Working with a List of Lists

We said in ​Lists Are Heterogeneous​ that lists can contain any type of data. That means that they can contain other lists. A list whose items are lists is called a nested list. For example, the following nested list describes life expectancies in different countries:

Here is the memory model that results from execution of that assignment statement:

Notice that each item in the outer list is itself a list of two items. We use the standard indexing notation to access the items in the outer list:

Since each of these items is also a list, we can index it again, just as we can chain together method calls or nest function calls:

We can also assign sublists to variables:

Assigning a sublist to a variable creates an alias for that sublist:

As before, any change we make through the sublist reference will be seen when we access the main list, and vice versa: