[Year 12 SofDev] Records and Arrays
Guy Flaherty
G.Flaherty at xavier.vic.edu.au
Thu Aug 19 20:03:09 EST 2010
Robert,
Actually, this is not what a record is. There is no requirement for a record to use any 'naming' for items it contains, only that it can contain items with different base data types. An array can contain items of only one data type. Meaning that what you have described is actually not correct according to the SD definitions. The commonly used term to describe both of these is 'array', but the SD definitions are quite clear in how they are different.
Usage is not so clear and, as you have demonstrated, can become confusing if you start to impose human logic/reason on top of computational data structures. An associative array is only an array if all items in the array are of the same type, otherwise it is a record. A record does not have to associate names to the items inside it. Both serve the same purpose with the only difference being the data types stored inside them. In essence, a record can be an array, but an array cannot be a record.
The fact that some programming languages do not follow this model leads me to believe that this will need to be explained to my students very carefully on a theoretical basis initially, and examples of real world usage must be chosen extremely carefully so as to not confuse them, given that in practice this is not what they will be seeing with their programming.
My understanding is also that a record does not have to be one dimensional. As I think Laurie was pointing out, this is quite a complex topic, if you look at real world usage. If you simply stick with the SD definitions it will be much easier to explain, and hence, the need for clear, open definitions in this instance is vital. Otherwise, how are we going to present the ideas for students to refer to as they complete the exam if we have no 'baseline' definitions on what is acceptable.
Guy Flaherty
>>> "Timmer-Arends" 19/08/10 6:51 PM >>>
Hello all
Don I think you touch on the most important point: the concept is the thing,
not how it is implemented (or not) in any given language.
So, (to meet the requirements of 'old' and 'new' languages) an array is a
named collection of elements which
may or may not be of the same type, and which are all individually accessed
via a numerical index or indices.
As for a 'record', if we are talking about named data structures that exist
(normally in memory) at program run time, then I agree it is an unfortunate
term in that context but only because we are so used to thinking of records
as part of a file on disk. I also agree that Pascal is probably the only
language to use the term in reference to a 'memory structure', but in the
context of Pascal it makes perfect sense, and, as Steven points out,
conceptually there isn't any difference. As I said, I think we have it so
ingrained in us to think of it as part of a file. Also creating further
confusion is the the term 'associative array' which looks an awful lot like
what Pascal calls a record, but throws in the word 'array'. (Although I
suspect that an associative array is in fact a broader generalisation that
encompassed both 'traditional arrays' and 'records').
So, a record (memory structure or not) is a named collection of elements
which may or may not be of the same type, and which are individually
accessed by, as Steven also points out, by an 'attribute name'. Also, a
record is 'one-dimensional' (Although you could have an array of records in
which case Pascal would say you have a file!)
Regards
Robert T-A
Brighton SC
----- Original Message -----
From: "Don Morelli (Optus)"
To: "'Year 12 Software Development Teachers' Mailing List'"
Sent: Thursday, August 19, 2010 4:45 PM
Subject: Re: [Year 12 SofDev] Records and Arrays
> This discussion is fantastic and I understand and agree with many of the
> issues raised. However, it again highlights the need for a set of
> definitions that we all adhere to, no matter what our practical
> experience.
> This type of discussion has been raised many times over different
> definitions. As teachers, we need to know what is the definition according
> to the assessing body and the people who are setting the exam, so that all
> students can have the same opportunity at getting exam questions correct
> no
> matter who the teacher is. I know that it would not happen, but I would be
> appalled if definitions changed as a result of discussion on this list or
> any other list, as these are subscribed to on an interest basis. It would
> disadvantage other teachers who are not members of the list (and their
> students).
> We need to remember that it is the algorithm and knowledge that we are
> teaching and will be tested, and not how it is implemented in a particular
> language.
> Adrian's definition is fine and in my opinion does not need to be changed,
> as it clearly distinguishes between two different types of data
> structures.
> Particularly if this is what the examiners are looking for in their
> definition of records and arrays. We are possibly using Python next year
> as
> we used good old VB6, but I will deal with the practical side of
> implementing these data structures and still teach the "old definitions".
> Anyway thanks for the discussion as it makes the job of teaching much more
> interesting.
