Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>It's puzzling that everyone demands justification to need a linked list. Linked lists are one of the most elementary data structures for a reason: they have properties that the other major data structures lack, and if you need those properties, you need a linked list or one of its close relatives. If you don't understand why linked lists are an important data structure that can't always be replaced with a deque or a binary tree, you should never have passed your "intro to data structures" class.</p> <p>Here's a quick implementation, supporting the usual stuff: constant-time insertion at any point given a reference to the node, splitting the list into two lists, and inserting a list into the middle of another list (splice). Generic Python interfaces are supported: push, pop, pushleft, popleft, extend, ordinary iteration, iteration over a slice (getiter).</p> <p>I just wrote this, so it's doctested but not production tested; there are probably still bugs.</p> <pre><code>def _ref(obj): """ weakref.ref has a bit of braindamage: you can't take a weakref to None. This is a major hassle and a needless limitation; work around it. """ from weakref import ref if obj is None: class NullRef(object): def __call__(self): return None return NullRef() else: return ref(obj) class _node(object): def __init__(self, obj): self.obj = obj self._next = None self._prev = _ref(None) def __repr__(self): return "node(%s)" % repr(self.obj) def __call__(self): return self.obj @property def next(self): return self._next @property def prev(self): return self._prev() # Implementation note: all "_last" and "prev" links are weakrefs, to prevent circular references. # This is important; if we don't do this, every list will be a big circular reference. This would # affect collection of the actual objects in the list, not just our node objects. # # This means that _node objects can't exist on their own; they must be part of a list, or nodes # in the list will be collected. We also have to pay attention to references when we move nodes # from one list to another. class llist(object): """ Implements a doubly-linked list. """ def __init__(self, init=None): self._first = None self._last = _ref(None) if init is not None: self.extend(init) def insert(self, item, node=None): """ Insert item before node. If node is None, insert at the end of the list. Return the node created for item. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.insert(1) &gt;&gt;&gt; b = l.insert(2) &gt;&gt;&gt; d = l.insert(4) &gt;&gt;&gt; l._check() [1, 2, 4] &gt;&gt;&gt; c = l.insert(3, d) &gt;&gt;&gt; l._check() [1, 2, 3, 4] """ item = _node(item) if node is None: if self._last() is not None: self._last()._next = item item._prev = _ref(self._last()) self._last = _ref(item) if self._first is None: self._first = item else: assert self._first is not None, "insertion node must be None when the list is empty" if node._prev() is not None: node._prev()._next = item item._prev = node._prev item._next = node node._prev = _ref(item) if node is self._first: self._first = item return item def remove(self, node): """ &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; d = l.append(4) &gt;&gt;&gt; e = l.append(5) &gt;&gt;&gt; l.remove(c) # Check removing from the middle 3 &gt;&gt;&gt; l._check() [1, 2, 4, 5] &gt;&gt;&gt; l.remove(a) # Check removing from the start 1 &gt;&gt;&gt; l._check() [2, 4, 5] &gt;&gt;&gt; l.remove(e) # Check removing from the end 5 &gt;&gt;&gt; l._check() [2, 4] """ if self._first is node: self._first = node._next if self._last() is node: self._last = node._prev if node._next is not None: node._next._prev = node._prev if node._prev() is not None: node._prev()._next = node._next node._next = None node._prev = _ref(None) return node.obj def __nonzero__(self): """ A list is true if it has any elements. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; bool(l) False &gt;&gt;&gt; l = llist([1]) &gt;&gt;&gt; bool(l) True """ return self._first is not None def __iter__(self): """ &gt;&gt;&gt; l = llist([1,2,3]) &gt;&gt;&gt; [i() for i in l] [1, 2, 3] """ return self.getiter(self._first, self._last()) def _check(self): if self._last() is None: assert self._last() is None return [] node = self._first ret = [] while node is not None: if node._next is None: assert node == self._last() if node._prev() is None: assert node == self._first if node._next is not None: assert node._next._prev() == node if node._prev() is not None: assert node._prev()._next == node ret.append(node.obj) node = node._next return ret def getiter(self, first, last): """ Return an iterator over [first,last]. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; l.append(1) node(1) &gt;&gt;&gt; start = l.append(2) &gt;&gt;&gt; l.extend([3,4,5,6]) &gt;&gt;&gt; end = l.append(7) &gt;&gt;&gt; l.extend([8,9]) &gt;&gt;&gt; [i() for i in l.getiter(start, end)] [2, 3, 4, 5, 6, 7] """ class listiter(object): def __init__(self, first, last): self.node = first self.final_node = last def __iter__(self): return self def next(self): ret = self.node if ret is None: raise StopIteration if ret is self.final_node: self.node = None else: self.node = self.node._next return ret return listiter(first, last) def append(self, item): """ Add an item to the end of the list. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; l.append(1) node(1) &gt;&gt;&gt; l.append(2) node(2) &gt;&gt;&gt; l._check() [1, 2] """ return self.insert(item, None) def appendleft(self, item): """ Add an item to the beginning of the list. