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Nested Loops
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En provenance du cours de University of Toronto
Learn to Program: The Fundamentals
2734 notes
À partir de la leçon
For Loops Over Indices, Parallel and Nested Lists and Strings, and Files
This module introduces how to use a for loop over the indexes of a list, how to nest lists, and how to read a write files.
Rencontrer les enseignants
Jennifer CampbellAssociate Professor, Teaching Stream
Department of Computer Science
Paul GriesAssociate Professor, Teaching Stream
Department of Computer Science
The bodies of loops can contain any statement including other loops.
We'll explore that in this lecture. Here we have a loop inside another loop.
The outer loop has I range over the values ten, eleven, and twelve, And for each of
those iterations j is going to range through one, two, three, and four.
When I visualize this I starts off referring to the value of ten.
J doesn't exist yet because we have just reached the four loop the first time.
This inner four loop. J is going to take on the value one in the
first iteration. I still refers to ten, And we're about to
print IJ, so we should see ten and one up here.
Notice that as soon as we printed tenant one, we went back to line two, not to line
one. And the reason for that is we are going to
completely execute this inner four loop until J has gone through the numbers 1,2,3
and four. So that means that J will take on the
value two next. And we're going to print tenant two.
Now J is going to take on the value three, J now refers to three, and we'll print
tenant three and four. Now we are done with this inner four loop, and so we are
going to jump back out to line one because we have completed execution of the body of
the outer four loop. Now, I is going to take on the value
eleven. J is going to take on the values one, two,
three, and four again. And now we're done with this inner loop,
so we'll go back out to the outer one. Now, we will do the last iteration of the
outer loop. Alright, it starts off as twelve and J is
going to go through the numbers one two three and four again.
And I will not change again. Now we are done with this inner loop.
We go back and check the outer loop once more.
And we are done with the program. Here is another example.
We will keep track of a list of list of numbers.
These are going to represent grades so for perhaps a student will have received a 70
a 75 and an 80 in one class. And in another class that their are taking
they have a 70 an 80 a 90 and a 100. And in the third class they have an 80 and
a 100. And, they want to know what the average is
for each of these list of grades. We are going to write a function to
calculate this list of averages. But before we do that we're going to start
with a similar problem. Just find the average of a list of
numbers. So.
Let's say that in one class, maybe English, the list of grades that we're
going to calculate the average of is 70, 75 and 80.
In order to calculate the average of this, we need to go and add up all the
assignment marks and then divide by how long that list is.
There's our accumulator, and then for each mark in English, what we're going to do is
we're going to add the mark to the total. Now,
The total is 225. We need to divide that by the length of
the list. Now that we know how to calculate the
average of a list of numbers, we would like to do this for each of the lists in
our grades list, and we would like to assemble the result into a list of these
averages. As we're developing this function, we need
examples. The first example we're going to do is
this list of grades that we have started with over on the left pane.
The average of 70, 75 and 80 is 75. The average of 70, 80, 90 and 100 is 85.
The average of 80 and 100 is 90. All of these are floats.
Let's make this window just a little bit wider.
The pipe contract is next. What we have is a list of list of numbers.
Now, these are all integers. But we can certainly imagine giving
floating point results for scores on a piece of work.
So, The pipe contract is going to be a list of
lists of number. And that is the, the type of the
parameter. And we are going to return a list of
float. We're fine with averages as the name, so
we leave it as such, and we're going to take in maybe grades as the name of our
parameter. .
In our description, this is going to return a new list in which each item is
the average of the grades in the inner list at the corresponding position.
. Of grades. Well, we know we're building a
new list. And we're going to be doing this nested
list by nested list. So we're going to be accumulating this
result. So we'll have maybe averages as the name
of our variable, which is going to be the list of averages at the end.
And when we're done, we know we want to return averages.
Well, we want to put into averages, the average of each of the lists inside our
grades lists. So,
For each of the sub lists in the grades list, what we want to do is calculate the
average of grades list and append it to averages.
Well, we have code that does that over on the left.
We know that we want start total off at zero.
And then for each mark in this particular sub grade list,
We would like to add it to total. And then once we are done with that,
We will summon up the values on the grades list.
We would like to append to averages the average.
And we're done. We will now call this in the visualizer
and watch it execute. We have added two statements at the bottom
of this file. The first one creates the, the nested
list, And the second one calls the averages
function passing in that list. Let's visualize this.
We first define our function. Next we are going to create about ten or
so objects. Grades refers to a list with three items
in it. The first one is a list with three items
in it, 70, 75 and 80. The second one refer this to a list that
has 70, 80, 90 and 100 in it. And the third item in index two in this
outer list refers to a list with two items in it, 80 and 90.
This first nested list contains 70, 75, and 80.
The second nested list contains 70,80,90. Where is it?
90 and 100. This third nested list contains 80, and
100. So indeed, the list structure that is
built in computer memory is a representation of what we have done on
this line. Now we're going to pass that list into the
averages function. We'll get a stack frame for function
averages containing parameter grades referring to that same list.
We will start, here's a fun one, we will start a new variable averages that is an
empty list. Notice we also have a function averages
outside. They're obviously two very different
things. Next we're going to go through this lists
inside this outer list. So grades list is initially going to refer
to the list that index at memory address x3.
Then we're going to go through 70, 75, and 80, add them up and divide by the length
of that list. We'll start total off at zero and for
every mark in the grades list, well mark is going to start off at 70 and the 75 and
then 80 each iteration here. We going to take that 70 here on line
seventeen and we're going to add it to the total which is zero.
So we'll get 70 of course 70 plus zero is zero.
We will assign that to total. On this next iteration, we're going to
have the marquee as 75. Now we're going to take 75 and add it to
70. 70 plus 75 is 145.
And we will assign 145 to total. On the next iteration we're going to
assign the 80 to mark. That's what this four loop does.
Grabs the last item in the list, mark now refers to 80.
On line seventeen we have an assignment statement. The right hand side evaluates
to 145 plus 80 or 225. We'll assign 225 to total.
We have now gone through all the items in that first nested list.
We've done the 70, the 75, and the 80, and so this for loop will now terminate.
We will calculate 225 divided by the length of grades list, which is three, and
we will append the result to the averages list.
Now averages refers to a list containing 75.
225 divided by three is 75. We have completed execution of the first
iteration of the outer four loop. We will now, on this next iteration, get
the second list, with four items in it. Grades list now refers to that second
list, with 70, 80, 90 and 100 in it. We will initialize total to zero,
Reset it, and then we're going to go through the four numbers,
Two, three, four and we're done. And now total is 340. The sum of 70 plus
80 plus 90 plus 100. We will append 340 divided by four to our
averages list. And that happens to be 85.
And now for the third nested list. Grades list.
The first of a list of two items in it, that's 80 and 100.
We'll reset total to zero. There's the 80 that we're dealing with.
Mark it now 80. We'll add 80 to zero, giving us 80, of
course. We will then get the 100.
And we'll add the two together. And then we will append 180 divided by the
length of that nested list, or two. So 180 divided by two is 90.
We have now gone through all three of our nested lists.
So this outer loop will terminate. And we're done.
We will now return, this averages list which as a reminder now contains 75, 85
and 90. We have to return that value.
We don't do anything with it in the main program, so when I click forward, the
program will just end.
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