a=b change b a==b true??

[rstupplebeen]

I do not have a clue what is happening in the code below.

>>a=[[2,4],[9,3]]
b=a
[map(list.sort,b)]

[[None, None]]

>>b

[[2, 4], [3, 9]]

>>a

[[2, 4], [3, 9]]

I want to make a copy of matrix a and then make changes to the
matrices separately. I assume that I am missing a fundamental
concept. Any help would be appreciated.

Rob Stupplebeen

[Diez B. Roggisch]

rs**********@gmail.com wrote:

I do not have a clue what is happening in the code below.

>>>a=[[2,4],[9,3]]
b=a
[map(list.sort,b)]

[[None, None]]

>>>b

[[2, 4], [3, 9]]

>>>a

[[2, 4], [3, 9]]

I want to make a copy of matrix a and then make changes to the
matrices separately. I assume that I am missing a fundamental
concept. Any help would be appreciated.

You are missing that the operation

b=a

is not a copying, but a mere name-binding. The object that a points to now
is also pointed at by b.

If you need copies, use the copy-module, with it's method deepcopy.

Diez

[Chris Mellon]

On 26 Feb 2007 05:50:24 -0800, rs**********@gmail.com
<rs**********@gmail.comwrote:

I do not have a clue what is happening in the code below.

>a=[[2,4],[9,3]]
b=a
[map(list.sort,b)]

[[None, None]]

>b

[[2, 4], [3, 9]]

>a

[[2, 4], [3, 9]]

I want to make a copy of matrix a and then make changes to the
matrices separately. I assume that I am missing a fundamental
concept. Any help would be appreciated.

this:

>>b=a

does not make a copy. It binds the name b to the same object that is
bound to the name a. If you want a copy, ask for a copy.

>>a = [1,2]
import copy
b = copy.copy(a) #shallow copy
a.append(3)
a,b

([1, 2, 3], [1, 2])

>>>

You may need to do something else depending on exactly what copy
semantics you want, read the docs on the copy module.

[Philipp Pagel]

rs**********@gmail.com wrote:

I do not have a clue what is happening in the code below.

>a=[[2,4],[9,3]]
b=a
[map(list.sort,b)]

[[None, None]]

>b

[[2, 4], [3, 9]]

>a

[[2, 4], [3, 9]]

I want to make a copy of matrix a and then make changes to the
matrices separately. I assume that I am missing a fundamental
concept.

The concept you are missing is the fact that b = a makes b another name
for your nested list. As you correctly stated you really want a copy.
This will do what you want:

>>import copy
a=[[2,4],[9,3]]
b = copy.deepcopy(a)
[map(list.sort,b)]

[[None, None]]

>>a

[[2, 4], [9, 3]]

>>b

[[2, 4], [3, 9]]

cu
Philipp

--
Dr. Philipp Pagel Tel. [phone]
Dept. of Genome Oriented Bioinformatics Fax. [phone]
Technical University of Munich
http://mips.gsf.de/staff/pagel

[bayer.justin]

If you want to copy lists, you do it by using the [:] operator. e.g.:

>>a = [1,2]
b = a[:]
a

[1, 2]

>>b

[1, 2]

>>b[0] = 2
a

[1, 2]

>>b

[2, 2]

If you want to copy a list of lists, you can use a list comprehension
if you do not want to use the copy module:

>>a = [[1,2],[3,4]]
b = [i[:] for i in a]
a

[[1, 2], [3, 4]]

>>b

[[1, 2], [3, 4]]

>>b[0][0] = "foo"
a

[[1, 2], [3, 4]]

>>b

[['foo', 2], [3, 4]]

[rstupplebeen]

All,
It works great now. Thank you for all of your incredibly quick
replies.
Rob

[Gabriel Genellina]

En Mon, 26 Feb 2007 11:26:43 -0300, <rs**********@gmail.comescribió:

It works great now. Thank you for all of your incredibly quick
replies.

Now that you have solved your immediate problem, you could read:
http://effbot.org/zone/python-objects.htm

--
Gabriel Genellina