Append a new item to a list within a list

问题:

I'm trying to append a new float element to a list within another list, for example:

list = [[]]*2
list[1].append(2.5)

And I get the following:

print list
[[2.5], [2.5]]

When I'd like to get:

[[], [2.5]]

How can I do this?

Thanks in advance.

回答1:

lst = [[] for _ in xrange(2)] (or just [[], []]). Don't use multiplication with mutable objects — you get the same one X times, not X different ones.

回答2:

list_list = [[] for Null in range(2)]

dont call it list, that will prevent you from calling the built-in function list().

The reason that your problem happens is that Python creates one list then repeats it twice. So, whether you append to it by accessing it either with list_list[0] or with list_list[1], you're doing the same thing so your changes will appear at both indexes.

回答3:

You can do in simplest way....

>>> list = [[]]*2
>>> list[1] = [2.5]
>>> list
[[], [2.5]]

回答4:

list = [[]]
list.append([2.5])

or

list = [[],[]]
list[1].append(2.5)

回答5:

[] is a list constructor, and in [[]] a list and a sublist is constructed. The *2 duplicates the reference to the inner list, but no new list is constructed:

>>> list[0] is list[1]
... True
>>> list[0] is []
... False

The solution is to have 2 inner lists, list = [[], []]

回答6:

As per @Cat Plus Plus dont use multiplication.I tried without it.with same your code.

>> list = [[],[]]
>> list[1].append(2.5)
>> list
>> [[],[2.5]]

回答7:

you should write something like this:

>>> l = [[] for _ in xrange(2)]
>>> l[1].append(2.5)
>>> l
[[], [2.5]]

回答8:

Your outter list contains another list and multiplying the outter list will have the resulting list's items all have the same pointer to the inner list. You can create a multidimensional list recursively like this:

def MultiDimensionalList(instance, *dimensions):
    if len(dimensions) == 1:
      return list(
         instance() for i in xrange(
          dimensions[0]
        )
      )
    else:
      return list(
        MultiDimensionalList(instance, *dimensions[1:]) for i
        in xrange(dimensions[0])
      )

print MultiDimensionalList(lambda: None, 1, 1, 0)

[[]]