# python – How to define a two-dimensional array?

## python – How to define a two-dimensional array?

Youre technically trying to index an uninitialized array. You have to first initialize the outer list with lists before adding items; Python calls this
list comprehension.

``````# Creates a list containing 5 lists, each of 8 items, all set to 0
w, h = 8, 5
Matrix = [[0 for x in range(w)] for y in range(h)]
``````

#You can now add items to the list:

``````Matrix = 1
Matrix = 3 # error! range...
Matrix = 3 # valid
``````

Note that the matrix is y address major, in other words, the y index comes before the x index.

``````print Matrix # prints 1
x, y = 0, 6
print Matrix[x][y] # prints 3; be careful with indexing!
``````

Although you can name them as you wish, I look at it this way to avoid some confusion that could arise with the indexing, if you use x for both the inner and outer lists, and want a non-square Matrix.

If you really want a matrix, you might be better off using `numpy`. Matrix operations in `numpy` most often use an array type with two dimensions. There are many ways to create a new array; one of the most useful is the `zeros` function, which takes a shape parameter and returns an array of the given shape, with the values initialized to zero:

``````>>> import numpy
>>> numpy.zeros((5, 5))
array([[ 0.,  0.,  0.,  0.,  0.],
[ 0.,  0.,  0.,  0.,  0.],
[ 0.,  0.,  0.,  0.,  0.],
[ 0.,  0.,  0.,  0.,  0.],
[ 0.,  0.,  0.,  0.,  0.]])
``````

Here are some other ways to create 2-d arrays and matrices (with output removed for compactness):

``````numpy.arange(25).reshape((5, 5))         # create a 1-d range and reshape
numpy.array(range(25)).reshape((5, 5))   # pass a Python range and reshape
numpy.array( * 25).reshape((5, 5))    # pass a Python list and reshape
numpy.empty((5, 5))                      # allocate, but dont initialize
numpy.ones((5, 5))                       # initialize with ones
``````

`numpy` provides a `matrix` type as well, but it is no longer recommended for any use, and may be removed from `numpy` in the future.

#### python – How to define a two-dimensional array?

Here is a shorter notation for initializing a list of lists:

``````matrix = [*5 for i in range(5)]
``````

Unfortunately shortening this to something like `5*[5*]` doesnt really work because you end up with 5 copies of the same list, so when you modify one of them they all change, for example:

``````>>> matrix = 5*[5*]
>>> matrix
[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
>>> matrix = 2
>>> matrix
[[0, 0, 0, 0, 2], [0, 0, 0, 0, 2], [0, 0, 0, 0, 2], [0, 0, 0, 0, 2], [0, 0, 0, 0, 2]]
``````