[Year 12 SofDev] Records and Arrays

Thu Aug 19 12:09:31 EST 2010

Your textbook will be very handy to have, Adrian, but if the examinations panel members do not agree with your definition, then we are all in deep yoghurt.

Maggie

Margaret Iaquinto
Information Technology Coordinator

St Leonard's College
Brighton Campus
t (+61 3) 9909 9300

x 529

www.stleonards.vic.edu.au

Alternate email: maggie at iaquinto.net

>>> Adrian Janson <janson.adrian.a at edumail.vic.gov.au> 19/08/2010 11:30 AM >>>

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

In computer science, an array data structure or simply array is a data structure consisting of a collection of elements (values or variables), each identified by one or more integer indices, stored so that the address of each element can be computed from its index tuple by a simple mathematical formula.[1][2] For example, an array of 10 integer variables, with indices 0 through 9, may be stored as 10 words 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]

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
Blog: 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
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" <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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