Lists can contain items of any type. In this lecture, we're going to work with

lists that have items of type lists. These lists are known as nested, with one

list inside another. Our first example of a nested list is one

in which each element has the name of an assignment.

For example, assignment one and a grade for that assignment.

So, each item will be a two-item list. The length of list grades is three.

It has three items. Each of the items is, itself, a

two-element list. We can see this using indexing.

Grades at index zero is the list with the string assignment one, and int 80.

Grades at index one is assignment two and 90.

And grades at index two is assignment three and 70.

The length of each of these items, also known as interlists is two.

And we can see that by passing the list at index zero as an argument to the length

function. Another way to access the items of grades

is to use a for loop, just like we would if the list had strings or ints as

elements. So, we can say, for each item in grades,

print that item. And the item is, that is printed will be a

two-element list. Since each item of grades is a list,

It can also be indexed. So, the list at index zero gives a two

element list with string assignment one and grade 80, and we can use a second

index to access each part. So, at grades at index zero and index zero

is assignment one, and grades at index zero and index one is 80.

Similarly, for the element of grades at index one, we can access the first value

using index zero, so grades at one, zero gives assignment two, and grades at one,

one is 90. Grades at two and zero is assignment

three, and grades at two and one is 70. Next, we'll write a function to calculate

the average of grades given a list of this form.

The function is named calculate average, and notice the type contract.

We've got one parameter, which is a list of list of stir number.

That describes the type of lists we've been working with.

This function will return a flow, which is the average of the grades in the given

list. To calculate the average, we'll sum the

grades and then divide by the number of grades.

The variable total will be used to keep track of the sum of the grades, and it

will have an initial value of zero. We're going to pass over the list of

assignment grades for item in assignment grades,

And each item is a two-element list with a string and an int.

So, to the total, we want to add the current value of total plus the list item

at index one, which is the grade. Once the for loop has executed, we will

divide the total by the number of grades, And that number of grades is equal to the

length of the list assignment grades, And then we can return that average.

Let's run the example call. First, run the module, and then, we'll

execute calculate average, confirming that it does return 85.0.

Nested Lists