Two DrRacket/scheme questions

1
votes

Programming language: DrRacket/scheme

Hey guys,

I am preparing for my first comp sci midterm, and have two quick questions that I'd love to get some input on:

(1) What exactly is the difference between a data definition and a structure definition?

I know that for a data definition, I can have something like:

;; a student is a 
;; - (make-student ln id height gradyear), where 
;; - ln is last name, and
;; - id is ID number, and
;; - height is height in inches, and
;; -gradyear is graduation year

but what is a structure definition?

(2) What exactly are the alphas and betas in contracts that come before functions, i.e. 

take : num α-list -> α-list

Thank you in advance!

2
schemeracket
This isn't a Scheme/Racket question, so much as it is a question about conventions when designing programs, and related terminology (it appears you are not using Typed Racket, in which case function contracts are only comments). If you are preparing for a midterm, the best people to answer this kind of question would be your teacher, a TA, or fellow classmates. - Dan Burton

2 Answers

4
votes

Quote from How to Design Programs (HtDP):

A DATA DEFINITION states, in a mixture of English and Scheme, how we intend to use a class of structures and how we construct elements of this class of data.

Given a problem to solve you as a programmer must decide how your input data is represented as values. In order for others to understand your program it is important to document how this is done in detail.

Some input data are simple and can be represented by a single number (e.g. temperature, pressure, etc).

Other types of data can be represented as fixed number of numbers/strings. (e.g. a cd can be represented as an author name (string), a title (string) and a price (number)). To pack a fixed number of values as one value one can represent this a structure.

If one needs to represent an unknown number of something, say cds, then one must use a list.

The data definition is simply your description of how the data are represented in your program.

To explain what a structure definition is, I'll quote from HtDP:

A STRUCTURE DEFINITION is, as the term says, a new form of definition. Here >is DrScheme's definition of posn:

(define-struct posn (x y))

Let us look at the cd example again. Since there are no builtin "cd values" in Racket, one must define what a cd value is. This is done with a structure definition:

(define-struct cd (author title price))

After the definition is made one can use make-cd to construct cd values. In order to explain that autor and title are expected to be strings, and price is expected to be a number, you must write down a data definition that explains how make-cd is supposed to be used.

I forgot to answer your second question:

(2) What exactly are the alphas and betas in contracts that come before functions, i.e.

take : num α-list -> α-list

The alpha is supposed to be replaced with a type.

If take get a integer-list (list of integers as input) then the output is an integer-list.

If take get a string-list (list of strings as input) then the output is an string-list.

In short if take gets a list of values with some type (alpha) as input, then the output is a list of values with the same type (alsp alpha).

2
votes

Jens Axel Soegaard's answer is correct, but does not elaborate on the relationship between the two, which I would state as follows.

A DATA DEFINITION describes to the reader how a value is going to be represented using Racket values.

Sometimes, the "built-in" values aren't enough, and we need to define a new kind of data, like the "CD" that Jens refers to. In order to define a new kind of data, we often use a STRUCTURE DEFINITION.

Put differently: some data definitions require structure definitions. Some do not.

If I were to elaborate any more, I would just be badly recapitulating HtDP; if what I've said thus far does not make sense, go read HtDP. :)