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- [Instructor] In this
self check exercise,

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we want you to go ahead and manipulate

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a two-dimensional list called t.

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And we've defined that list for you here.

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It has two rows, and in each
row we have three columns each.

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Now, what we'd like you to do

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with this particular list,

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is first, go ahead and
figure out the average

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of t's elements using
nested for statements

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to walk your way through the
list, calculate the total,

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and then eventually calculate the average.

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And then separately,
we'd like you to do that

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a little bit differently,

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we'd like you to use a for statement

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that displays the average of t's elements

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using reductions like, sum and len

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to calculate the sum
of each row's elements

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and the number of elements
in each row respectively,

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and then move on to calculate the average

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as a result of all of that, as well.

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So, go ahead and give that a shot,

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and after you come back
we'll show you the answer.

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Okay, so first of all, of course,

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we need to define the list

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that we were talking about up above.

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And the first the thing we want to do

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is used nested for loops to calculate

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the total and average for these elements.

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So, as you can see, we've
defined a couple of counters here

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to help us keep track of the
total of all the elements

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and to keep track of the number
of items there are, as well.

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And we have a nested for loop,

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just like we showed you
in the preceding video,

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but for each item this time,

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we're going to add the item to the total

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and we're going to increment items by one.

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And again, for those of you

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coming from other C-based languages,

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we do not have a plus
plus operator in Python.

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So, let me go ahead and execute that.

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And, of course, once we have the total

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and the number of items,

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we can calculate the average very easily

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by just dividing the total
by that number of items.

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Now, of course, we've seen
a number of reductions

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in prior examples and lessons.

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And so there's easier ways to implement

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what we just showed you there.

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Let's go ahead and rest total
to zero and items to zero.

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And take a look at this for loop.

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This is not a nested for loop,

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it's just a single for loop

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that says for every row in the array t,

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or in the list t, excuse me,

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let's go ahead and sum up
the elements within that row

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and add that entire
row's sum to the total.

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And let's count the number
of items in that row

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by checking its length,

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and add that to the total number of items.

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And then, of course, after
executing that we can calculate

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total divided by items to get our average.