How to initialize a two-dimensional array in Python?

Posted 2020-10-09

【wrong way:】

m=[[element] * numcols] * numrows

for example:

>>> m=[[‘a‘] *3] * 2
>>> m
[[‘a‘, ‘a‘, ‘a‘], [‘a‘, ‘a‘, ‘a‘]]
>>> m[1][1]
‘a‘
>>> m[1][1]=‘b‘
>>> m
[[‘a‘, ‘b‘, ‘a‘], [‘a‘, ‘b‘, ‘a‘]]

注意：此时m[0][1] 也被修改了， because * is copying the address of the object (list)，m[0][1]和m[1][1] reference to same object.

【right way:】

m = [[element for x in range(numcols)] for y in range(numrows)]
for example:

>>> m=[[‘a‘ for x in range(3)] for y in range(2)]
>>> m
[[‘a‘, ‘a‘, ‘a‘], [‘a‘, ‘a‘, ‘a‘]]
>>> m[1][1]
‘a‘
>>> m[1][1]=‘b‘
>>> m
[[‘a‘, ‘a‘, ‘a‘], [‘a‘, ‘b‘, ‘a‘]]