> Cheers Don
>
> -----Original Message-----
> From: sofdev-bounces at edulists.com.au
> [mailto:sofdev-bounces at edulists.com.au]
> On Behalf Of Guy Flaherty
> Sent: Thursday, 19 August 2010 11:48 AM
> To: 'Year 12 Software Development Teachers' Mailing List'
> Subject: Re: [Year 12 SofDev] Records and Arrays
>
> Laurie,
>
> I only mentioned it as Adrian had talked about it and I feel it is
> important
> but not part of the ARRAY / RECORD discussion. Wasn't trying to belabor a
> point ;)
>
> As for RECORD, I think any definition needs to very clearly differentiate
> between the 'common' use of the term and the SD use of the term, as they
> are
> quite different. The SD definition is the 'base' functionality that the
> type
> provides, allowing for data of different types to be stored as a group
> that
> can be referenced using a single pointer. The common use is how we, as
> humans, have come to use such a structure, but it in no way is required
> that
> the 'RECORD' type be used in such a way. To give a very poor analogy, a
> hammer is usually used for hitting nails and is probably how most people
> would think of one if they had to describe it, but it is not required to
> hit
> nails and can be used for other things as well. The problem with trying to
> teach what a 'record' is comes when you start to confuse the common use,
> such as for database rows and fields, and what it actually is according to
> the SD definition, a simple data structure.
>
> The more I think about it the more I feel the current definitions are
> simple
> to understand and explain the differences the best. The trouble is they
> are
> not accurately reflecting what many students will be seeing as they learn
> about programming languages. I agree with John, in that if we have to have
> them keep them as they are. To answer Adrian's question, I simply feel he
> needs to include something that explains how these definitions may not
> apply
> in real world applications. I will have to make a point of explaining to
> my
> students that even though the vast majority of literature states something
> is an array (depending on the language you are using), in SD terms it is a
> record! Makes sense, right?!
>
> Guy Flaherty
>
> PS. Sorry Adrian, I just got your latest message now :D
>
>>>> "Laurie Savage" 19/08/10 11:10 AM
>>>> >>>
>
> Guy,
>
> I realised I had misused "typing" after sending the messages. I was using
> it
> as shorthand for what we were discussing, which was pretty dumb of me.
> Thanks for the extensive coverage of real world usages. I think the
> concept
> of a RECORD has a use in the pencil and paper stages but that is very
> different from the SD definition.
>
> Laurie
>
> -----Original Message-----
> From: sofdev-bounces at edulists.com.au
> [mailto:sofdev-bounces at edulists.com.au]
> On Behalf Of Guy Flaherty
> Sent: Thursday, 19 August 2010 10:51 AM
> To: 'Year 12 Software Development Teachers' Mailing List'
> Subject: Re: [Year 12 SofDev] Records and Arrays
>
> Adrian,
>
> An interesting question you are asking there! If you are looking for a
> 'standard' definition, I am unsure how you can provide one when half of
> the
> approved languages work one way and the other half work in a different
> way.
> If your definition is based on common usage you would likely have to
> conclude that there is no 'record' type in real world usage as it is
> currently defined, as only Pascal appears to use this structure and names
> it
> as a 'record'. Other languages have similar structures but do not call
> them
> 'records', and other languages treat everything as a 'record' according to
> this definition and can also use the same structure as an array. In
> current
> usage, an 'array' can only be defined relative to any one particular
> language, as what an array can actually do depends on the language.
>
> If I were a student and I saw a definition for an array that stated that
> everything I had learned about programming 'arrays' in the language I was
> using was not correct, well, I think I would quickly become confused.
>
> If you look at it in terms of language popularity:
>
> 1. Java - has array but no record like type (must use a class, so not
> very
> helpful)
> 2. C - has array but doesn't have 'record' as such, as it is named as
> 'struct' but fulfills the same purpose
> 3. C++ - same as C
> 4. PHP - Has only 'record' type but known only as 'array'. Can be used as
> both an 'array' or as a 'record'
> In no order (roughly similar real world usage)
> C# - very similar to C
> VB - very similar to C#
> Python - has 'list', 'tuple', & 'dictionary' but all commonly referred
> to as 'arrays' when they are in fact either depending on use, according to
> SD definition
> Perl - very similar to PHP
> Objective C - Allows for some use of C 'structs' but names them as
> 'arrays' when in fact it would be defined as a 'record' in SD (Can be used
> as an 'array')
> Ruby - very similar to PHP
> 11. Pascal - Follows the SD definitions fully, and is probably where they
> originated as it is the oldest of the languages
> Not even used??? Visual Fox Pro - similar to PHP
>
> So, merging that down into a simple definition, such as we are using in
> the
> current study design, of ARRAY and RECORD would seem rather pointless for
> the 'newer' languages and completely appropriate for the more
> 'established'
> languages.
