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Hello, welcome back to the
challenge 2 solution for section 20.

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So I'm in the section 20
workspace, and I'm in the

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challenge_2_solution project.

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And what I'd like to do is
just go over my solution.

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This is a really, really
straightforward program.

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If you're not used to thinking
abstractly with stl containers

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and iterators and so forth, then
it becomes much more difficult.

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But once you really take advantage
of using the power of the stl,

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it makes much, much easier.

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So here's what I've got going on here.

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I've got my song class, which
we've already talked about.

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It's very straightforward.

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I talked about that
in the previous video.

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And I'm going to scroll down to the
methods that we're going to implement.

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So here is the display
menu which i gave you.

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And here's that very first
method you had to implement,

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play the current song.

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So I'm being passed in a song object.

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What do I do?

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I just say playing,
and I display the song.

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Simple as that, the song class
overloads my stream insertion operator

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so it becomes basically a one-liner.

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The next one is displaying
the actual playlist.

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And again, really straightforward.

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What am I getting, I'm
getting the playlist, which

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is a list of song objects.

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And I'm also getting the current
song because I want to display both

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of them that way I have all the
information I need in the display.

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All I'm doing is arranged based
for loop over the playlist.

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I'm grabbing the song that's there and
displaying it, just like we did here.

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So we're using the overloaded
insertion operator.

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That's going to display all the
songs in the playlist in the loop.

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And then I'm just going to
say the current song that's

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playing is current song.

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This code is the same as this code.

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I could have called this function
from there, either way is fine.

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And now let's look at the main.

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That's the real workhorse.

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You can see that I am
defining playlist right here.

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And this of course is main.

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I'm initializing it to these six songs.

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You could choose any songs you like.

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This right here is so important.

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If you forget to do this,
you can run into issues.

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That's why I gave you
that one line in the code.

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Current song is an iterator
to a song in a std list.

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And I'm setting it to the beginning.

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So in other words, I'm always starting
right at that first song up there.

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And then I'm displaying
the playlist what.

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Am I passing in I'm passing in the
playlist, and I'm passing in the song.

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Remember, current song is an iterator.

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So I need to de-reference
that iterator to get to the

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song that it's pointing to.

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To be double safe and to use defensive
programming, you always want to make

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sure that that is actually pointing
to the list and not off the list.

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But in this case, I'm sure I'm not
off the list anywhere in this program.

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So I'm not doing it, but defensive
programming probably would be a

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good idea to check to make sure that
iterator is not pointing off the list.

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Okay, so now we're in the loop.

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And we've got a character
variable that's where I'm going

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to read the user selection into.

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I'm displaying the menu.

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I'm reading their selection.

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I'm making the selection uppercase
that way I don't have to use

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ors here in my if statements.

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And if the selection is f, that
means we're playing the first song.

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I'm just displaying this little message
right here, just playing for a song.

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And I'm setting the iterator
current song equals playlist.begin.

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That's the first element in
the list, the first song.

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That's it, and I'm
calling play current song.

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And I'm dereferencing that
iterator and passing this song in.

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That'll display that,
that song is playing.

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And these little messages here I've
got one here and I've got one here.

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You don't really don't need
those, but I just put them in

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there that way you can explicitly
see these functions working.

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So let's look at next.

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I'll scroll up just a bit.

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So here we're going
to play the next song.

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What do I do.

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I increment the iterator.

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Now it's possible that I was on the
last song, and now I'm off the list.

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So I really need to
be -- I need to check here.

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So if the current song is
equal to playlist end, that

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means I'm off the list, right.

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I just pressed next, and
I was on the last song.

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So what I need to do here is wrap to
the start of the playlist and then

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set my iterator to the beginning, and
then we probably press current song.

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Now if I'm not at the end, then I
was somewhere at the beginning or

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in the middle, it's not a big deal.

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That's it, that's the logic right here.

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For previous, it's very similar.

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So I'm displaying
playing previous song.

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And I'm checking to see if the current
song is equal to the first song,

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then I need to wrap around, right.

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So in this case, I need to set
the current song to the end.

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Now notice what happens here.

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Remember, here's my list.

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And this is the end of the list, right.

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This is just a placeholder.

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There's no song there.

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So I want to set it to there because
in the next line, I want to decrement

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it and bring it right back to here.

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So that way I don't have to
duplicate code all over the place.

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So that's it.

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I decrement the song.

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And I call play current
song with the song.

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And now it's going to be
playing the last song if I was

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at the beginning of the list.

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Otherwise, it's just going
to play the previous one.

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The edition right here
where we're adding songs.

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I've got some variables.

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I've got my name, my artist,
which is strings and the rating.

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I've got this line of code where
if I provided in your description

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this just clears out anything
that's in standard in or cin.

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In case, you've got garbage characters
in there or extra returns from

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processing, this is just a good
practice to get into whenever you

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really need to just clear out stuff
I don't want anything in there.

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So I'm just clearing everything out,
and then I'm adding and playing a new

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song, I'm displaying that message.

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And I'm asking the user to
enter the name of the song,

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the artist and the rating.

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And once I have that, I'm creating a
new song based on that information.

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And I'm inserting using
playlist.insert and current song,

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remember, that's my iterator.

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So I'm inserting it before that.

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That's the behavior.

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I could use in place here
as well, whatever you like.

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Now what happened was,
remember, here was my iterator,

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which was current song.

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It's pointing here.

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And I just inserted that song here.

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So I need to decrement
that operator to play it.

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So right here is where I'm doing that.

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I'm decrementing the iterator
right here and I'm playing

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the song that I just inserted.

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All right, now the last
couple is just real simple.

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Here list, I'm just calling display
playlist, and I'm passing in the

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playlist in the current song.

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Q, I'm quitting anything
else is an illegal choice.

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That's it, you can see it's
really straightforward, right.

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I've got no looping variables.

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I'm not worried about
keeping track of stuff.

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The only thing I'm keeping track
of is I've got my list here.

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And all I'm doing is I've got
that current song that's the only

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thing that really concerns me.

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I've got that current song iterator.

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And all I'm doing is moving that
guy back and forth along the list.

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Really, really simple.

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It never invalidates because
when I'm inserting something, I'm

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inserting something before it.

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So all i need to do
is just move it back.

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So I'm never going to invalidate that.

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And the way that the code is written,
it's never going to go off the list.

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So that's why I'm not checking
to make sure it's in the

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list but I certainly could.

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All right, I hope you
had fun with this.

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I hope you've extended it as well.

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If you have your solution, you think
it's really cool, post it in the Q&A.

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We'll all comment on it.

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All right, I'll see you in the
next video where we're going

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to start talking about sets.