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[MUSIC PLAYING]

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SPEAKER: Well, hello, one and all,
and welcome to our short on for loops.

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If you're familiar with the
Mario video game series,

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you might be familiar with Princess
Peach and her castle, Peach's Castle.

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And we'll imagine together that Princess
Peach is inviting some guests to join

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her at a ball at Peach's
Castle at 7:00 PM this evening.

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Now, thankfully, Peach
has automated the process

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of writing letters to her guests.

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In fact, we have here a
function called write_letter,

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which takes in two arguments, one
called receiver and one called sender.

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And notice how the
purpose write_letter is

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to return to us a fully
written and formatted letter

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to a receiver from the sender.

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In particular, we're using
Python f-strings here

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to interpolate the values
of receiver and sender.

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For instance, if our receiver were
to be Mario, as it is on line 2 here,

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and our sender were to be Princess
Peach, well, we should see,

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at the end of this, a letter
that says "Dear Mario,"

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and then some text down below,
"Sincerely, Princess Peach."

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And then we'll take that letter
and print it out to our terminal

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to make sure everything is all correct.

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So let's see what happens
if we run this program.

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I'll go ahead and open
up my terminal here.

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And I'll run python of letters.py.

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I'll give us a bit more space so we
can see all the letters fly by here.

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If I hit Enter, we'll see some letters
being printed out to each of our guests

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here.

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We see we invited Mario first.

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That seems good.

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Invited Luigi next, Daisy
as well, and finally Yoshi.

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So this program seems to work.

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But there's a question of
how well it's designed.

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I were to show you again
what this program looks like,

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let's think of some scenarios where
this might get a little tedious.

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Let's say we're only
inviting four guests now,

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but we also want to invite other folks,
too, in which case, our guest list

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gets pretty long.

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And to do that, I need
to copy and paste, copy

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and paste to add new letters
to new characters as well.

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Now, if you find yourself
in this kind of situation

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where you're repeating something
for each maybe guest you have,

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you might think to use
something like a for loop.

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A for loop, in fact,
is great when you know

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how many times you want to iterate or
you want to do something, like write

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a letter, for each person or each thing
you have in some list, like a guest

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list, in this case.

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So let's actually make ourselves
a guest list and write a for loop

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to write a letter for each
person on that guest list.

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Well, in main here, why don't I
go ahead and update this program

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to instead keep track of a list
that we're going to call names.

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And this names list will be
our guest list for the ball.

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I'll go ahead and maybe add a
few names to the guest list.

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I'll go ahead and add Mario here
and Luigi as well, Daisy too--

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Daisy-- and and Yoshi, just like this.

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And now I have a guest list
of names I'm hoping to invite.

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Well, now what should we do?

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We know we want to write a
letter for each person that

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is going to be in this list.

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So I think I can actually get
rid of this code down below here

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and instead think about
how I could use a for loop.

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Well, one way to use a for
loop, as we saw in lecture,

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is to use numbers and iterate over
every number in some range of numbers.

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So I could write some
code like this-- for i,

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which is usually the name we give
to some variable that is iterating,

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changing on each iteration-- for i in--

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and then the Python range function.

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And range will give us a list of numbers
from 0 up to but not including the value

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we pass in as input to range.

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So in effect, maybe I want i here
to iterate, let's say, from 0, maybe

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the zeroth index of this list, to
1, the first index of this list,

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to 2, the second index of this list, and
then to 3, the third index of this list,

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again, indexing from 0 with Mario here.

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So to do that, I can actually pass in
to range here the length of our list.

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Here, I'm saying, len of
names, which in this case is 4.

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But range will give us 0, 1, 2, and 3.

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Let's try this out though.

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I'll go ahead and print i down below.

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And ideally, I'll see 0, 1, 2, and 3,
each of the indices in this list here.

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I'll go ahead and run
python of letters.py.

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And we'll in fact see 0, 1, 2, and 3.

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That seems to work just as well.

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But of course, we don't want to
invite these numbers to our party.

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We want to invite Mario,
Luigi, Daisy, and Yoshi.

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So one way to approach this is
to use i to index into our list.

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I could do something like this.

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I could say names bracket i.

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And on each iteration,
again, i will change.

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First, it will be 0,
then 1, then 2, then 3,

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giving us different
names in our list here.

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I wanted to go ahead and
run letters.py again.

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And we'll see, now, the
names we're hoping to invite.

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And this is promising because I think
I can actually use these as input

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to the write_letter function.

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Let's try this.

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If I were to go ahead and print not
just names bracket i but the result

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write_letter given names bracket i
as the receiver and "Princess Peach"

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as the sender, I think we'd start to
get the same letters we saw last time.

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Notice a few things first here, though.

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Again, i will update on
each iteration of our loop--

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that is, from one traversal or
time, going from top to bottom

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in this indented code here.

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What will actually stay the
same is "Princess Peach" here,

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the name that will be the sender.

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That will not change on each
iteration, because it doesn't

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depend on anything in our loop itself.

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So let's go ahead and run this.

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I'll run Python of letters.py.

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And we'll see the same letters
down below here as well.

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So same results, but what
have we given ourselves here?

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I'm not able to make this program much
more flexible, much more adaptable.

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How would you invite additional guests?

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We could very quickly, let's
say, add a guest, like Bowser.

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Everyone is invited here.

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And we could print out a
letter for Bowser, just

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like that, no copying and pasting, just
adding Bowser's name to our guest list.

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Now let's uninvite Bowser, actually,
and think through some ways

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to improve the design of even
this little segment of code here.

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Now, this works.

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It's actually pretty well designed.

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But I think we could make it a little
more readable, maybe stylistically

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better too.

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And we could do that using a
feature of Python, which is we

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can actually iterate over
any kind of list directly.

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This is a list here.

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Names is a list of, in this case, names.

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So we can actually use it as
the second part of our for loop.

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We could say, for i in names.

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And in this case, i will
actually temporarily

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refer to each of the items in our
list, or the names in our names list.

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i will first be Mario, then
Luigi, then Daisy, then Yoshi,

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updating on each iteration.

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But I don't need to use i.

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In fact, I can give
it any name I like as

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long as I am consistent with that name.

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I could say something more
English-like, like, for name in names,

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because we have a list of names, plural.

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But each item is a
single name, singular.

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So now on the inside
of my for loop, well,

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there's no longer an i variable
to use to index into names.

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But I've conveniently given
myself now this variable called

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name that I can just use.

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Again, first name will be Mario,
then Luigi, then Daisy, then Yoshi,

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updating on each iteration as
we loop through our for loop.

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And once we get to the end,
once there are no more names,

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we'll simply stop looping
and exit our program.

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Let's try it here.

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I'll go ahead and run
python of letters.py.

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And we'll see the same results but
now with a bit better English syntax

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and probably better stylistically
as writing programs like these.

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So for loops, again, are powerful when
you know how many times you want to loop

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or when you want to do
something for each item you have

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in some list or, more
generally, some iterable,

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something can be iterated over.

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This was our short on for loops.

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We'll see you next time.

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