How Do I Properly Define and Initialize Two-Dimensional Arrays in Python?

Defining Two-Dimensional Arrays in Python

You may encounter an IndexError when attempting to define a two-dimensional array without initializing its length, as in the syntax:

Matrix = [][]

This error arises because Python requires the outer list to be initialized with inner lists initially. Python refers to this process as "list comprehension," demonstrated in the following code:

# Create 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)] 

Once you have initialized the array, you can add items to it. For instance:

Matrix[0][0] = 1
Matrix[6][0] = 3 # error! range... 
Matrix[0][6] = 3 # valid

Note that the array is "y"-address major, meaning the "y index" precedes the "x index". For example:

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

To avoid any potential confusion in indexing, consider using "x" for the inner list and "y" for the outer list, especially for non-square arrays.

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