How to Define a Two-Dimensional Array in Python Without Predefined Dimensions?

Defining a Two-Dimensional Array without an Initialized Length

To define a two-dimensional array without an initialized length, it is necessary to first initialize the outer list with lists using 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)]

Once the outer list is initialized, individual values can be added to the array:

# Adds 1 to the top-left corner of the array, and 3 to the bottom-right
Matrix[0][0] = 1
Matrix[h - 1][0] = 3  # Error! Index out of range
Matrix[0][w - 1] = 3

Note that the array is "y" address major, meaning the "y index" comes before the "x index" when accessing elements:

# Prints 1 from the top-left corner
print(Matrix[0][0])

# Prints 3 from the bottom-right corner
x, y = 0, w - 1
print(Matrix[x][y])

While the inner and outer lists can be named arbitrarily, it is recommended to use different names to avoid confusion during indexing, especially when dealing with non-square arrays.

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