b | o |<-- | c | o | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------
This function sorts list stably, though destructively, and returns the sorted list. It compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria.
The argument predicate must be a function that accepts two
arguments. It is called with two elements of list. To get an
increasing order sort, the predicate should return non-
nil if the
first element is “less than” the second, or
nil if not.
The comparison function predicate must give reliable results for
any given pair of arguments, at least within a single call to
sort. It must be antisymmetric; that is, if a is
less than b, b must not be less than a. It must be
transitive—that is, if a is less than b, and b
is less than c, then a must be less than c. If you
use a comparison function which does not meet these requirements, the
sort is unpredictable.
The destructive aspect of
sort is that it rearranges the cons
cells forming list by changing CDRs. A nondestructive sort
function would create new cons cells to store the elements in their
sorted order. If you wish to make a sorted copy without destroying the
original, copy it first with
copy-sequence and then sort.
Sorting does not change the CARs of the cons cells in list;
the cons cell that originally contained the element
list still has
a in its CAR after sorting, but it now
appears in a different position in the list due to the change of
CDRs. For example:
(setq nums '(1 3 2 6 5 4 0)) ⇒ (1 3 2 6 5 4 0)
(sort nums '<) ⇒ (0 1 2 3 4 5 6)
nums ⇒ (1 2 3 4 5 6)
Warning: Note that the list in
nums no longer contains
0; this is the same cons cell that it was before, but it is no longer
the first one in the list. Don’t assume a variable that formerly held
the argument now holds the entire sorted list! Instead, save the result
sort and use that. Most often we store the result back into
the variable that held the original list:
(setq nums (sort nums '<))
See Sorting, for more functions that perform sorting.
documentation in Accessing Documentation, for a
useful example of