[Year 12 SofDev] Records and Arrays

Thu Aug 19 10:50:54 EST 2010

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" <janson.adrian.a at edumail.vic.gov.au> 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/> 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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