levent wrote:
> Thanks for the answers. Enumerating in reverse is indeed quite a smart
> idea.
>
> The fact is though, I overly simplified the task in the super-hero
> example. In the real case, the dictionary keys are not necessarily the
> indices for inserts; that is to say, the inserts do not necessarily
> take place in some sorted order.
>
> I think I was thinking more of a linked-list idea, where you do not
> store the indices as integers to some random access array but rather as
> pointers into list's nodes. Then the subsequent inserts would not hurt
> previously stored pointers. For those who know a bit C++/STL here is a
> sketch of the idea:
>
> name_list heros;
> heros.push_back("clark");
> // ... add the rest
> indexed_name_list surnames;
> surnames.push_back(
> make_pair( find( heros.begin(), heros.end(), "clark"), "kent") )
> ); // the find function returns an iterator to appropriate location
> // ... add the rest
>
> for_each(surnames.begin(), surnames.end(), insert_surnames)
> // insert_surnames is a callback that receives a single indexed surname
> // at a time and does the job, without affecting outer iterators.
>
>
> I was wondering how to make indices as *robust* in Python... Any ideas?
Python's list is technically a vector. You can therefore either search for
the insertion point in a timely fashion:
heros = ["super", "clark", "spider", "peter", "bat", "bruce"]
names = [("clark", "kent"), ("peter", "parker"), ("bruce", "wayne")]
for first, last in names:
heros.insert(heros.index(first)+1, last)
print heros
or you have to snatch a real (linked) list from somewhere. Here's an ad-hoc
implementation:
class Node(object):
def __init__(self, value, next=None):
self.value = value
self.right = next
def __str__(self):
return "List(%s)" % ", ".join(repr(n.value) for n in self)
def __iter__(self):
item = self
while item:
yield item
item = item.right
def find(self, value):
for item in self:
if item.value == value:
return item
raise ValueError("%r not found" % (value,))
def insert_after(self, value):
self.right = Node(value, self.right)
@staticmethod
def from_list(items):
items = iter(items)
try:
first = items.next()
except StopIteration:
raise ValueError("empty lists not supported")
cur = head = Node(first)
for value in items:
node = Node(value)
cur.right = node
cur = node
return head
if __name__ == "__main__":
heros = ["super", "clark", "spider", "peter", "bat", "bruce"]
heros = Node.from_list(heros)
names = [("clark", "kent"), ("peter", "parker"), ("bruce", "wayne")]
surnames = [(heros.find(f), s) for f, s in names]
print heros
for node, value in surnames:
node.insert_after(value)
print heros
Peter
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