I have a list of lists in python.
z=[[0]*6]*7
Since all elements are 0. But now if i want to change an individual element within a list e.g
z[0][0]=2
This should change the first element of the first list. But it actually changes the first element of all elements. This is so illogical to me. Can some one help.
[2, 0, 0, 0, 0, 0],
[2, 0, 0, 0, 0, 0],
[2, 0, 0, 0, 0, 0],
[2, 0, 0, 0, 0, 0],
[2, 0, 0, 0, 0, 0],
[2, 0, 0, 0, 0, 0]]
That's a classical "programmer's trap" in python.
l = [ 0 ] * 6
creates a list containing six times a reference to the same python object, here, the numerical constant 0.
Now, setting
l[0] = 1
will replace the first reference to the const 0 object by a reference to another object, the const 1 object.
Now, let's look at
z=[[0]*6]*7
This first creates a list containing six references to the const 0 object, and then it creates a list containing seven references to that single six-element list.
Now,
z[0]
will give you the first element from z, which is simply the same as all the six other elements.
Proof:
print id(z[0]) == id(z[1])
different objects have different IDs.
So you never actually made a list of different lists, but just the same list all over.
You can actually create different sublists and put them into one by doing something like
z = [ [0]*6 for _ in range(7) ]
However, I get the feeling you actually might want to use matrices – if that's really the case, numpy's ndarray is what you're after, because:
import numpy
z = numpy.ndarray((7,6))
actually gives you a matrix with 42 numeric elements.
