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At the beginning of the week, I showed 
you this picture. 

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And I talked about how the structure of 
this part of the course is really 

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going to be defined. 
By the cross product of recipes, like how 

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to design functions and how to design 
world programs, with data definitions 

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with different structure. 
During the week we saw how to design 

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simple atomic data definitions, interval 
data definitions, enumerations, and 

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itemizations. 
And then we designed one function using 

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each of those data definitions. 
We basically completed the red box in 

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this week. 
Next week we'll do the green box. 

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That point that you can think about the 
organization of the first part of the 

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course in terms of a cross product 
between the form of the data and the how 

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to design functions recipe. 
That point's important but there's a much 

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more important point that underlies that. 
Think about the examples we've seen this 

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week. 
Think about the city name example and the 

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best function. 
The inherent structure of the domain 

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information in that case was atomic. 
So we represented it using simple atomic 

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data. 
And that meant the template had a certain 

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form, dot, dot, dot x. 
And that told us something about the 

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function and something about the test as 
well. 

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And that happened with all of the data 
definitions. 

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With seednum, the inherent structure of 
the information is a contiguous range of 

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numbers. 
So in that case, we use an interval. 

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That formed the template a certain way, 
it formed the body of the function a 

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certain way, and it told us something 
about the test, in that case, that there 

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should be three of them. 
Every example we saw this week, it was 

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the same phenomenon. 
Which is that once we identify the 

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structure of the information, the 
inherent structure of the information, 

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that gives us the structure of the data 
used to represent it. 

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Which gives us the structure of the 
template. 

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Which gives us the structure of the 
function and also we get guidance about 

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the tests for that function. 
So identifying the structure of the 

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information is really a key step or maybe 
the key step in the program design. 

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At least for many kinds of programs, the 
ones we're going to look at for the first 

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part of this course. 
As data definitions get more 

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sophisticated, what you're going to see 
is that there are different choices you 

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can make about the data that you'll use 
to represent information. 

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And by making different choices, you 
actually have a lot of influence on the 

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rest of the program. 
Because of this fact that the structure 

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of the data determines the structure of 
the template, determines the structure of 

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the functions. 
So, designing data is a very high point 

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of leverage for designing programs. 
There are some programs for which that's 

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less true but for very many programs, 
this approach that we're taking in this 

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first part of the course, this data 
driven approach, does a very good job of 

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producing the structure of large parts of 
the program. 

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What we'll see later in the course is 
some examples where the control structure 

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becomes dominant and what we'll learn in 
that part of the course is how to blend 

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templates based with control structure 
together with templates based on type 

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comments or templates based on a form of 
data to produce the dominant structure of 

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our program. 
But even in those programs, the form of 

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the information determines the form of 
the data and the form of the data 

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determines the form of many of the 
functions that operate on the data. 

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So identifying the form of the 
information remains a key step in the 

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design of programs. 
That's going to be true throughout the 

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course and throughout all programs you 
write. 

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So have fun with the homework 
assignments, and I'll see you next week.