1. Two-dimensional arrays
Lists in Python
Two-dimensional arrays

2. Two dimensions
You will sometimes have data which has a two-dimensional structure (days and hours for example, or rows and columns)
You could store the data in one big list and keep track of the structure some other way but most modern languages provide a way to express the two dimensions directly

3. List of lists
In Python you can think of this structure as a list of lists
mat = [[1,2,3], [5, 9, 2], [7, 8, 3], [6, 7, 0]]
Of course in Python any of these lists can be any size (they don’t all have to be 3 elements long!)
Typically a 2-d array is thought of as having so many rows and so many columns (a grid like Excel)

4. Syntax of 2-d arrays
One way to create a 2-d array is as given on the last slide, just write it out
Another that is quicker if you want to initialize all elements to one value:
mat2 = [[0] * 3 for i in range(4)]
creates a 2-d array with 4 rows, 3 columns full of zeros O

5. Accessing elements of a 2-d array
You use two subscripts, each in a pair of []
mat [2][1] is the element in the third row and second column – just like 1-d arrays, they start numbering at [0][0]