Doubly-linked list/Element definition: Difference between revisions

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 P = P => back_pointer; /* P now points at the previous node.        */
 </lang>
+=={{header|PicoLisp}}==
+We use (in addition to the header structure described in
+[[Doubly-linked list/Definition#PicoLisp]])
+two cells per doubly-linked list element:
+         +-----+-----+     +-----+-----+
+         | Val |  ---+---> |  |  |  ---+---> next
+         +-----+-----+     +--+--+-----+
+                              |
+                     prev <---+
+With that, 'cddr' can be used to access the next, and 'cadr' to access the
+previous element.
+<lang PicoLisp># 'cons' an element to a doubly-linked list
+(de 2cons (X DLst)
+   (let L (car DLst)                  # Get current data list
+      (set DLst (cons X NIL L))       # Prepend two new cons pairs
+      (if L                           # Unless DLst was empty
+         (set (cdr L) (car DLst))     # set new 'prev' link
+         (con DLst (car DLst)) ) ) )  # otherwise set 'end' link
+# We prepend 'not' to the list in the previous example
+(2cons 'not *DLst)</lang>
+For output of the example data, see [[Doubly-linked list/Traversal#PicoLisp]].
 =={{header|Pop11}}==