[BLANK_AUDIO]. 
In this video, I'm going to work through 
a relatively straightforward how to 
design functions problem. 
So this video would be a really good 
chance for you to practice what you saw 
in the HTDF full speed and slow motion 
videos. 
Just get the starter file and start 
working the problem yourself. 
And every now and then, run the video to 
kind of catch up to where you are in the 
problem, so that you could see that 
you're doing it kind of the way that I'm 
doing it in the video myself. 
 >> Okay. 
This first function is going to be 
simple, a lot like [UNKNOWN]. 
You need to design a function called Yell 
that consumes strings like hello and 
produces strings like hello bang, where 
the exclamation mark has been added to 
the end. 
So let's see, the signature of this 
function is it consumes string and it 
produces string. 
And what it's supposed to do is produce 
[SOUND] string with bang added to end of 
supplied string. 
And that's a little cumbersome. 
purpose statements are often cumbersome 
the first time you write it, let's see if 
you can do better than that. 
What if we just say, add bang to the end 
of s, where s is the name of the 
parameter in the stub. 
So that's simple. 
We'll add bang to the end of s. 
Okay, let's write a check-expect. 
Check-expect. 
Yell with hello and better produce hello 
bang like that. 
And check-expect yell with bye. 
It better produce bye, bang. 
Like that. 
Now we run the test, run the 
check-expects. 
And both are failing but the key thing is 
that both are running. 
So that tells me that they are well 
formed. 
So I'll just ignore the fact that they're 
failing for now. 
I really just want to know if they were 
running. 
And now what we'll do is we'll comment 
out this stub and we'll label it 
[UNKNOWN] stub. 
And let's see. 
The template for this function is dif-, 
is defined yell of s. 
Body of the template is dot dot dot s. 
What I'm going to do is select the 
template, make a copy of it. 
Paste a copy here. 
Comment out the template and label it as 
being the template. 
Later in the course, you won't have to 
leave the template behind. 
But for now we find that it's helpful. 
There is the template. 
And now I need to edit it. 
And, oh yeah, what this function does is 
it produces a new string by adding bang 
to the end of s. 
And so that's just going to be 
string-append S and bang like that. 
Now, we'll want it again and both tests 
passed. 
So there you go. 
That's the design of the yell function 
following the HTDF recipe.