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; l.appendleft(1) node(1) &gt;&gt;&gt; l.appendleft(2) node(2) &gt;&gt;&gt; l._check() [2, 1] """ return self.insert(item, self._first) def pop(self): """ Remove an item from the end of the list and return it. &gt;&gt;&gt; l = llist([1,2,3]) &gt;&gt;&gt; l.pop() 3 &gt;&gt;&gt; l.pop() 2 &gt;&gt;&gt; l.pop() 1 &gt;&gt;&gt; l.pop() Traceback (most recent call last): ... IndexError: pop from empty llist """ if self._last() is None: raise IndexError, "pop from empty llist" return self.remove(self._last()) def popleft(self): """ Remove an item from the beginning of the list and return it. &gt;&gt;&gt; l = llist([1,2,3]) &gt;&gt;&gt; l.popleft() 1 &gt;&gt;&gt; l.popleft() 2 &gt;&gt;&gt; l.popleft() 3 &gt;&gt;&gt; l.popleft() Traceback (most recent call last): ... IndexError: popleft from empty llist """ if self._first is None: raise IndexError, "popleft from empty llist" return self.remove(self._first) def splice(self, source, node=None): """ Splice the contents of source into this list before node; if node is None, insert at the end. Empty source_list. Return the first and last nodes that were moved. # Test inserting at the beginning. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; l2 = llist([4,5,6]) &gt;&gt;&gt; l.splice(l2, a) (node(4), node(6)) &gt;&gt;&gt; l._check() [4, 5, 6, 1, 2, 3] &gt;&gt;&gt; l2._check() [] # Test inserting in the middle. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; l2 = llist([4,5,6]) &gt;&gt;&gt; l.splice(l2, b) (node(4), node(6)) &gt;&gt;&gt; l._check() [1, 4, 5, 6, 2, 3] &gt;&gt;&gt; l2._check() [] # Test inserting at the end. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; l2 = llist([4,5,6]) &gt;&gt;&gt; l.splice(l2, None) (node(4), node(6)) &gt;&gt;&gt; l._check() [1, 2, 3, 4, 5, 6] &gt;&gt;&gt; l2._check() [] # Test inserting a list with a single item. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; l2 = llist([4]) &gt;&gt;&gt; l.splice(l2, b) (node(4), node(4)) &gt;&gt;&gt; l._check() [1, 4, 2, 3] &gt;&gt;&gt; l2._check() [] """ if source._first is None: return first = source._first last = source._last() if node is None: if self._last() is not None: self._last()._next = source._first source._first._prev = self._last self._last = source._last if self._first is None: self._first = source._first else: source._first._prev = node._prev source._last()._next = node if node._prev() is not None: node._prev()._next = source._first node._prev = source._last if node is self._first: self._first = source._first source._first = None source._last = _ref(None) return first, last def split(self, start, end=None): """ Remove all items between [node, end] and return them in a new list. If end is None, remove until the end of the list. &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; d = l.append(4) &gt;&gt;&gt; e = l.append(5) &gt;&gt;&gt; l._check() [1, 2, 3, 4, 5] &gt;&gt;&gt; l2 = l.split(c, e) &gt;&gt;&gt; l._check() [1, 2] &gt;&gt;&gt; l2._check() [3, 4, 5] &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; d = l.append(4) &gt;&gt;&gt; e = l.append(5) &gt;&gt;&gt; l2 = l.split(a, c) &gt;&gt;&gt; l._check() [4, 5] &gt;&gt;&gt; l2._check() [1, 2, 3] &gt;&gt;&gt; l = llist() &gt;&gt;&gt; a = l.append(1) &gt;&gt;&gt; b = l.append(2) &gt;&gt;&gt; c = l.append(3) &gt;&gt;&gt; d = l.append(4) &gt;&gt;&gt; e = l.append(5) &gt;&gt;&gt; l2 = l.split(b, d) &gt;&gt;&gt; l._check() [1, 5] &gt;&gt;&gt; l2._check() [2, 3, 4] """ if end is None: end = self._last() ret = llist() # First, move the region into the new list. It's important to do this first, or # once we remove the nodes from the old list, they'll be held only by weakrefs and # nodes could end up being collected before we put it into the new one. ret._first = start ret._last = _ref(end) # Hook our own nodes back together. if start is self._first: self._first = end._next if end is self._last(): self._last = start._prev if start._prev() is not None: start._prev()._next = end._next if end._next is not None: end._next._prev = start._prev start._prev = _ref(None) end._next = None return ret def extend(self, items): """ &gt;&gt;&gt; l = llist() &gt;&gt;&gt; l.extend([1,2,3,4,5]) &gt;&gt;&gt; l._check() [1, 2, 3, 4, 5] """ for item in items: self.append(item) if __name__ == "__main__": import doctest doctest.testmod() </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POPython linked list O(1) insert/remove
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USGlenn Maynard
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThanks a lot, looks similar to the one I wrote last night. Don't get me started on the whole Python religion, I am exasperbated by the constant calls for justification for an extremely simple question. Thanks again for the reasoned and thoughtful reply, unfortunately a rare thing these days.
      singulars
      1. PO
      1. USeclectocrat
    2. COAnd, of course, that's why this type being missing from the standard library is such a problem: everyone ends up having to roll their own, which is a waste of time and results in different, incompatible interfaces. A linked list isn't an obscure, specialty data type; it's data structures 101. Binary trees are an equally significant gap in Python's standard library.
      singulars
      1. PO
      1. USGlenn Maynard
    3. COIt's likely that people are questioning motivation because, after passing "intro to data structures", it's quite unusual to *actually* need a linked list specifically for its performance characteristics. The vast majority of the time, if the data is large enough to worry about time complexity, sorting, indexing, or searching will be needed, and linked lists are *spectacularly bad* at all of those. Without context, insisting on a linked list sounds suspiciously like a design mistake, especially when requests for clarification are met with unwarranted hostility.
      singulars
      1. PO
      1. USC. A. McCann
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload