How to Define and Initialize a Two-Dimensional Array in Python Without Predefined Lengths?

Defining a Two-Dimensional Array Without Initialized Length

When attempting to define a two-dimensional array without specifying its length, you may encounter an "IndexError: list index out of range." This occurs due to the uninitialized outer list.

To resolve this, use Python's "list comprehension" to first initialize the outer list with empty lists:

w, h = 8, 5
Matrix = [[0 for x in range(w)] for y in range(h)] 

This creates a list containing "h" lists of "w" items, all initialized to 0.

Now, you can assign values to the array elements:

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."

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

Although you can customize the variable names, using "x" and "y" for the inner and outer list indices, respectively, helps avoid confusion with non-square arrays.

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