1
23:59:59,500 --> 00:00:06,054
[BLANK_AUDIO]. 

2
00:00:06,054 --> 00:00:09,712
In this video, I'm going to work through 
a relatively straightforward how to 

3
00:00:09,712 --> 00:00:13,842
design functions problem. 
So this video would be a really good 

4
00:00:13,842 --> 00:00:17,937
chance for you to practice what you saw 
in the HTDF full speed and slow motion 

5
00:00:17,937 --> 00:00:22,326
videos. 
Just get the starter file and start 

6
00:00:22,326 --> 00:00:25,962
working the problem yourself. 
And every now and then, run the video to 

7
00:00:25,962 --> 00:00:29,082
kind of catch up to where you are in the 
problem, so that you could see that 

8
00:00:29,082 --> 00:00:34,259
you're doing it kind of the way that I'm 
doing it in the video myself. 

9
00:00:36,289 --> 00:00:37,486
 >> Okay. 
This first function is going to be 

10
00:00:37,486 --> 00:00:41,144
simple, a lot like [UNKNOWN]. 
You need to design a function called Yell 

11
00:00:41,144 --> 00:00:45,434
that consumes strings like hello and 
produces strings like hello bang, where 

12
00:00:45,434 --> 00:00:49,990
the exclamation mark has been added to 
the end. 

13
00:00:49,990 --> 00:00:56,710
So let's see, the signature of this 
function is it consumes string and it 

14
00:00:56,710 --> 00:01:03,834
produces string. 
And what it's supposed to do is produce 

15
00:01:03,834 --> 00:01:10,980
[SOUND] string with bang added to end of 
supplied string. 

16
00:01:10,980 --> 00:01:16,836
And that's a little cumbersome. 
purpose statements are often cumbersome 

17
00:01:16,836 --> 00:01:19,730
the first time you write it, let's see if 
you can do better than that. 

18
00:01:19,730 --> 00:01:26,825
What if we just say, add bang to the end 
of s, where s is the name of the 

19
00:01:26,825 --> 00:01:34,753
parameter in the stub. 
So that's simple. 

20
00:01:34,753 --> 00:01:42,051
We'll add bang to the end of s. 
Okay, let's write a check-expect. 

21
00:01:42,051 --> 00:01:48,757
Check-expect. 
Yell with hello and better produce hello 

22
00:01:48,757 --> 00:02:02,536
bang like that. 
And check-expect yell with bye. 

23
00:02:02,536 --> 00:02:06,498
It better produce bye, bang. 
Like that. 

24
00:02:06,498 --> 00:02:10,043
Now we run the test, run the 
check-expects. 

25
00:02:10,043 --> 00:02:15,320
And both are failing but the key thing is 
that both are running. 

26
00:02:15,320 --> 00:02:18,150
So that tells me that they are well 
formed. 

27
00:02:19,520 --> 00:02:22,780
So I'll just ignore the fact that they're 
failing for now. 

28
00:02:22,780 --> 00:02:24,530
I really just want to know if they were 
running. 

29
00:02:24,530 --> 00:02:29,814
And now what we'll do is we'll comment 
out this stub and we'll label it 

30
00:02:29,814 --> 00:02:34,006
[UNKNOWN] stub. 
And let's see. 

31
00:02:34,006 --> 00:02:41,800
The template for this function is dif-, 
is defined yell of s. 

32
00:02:41,800 --> 00:02:48,390
Body of the template is dot dot dot s. 
What I'm going to do is select the 

33
00:02:48,390 --> 00:02:53,572
template, make a copy of it. 
Paste a copy here. 

34
00:02:53,572 --> 00:02:58,250
Comment out the template and label it as 
being the template. 

35
00:02:58,250 --> 00:03:00,970
Later in the course, you won't have to 
leave the template behind. 

36
00:03:00,970 --> 00:03:05,563
But for now we find that it's helpful. 
There is the template. 

37
00:03:05,563 --> 00:03:09,984
And now I need to edit it. 
And, oh yeah, what this function does is 

38
00:03:09,984 --> 00:03:14,410
it produces a new string by adding bang 
to the end of s. 

39
00:03:14,410 --> 00:03:21,690
And so that's just going to be 
string-append S and bang like that. 

40
00:03:21,690 --> 00:03:25,241
Now, we'll want it again and both tests 
passed. 

41
00:03:25,241 --> 00:03:29,864
So there you go. 
That's the design of the yell function 

42
00:03:29,864 --> 00:03:33,094
following the HTDF recipe.