What is the difference between x[:]=y and x=y[:]?
jfong at ms4.hinet.net
jfong at ms4.hinet.net
Wed Apr 12 04:08:07 EDT 2017
I have a list of list and like to expand each "list element" by appending a 1 and a 0 to it. For example, from "lr = [[1], [0]]" expand to "lr = [[1,1], [0,1], [1,0], [0,0]]".
The following won't work:
Python 3.4.4 (v3.4.4:737efcadf5a6, Dec 20 2015, 19:28:18) [MSC v.1600 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> lr = [[1], [0]]
>>> lx = []
>>> for i in range(len(lr)):
... lx[:] = lr[i]
... lx.append(0)
... lr[i].append(1)
... lr.append(lx)
...
>>> lr
[[1, 1], [0, 1], [0, 0], [0, 0]]
>>>
But the following does:
>>> lr = [[1], [0]]
>>> lx = []
>>> for i in range(len(lr)):
... lx = lr[i][:]
... lx.append(0)
... lr[i].append(1)
... lr.append(lx)
...
>>> lr
[[1, 1], [0, 1], [1, 0], [0, 0]]
>>>
After stepping through the first one manually:
>>> lr = [[1], [0]]
>>> lx[:] = lr[0]
>>> lx.append(0)
>>> lx
[1, 0]
>>> lr[0].append(1)
>>> lr
[[1, 1], [0]]
>>> lr.append(lx)
>>> lr
[[1, 1], [0], [1, 0]]
>>>
So far so good...
>>> lx[:] = lr[1]
>>> lx.append(0)
>>> lx
[0, 0]
>>> lr[1].append(1)
>>> lr
[[1, 1], [0, 1], [0, 0]]
>>>
Woops! What's wrong?
--Jach Fong
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