Appending two lists together?

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October 31st, 2012, 12:57 PM

norm850

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Appending two lists together?

Hey guys,

We use sage to code with python, and I am not sure if it's sage or if it's just python, but what I want to do is:

list = [[0, 0], [0, 0], [0, 1], [0, 1]]
list[0].append(0)

and I want for the list to now be:

[[0,0,0], [0,0], [0,1], [0,1]]

But it's doing this:
[[0,0,0], [0,0,0], [0,1], [0,1]]

Any help with this??

October 31st, 2012, 01:46 PM

Lux Perpetua

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On Python 2.6.5 (but other versions are not likely to differ):

Code:

>>> list = [[0,0], [0,0], [0,1], [0,1]]
>>> list[0].append(0)
>>> list
[[0, 0, 0], [0, 0], [0, 1], [0, 1]]

Please post an actual code example that shows the behavior in your post.

October 31st, 2012, 03:12 PM

b49P23TIvg

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Doesn't look like the problem involves Macsyma, but it might.

You probably created your original list equivalently thus:

Code:

>>> ZZ=[0,0]
>>> ZO=[0,1]
>>> LIST = [ZZ,ZZ,ZO,ZO]
>>> LIST
[[0, 0], [0, 0], [0, 1], [0, 1]]
>>> LIST[0].append('references the same object')
>>> LIST
[[0, 0, 'references the same object'], [0, 0, 'references the same object'], [0, 1], [0, 1]]
>>>

The recent posts in this thread contain expanations.

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October 31st, 2012, 03:28 PM

norm850

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Quote:

Originally Posted by b49P23TIvg

Doesn't look like the problem involves Macsyma, but it might.

You probably created your original list equivalently thus:

Code:

>>> ZZ=[0,0]
>>> ZO=[0,1]
>>> LIST = [ZZ,ZZ,ZO,ZO]
>>> LIST
[[0, 0], [0, 0], [0, 1], [0, 1]]
>>> LIST[0].append('references the same object')
>>> LIST
[[0, 0, 'references the same object'], [0, 0, 'references the same object'], [0, 1], [0, 1]]
>>>

The recent posts in this thread contain expanations.

I will post my code after class, be on the lookout within the next hour or so, thanks for the help!

November 4th, 2012, 08:15 AM

Dietrich

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When you work with mutable objects like lists you have to be careful about doing an alias copy:

Code:

# testing aliased (same address) objects
q = [0, 0]

mylist = [q, q]

# mylist[0] and mylist[1] have the same address
print(id(mylist[0]), id(mylist[1]))

'''possible result (same id address) >>>
(40117872, 40117872)
'''

# q[:] makes a true copy of list q and avoids alias copy
# [:] is a slicing operator
mylist = [q, q[:]]

# test it ...
print(id(mylist[0]), id(mylist[1]))

'''possible result (different id address) >>>
(40117872, 40119112)
'''

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Last edited by Dietrich : November 4th, 2012 at 09:04 AM.

Appending two lists together?

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