>
> To me, RECORD seems totally irrelevant, as it is only ever named that in
> Pascal and no other language refers to their type as a 'Record'. An ARRAY
> can only be defined relevant to the language you are using, and this is
> how
> I think you should explain it in your book, for real world usage. If the
> new
> study design insists that we continue with the current definitions then
> that
> should be the main explanation given, with an aside about real world usage
> and language difference. That would seem to be very relevant for students
> in
> my opinion.
>
> Finally, typing has no relevance in the definitions. It does not matter if
> a
> language uses static or dynamic typing when comparing the current
> definitions of ARRAY and RECORD. If you start talking about typing then I
> believe you are just going to confuse students even more by including a
> totally separate issue.
>
> Guy Flaherty
>
>>>> "Adrian Janson" 19/08/10 10:01 AM
>>>>
> Hi everyone,
>
>
>
> As the one writing the text book for 2011+, here is what I have written
> for
> the definitions of array and record (there is more detail and examples
> following these definitions):
>
>
>
>
>
> One-Dimensional Arrays
>
>
>
> A one dimensional (or 1D) array is a data structure in which variables of
> the same type are grouped together under the same name.
>
> ...
>
>
>
>
>
> Records
>
>
>
>A record is a structure that can be used to group together a variety of
> information for a particular purpose. Records are similar to arrays but
> whereas an array has to contain elements all of the same type, the
> variables
> within a record can be of different types.
>
>
>
> ...
>
>
>
> I do think that there is a place for a standard definition - as students
> will probably not use 1 language in their lives (assuming that they
> progress
> to an IT course of some description), and even if they don't, the standard
> definition can set a context for a discussion of how the language differs
> from this (if it is not strongly typed). I don't think it would hurt to
> say
> 'the definition of an array is XYZ, however in PHP, an array works in this
> way....' Interesting to note that there are 6 languages for which the
> definitions above do not strictly hold!
>
>
>
> Is the concensus of this community that you would like me to include more
> in
> these definitions about the distinction of strongly typed languages? Or
> what is the general feeling about the definitions that I have written
> above?
>
>
>
> Cheers,
>
> Adrian
>
>
>
> Adrian Janson B.Sc, Dip.Ed, M.Ed
> Director of ICT
> Melbourne High School, Forrest Hill, South Yarra, Victoria 3141 Australia.
> Phone: 03 9826 0711 International: +61 3 9826 0711
> Fax: 03 9826 8767 International: +61 3 9826 8767
> E-mail: janson.adrian.a at edumail.vic.gov.au
>
> Website: http://www.mhs.vic.edu.au
>
> Blog: http://jansona.edublogs.org
>
> -----Original Message-----
> From: sofdev-bounces at edulists.com.au
> [mailto:sofdev-bounces at edulists.com.au]
> On Behalf Of Laurie Savage
> Sent: Thursday, 19 August 2010 9:23 AM
> To: 'Year 12 Software Development Teachers' Mailing List'
> Subject: Re: [Year 12 SofDev] Records and Arrays
>
>
>
> Guy,
>
>
>
> You are right, and of course I have taught this distinction. As I
> mentioned
>
> to Maggie, I do object to teaching things that students need to unlearn or
>
> that are irrelevant.
>
>
>
> Laurie
>
>
>
> -----Original Message-----
>
> From: sofdev-bounces at edulists.com.au
> [mailto:sofdev-bounces at edulists.com.au]
>
> On Behalf Of Guy Flaherty
>
> Sent: Thursday, 19 August 2010 9:19 AM
>
> To: 'Year 12 Software Development Teachers' Mailing List'
>
> Subject: Re: [Year 12 SofDev] Records and Arrays
>
>
>
> Laurie,
>
>
>
> Typing is not really the issue, and Steven's explanation is quite likely
> to
>
> cause more confusion in my opinion. Let me explain my thinking:
>
>
>
> The difference between a record and an array is simply that one allows
> only
>
> one type of data to be used, the other allows multiple types of data to be
>
> used (some languages require type to be declared, others don't).
>
>
>
> Associative 'arrays' are called 'arrays' in their language literature, but
>
> according to the study design they are in fact 'records' and not 'arrays'.
