CPSC120B - Day 30 Notes

We already know that lists can contain other lists
```
a_list = [[1], [2, 3], [4, 5, 6]]
```
2D lists are lists containing lists with additional restrictions
All sub lists are the same size and all elements are the same type
So the above lists is not a 2D list, but the following is
```
a_list = [[1, 2, 3], [4, 5, 6]]
```
There are two sublists, both of length 3 and both only containing ints
In memory, a 2D list looks more like the way we defined it above
But working with them can be simplified if we think of them as a grid, or matrix
```
a_list = [[1, 2, 3],
          [4, 5, 6]]
```
The grid consists of rows and columns
So to access an element of the 2D you must specify the row and column of the element
```
row = 1
col = 2
print(a_list[row][col])
```

Because each sub list is a row, can get an entire row of a 2D list with array access
```
a_row = a_list[1]
```
Can also get the number of columns by getting the length of a row
```
num_cols = len(a_list[0])
```

And can use loops to iterate over a row of the list

row = 1
for col in range(len(a_list[0])):
    print(a_list[row][col])

Because each sub-list is a row, the number of rows is the length of the array
```
num_rows = len(a_list)
```

Iterating over a column similar to iterating over a row

col = 1
for row in range(len(a_list)):
    print(a_list[row][col])