how to make a nested list

[Stef Mientki]

hello,

I need a nested list, like this

A= [ [None,None], [None,None], [None, None] ]
A[2][0] =77
A

[[None, None], [None, None], [77, None]]


Because the list is much larger, I need a shortcut (ok I can use a for loop)
So I tried

B = 3 * [ [ None, None ]]
B[2][0] = 77
B

[[77, None], [77, None], [77, None]]

which doesn't work as expected.

any suggestions ?

thanks,
Stef

 

[Peter Pearson]

On Thu, 15 Sep 2011 18:57:24 +0200, Stef Mientki wrote:
[snip]

Because the list is much larger, I need a shortcut (ok I can use a for loop)
So I tried

B = 3 * [ [ None, None ]]
B[2][0] = 77
B

[[77, None], [77, None], [77, None]]

which doesn't work as expected.

any suggestions ?

b = [[None, None] for i in range(3)]
b[2][0] = 77
b

[[None, None], [None, None], [77, None]]

If you don't understand why the other approach resulted in
[[77, None], [77, None], [77, None]], feel free to ask. It's
a mistake that I think everybody makes at least once.

 

[John Ladasky]

Stef,

Are your bottom-level lists always of length 2? If so, then you could
use an array, instead of a list of lists.

Python ships with a module called array, but it doesn't allow you to
put non-numeric types into arrays, and it looks like you want the
NoneType. I use the popular numpy module, which does allow non-
numeric types.

You might also like the slice notation that numpy uses for referencing
items in the array. The indices go inside a single set of square
brackets, and are separated by commas.
array([[None, None],
[None, None],
[None, None]], dtype=object)

B[2,0] = 77
B

array([[None, None],
[None, None],
[77, None]], dtype=object)