>
>
>
> Java does not even have a record type, you have to create a class with no
>
> instance methods to represent a 'record' type, so now you have to include
>
> classes if you start thinking about Java. Additionally, other languages,
>
> such as PHP, Python, Ruby etc can do the same thing as Java but are not
>
> required to as their 'array' types can work as 'arrays' or 'records'.
>
>
>
> Ruby names their 'record' type an 'array' and their 'associative array' a
>
> 'hash', making it even more difficult for students learning Ruby to
>
> understand the Study Design definitions.
>
>
>
> Thinking about all this, I am simply going to instruct my students that
> for
>
> the purpose of completing any exam questions, the difference between a
>
> record and an array IS that one can contain multiple types of data and the
>
> other can only contain one type of data. Despite the fact that this is not
>
> what they are seeing when they do their programming, this is how it would
>
> seem to be best to approach it.
>
>
>
> Guy Flaherty
>
>
>
>>>> "Laurie Savage" 19/08/10 08:39 AM
>>>> >>>
>
>
>
> Well yes, and no. Thanks for the very clear description of a record which
> IS
>
> an array (is ISBN an integer or a string? PHP will treat it contextually)!
>
> However this does not take away from the fact that some major modern
>
> languages are not strongly typed and so the study guide distinction does
> not
>
> apply and this disadvantages students working in those languages.
>
>
>
>
>
>
>
> Laurie
>
>
>
>
>
>
>
> From: sofdev-bounces at edulists.com.au
> [mailto:sofdev-bounces at edulists.com.au]
>
> On Behalf Of Steven Bird
>
> Sent: Wednesday, 18 August 2010 8:58 PM
>
> To: Year 12 Software Development Teachers' Mailing List
>
> Subject: Re: [Year 12 SofDev] Records and Arrays
>
>
>
>
>
>
>
> The following might help you to distinguish these concepts.
>
>
>
> Consider any kind of real world entity, e.g. a student, a school, a book,
> a
>
> patient, etc. Now think about its attributes. For a book this might be
>
> title, author, year, isbn, price... When we model such an entity using a
>
> computer, we specify a set of attributes. This is a "record". It is just
> a
>
> collection of attributes describing an entity.
>
>
>
> A record could be stored on disk (e.g. as a row of a relational database
>
> table, or as a row of a CSV file), or represented in volatile memory in a
>
> data structure in a running program. Perhaps the most common data type
> used
>
> for representing a record is an "associative array" ("hash" in Perl;
>
> "dictionary" in Python; "array" (!) in PHP, "map" in C++); some languages
>
> support a "tuple" type which is also appropriate for representing records.
>
> However, at one level these details don't matter. What's important is
> just
>
> that we have a set of attributes.
>
>
>
> Now consider any collection of entities of the same type, e.g. students,
>
> schools, books, patients, etc. When we model these in a computer, we
>
> specify a collection of like entities. This is just a set or list of
>
> entities.
>
>
>
> This list could be stored on disk (e.g. as a relational table, or a CSV
>
> file), or represented in a running program. The most common data type is
> an
>
> "array", and that's what it is called in most languages (but it is "list"
> in
>
> Python). Some languages encourage you to define array elements to be all
> of
>
> the same type, but not all. In general, I think this is a good practice.
>
>
>
> PHP blurs the distinction between records and arrays. However, a
> programmer
>
> can use PHP in such a way to keep them quite distinct:
>
>
>
> my_record1 = array('isbn'=>1441412050, 'title'=>'Alice in Wonderland',
>
> 'author'=>'Lewis Carroll');
>
>
>
>
>
>
>
>
>
>
>
>
>
> my_record2 = array('isbn'=>1441412050, 'title'=>'Through the Looking
> Glass',
>
> 'author'=>'Lewis Carroll');
>
>
>
>
>
>
>
>
>
>
>
> my_array = array(my_record1, my_record2);
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> my_record1['isbn']; /* access a record's attribute by name */
>
>
>
>
>
> my_array[1]; /* access an array's element by index */
>
>
>
> I hope this helps!
>
>
>
> --
>
> Steven Bird
>
> http://stevenbird.me/
>
>
>
>
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> http://www.vcaa.vic.edu.au/vce/studies/infotech/softwaredevel3-4.html
> http://www.vitta.org.au - VITTA Victorian Information Technology Teachers
> Association Inc
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Date: 2010-08-19 18:50:12.342
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