[Year 12 SofDev] Records and Arrays

Thu Aug 19 11:38:03 EST 2010

That’s great! But I guess our students will need to know the present
“definition” this year.

Laurie

From: sofdev-bounces at edulists.com.au [mailto:sofdev-bounces at edulists.com.au]
On Behalf Of Adrian Janson
Sent: Thursday, 19 August 2010 11:30 AM
To: 'Year 12 Software Development Teachers' Mailing List'
Subject: Re: [Year 12 SofDev] Records and Arrays

Hmm... the definition from Wikipedia also does not include a reference to
arrays needing to be the same type:

In computer <http://en.wikipedia.org/wiki/Computer_science>  science, an
array data structure or simply array is a data
<http://en.wikipedia.org/wiki/Data_structure>  structure consisting of a
collection of elements (values
<http://en.wikipedia.org/wiki/Value_(computer_science)>  or variables
<http://en.wikipedia.org/wiki/Variable_(programming)> ), each identified by
one or more integer
<http://en.wikipedia.org/wiki/Integer_(computer_science)>  indices
<http://en.wikipedia.org/wiki/Index_(information_technology)> , stored so
that the address <http://en.wikipedia.org/wiki/Physical_address>  of each
element can be computed from its index tuple
<http://en.wikipedia.org/wiki/Tuple>  by a simple mathematical formula.[1]
<http://en.wikipedia.org/wiki/Array_data_structure#cite_note-0> [2]
<http://en.wikipedia.org/wiki/Array_data_structure#cite_note-knuth-1>  For
example, an array of 10 integer variables, with indices 0 through 9, may be
stored as 10 words <http://en.wikipedia.org/wiki/Word_(computing)>  at
memory addresses 2000, 2004, 2008, 
 2036 (this memory allocation can vary
because some computers use other than 4 bytes to store integer type
variables); so that the element with index i has address 2000 + 4 × i.[3]
<http://en.wikipedia.org/wiki/Array_data_structure#cite_note-2> 

I will change my definition and remove the reference to variable type.  I
will also not talk about ‘strongly typed languages’ etc – as will definitely
cause confusion.  The ultimate goal of the text is to support the study –
and as such does need to state a definition that students would be expected
to know – teachers can then provide the icing on this cake.  Guy – your
breakdown of arrays/records in different languages is great – and it may be
that we share some materials that flesh out these points.  In the context of
the text again, adding this sort of detail will probably add to confusion.  

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 <http://www.mhs.vic.edu.au/> 

Blog: http://jansona.edublogs.org <http://jansona.edublogs.org/> 

From: sofdev-bounces at edulists.com.au [mailto:sofdev-bounces at edulists.com.au]
On Behalf Of Laurie Savage
Sent: Thursday, 19 August 2010 10:32 AM
To: 'Year 12 Software Development Teachers' Mailing List'
Subject: Re: [Year 12 SofDev] Records and Arrays

I’m sorry, but “an array has to contain elements all of the same type” is
neither generally nor necessarily true so should not be taught. When I teach
Physics I do not teach the Geocentric model, and in Geology I do not teach
the Biblical flood. Why? Because they are neither useful nor predictive and
are refuteable.

Laurie

From: sofdev-bounces at edulists.com.au [mailto:sofdev-bounces at edulists.com.au]
On Behalf Of Adrian Janson
Sent: Thursday, 19 August 2010 10:01 AM
To: 'Year 12 Software Development Teachers' Mailing List'
Subject: Re: [Year 12 SofDev] Records and Arrays

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 <http://www.mhs.vic.edu.au/> 

Blog: http://jansona.edublogs.org <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" <savage.john.l at edumail.vic.gov.au> 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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Date: 2010-08-19 08:45:37.693

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