Masoud Abasian Masoud Abasian - 6 months ago 8
Python Question

How to define two-dimensional array in python

I want to define a two-dimensional array without an initialized length like this :

Matrix = [][]


but it does not work.

I tried this, but it is wrong, too:

>>> Matrix = [5][5]
Traceback ...

IndexError: list index out of range


What is my mistake?

Answer

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

print Matrix[0][0] # 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.

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