How to define a two-dimensional array?

Question

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

Matrix = [][] 

but it does not work...

I've tried the code below, but it is wrong too:

Matrix = [5][5] 

Error:

Traceback ...  IndexError: list index out of range 

What is my mistake?

Answer 1

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 

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