I have an error on array substitution.
way=[[0]*4]*2
arri=[1]*4
for i in range(0,2):
arri[i]=0
way[i]=arri
print(way)
I thought that
[[0, 1, 1, 1], [0, 0, 0, 0]]
[[0, 1, 1, 1], [0, 0, 1, 1]]
will be printed on the console, but actually:
[[0, 1, 1, 1], [0, 0, 0, 0]]
[[0, 0, 1, 1], [0, 0, 1, 1]]
This got printed on console.
When I fix way[i] to way[1],
[[0, 0, 0, 0], [0, 1, 1, 1]]
[[0, 0, 0, 0], [0, 0, 1, 1]]
This got printed.
What's the point I'm missing here?
Because way=[[0]*4]*2 gives you two references pointing to the same list [0, 0, 0, 0], which can be proved by their identical id:
In [4]: way=[[0]*4]*2
In [5]: way
Out[5]: [[0, 0, 0, 0], [0, 0, 0, 0]]
In [6]: way[0][0] = 1
In [7]: way
Out[7]: [[1, 0, 0, 0], [1, 0, 0, 0]]
In [8]: id(way[0])
Out[8]: 39479992
In [9]: id(way[1])
Out[9]: 39479992
You may use list comprehension to create your lists like way = [[0]*4 for i in range(2)]
You may ask why [0]*4 gives you a normal [0, 0, 0, 0]:
In [16]: way
Out[16]: [0, 0, 0, 0]
In [17]: way[0] = 1
In [18]: way
Out[18]: [1, 0, 0, 0] # not [1, 1, 1, 1]
That's because 0 is an integer, which is of an immutable type int, and [0, 0, 0, 0] is of a mutable type list. You can see this link to get more information.
