New To Smalltalk. Date format.

Greetings,

I have recently decided to dive into the Smalltalk 6.0 world of
programming and have started off by playing around with the Date class
for concept building.


What i am trying to do is change the display of the Date in the
workspace.


for example:
-------


Date today.


------
will return: Tuesday, August 29, 2006


Now, what I would like to do is change the display to: yyDDmm


I believe I found the code, but I am not sure what astream is or how to

put it in.


Code I am trying to figure out:
-----


Date new printOn: aStream format: 'yyMMdd'.


-----


now if you run it, you can debug and I can see that yyMMdd displays:
060829
which is what i want to display.


can someone explain aStream to me as well as how I put it into the code

above?

see my response in comp.lang.smalltalk...

"Rich" <[hidden email]> wrote in message
news:[hidden email]...

Rich,

>I have recently decided to dive into the Smalltalk 6.0 world of
>programming

A wise choice :-)    Welcome.

>What i am trying to do is change the display of the Date in the
>workspace.
>
>Date today.

The first step is to understand what you are doing here.

You are sending a message, #today, to the Date class.  It uses the
Windows api to find out what the current date is and then answers an
instance of the Date class which contains that information.

That's all it does.  If you just evaluate the above statement then the
instance of Date that is answered is just discarded.  

However, what you are probably doing is using "Display it" or "Inspect
it" from a workspace to see the value of the Date object.  This
involves another message send, one performed automatically by the
workspace, which asks the Date object to describe itself as a
printable String.

You can ask _any_ object to describe itself by sending it the
#printString or #displayString message (which end up sending the
#printOn: message).  The default behaviour of a Date object is to
answer a String formatted using the "Long Picture" - i.e. "Tuesday,
August 29, 2006"

You can change that behavior to tell the Date class to default to
using the "Short Picture"  In a workspace enter

Date defaultLongPicture: false.
Date today  

Highlight it all and then "Display it" .  The default formatting for a
String answered by Date  (in my locale) now shows as

29/08/2006

Set the format back by using

Date defaultLongPicture: true.


OK, that's dealt with the default formatting of a Date object.  The
Date class obtains these default formats from the Locale information
for your system - which is why my short date is formatted DD/MM/YYYY.

 What you are looking for is a date format that is not a default for
your locale (an assumption on my part).  To do that you have to tell
the Date how you want it to be formatted.  

Aside - a Stream is just a Smalltalk class that allows you to store
and retrieve objects (characters in this case) sequentially.  You
don't really need to understand it here - but you will in the future
as it's a very imporatnt part of the class hierarchy.  

Enter the following into a Workspace - comments interspersed"

"Create a new Stream that streams over a String - i.e. a sequence of
characters"
aStream := String writeStream.

"get a new instance of the Date class containing todays date"
aDate := Date today.

"ask this Date object to describe itself, in a specific format, on the
Stream object that we created.
aDate printOn: aStream format: 'yyMMdd'.

"now ask the stream for the sequence of character that it contains"
aStream contents inspect


If you select all of that and evaluate it you should get an Inspector
open up on the String "060829"



OK that's how you get a formatted String using a workspace.  If you
are going to want to do this a lot then one way of making it easier is
to add a method to the Date class that does it for you.  For example,
you could implement an instance method

Date>>printStringYYMMDD
        | stream |
        stream := String writeStream.
        self printOn: stream format: 'yyMMdd'.
        ^stream contents


You could then just evaluate

Date today printStringYYMMDD

and it would answer a correctly formatted String.


Note:  If you want to change the behaviour for Date, so that it
defaults to answering a String in the YYMMDD format, then it is
possible but more complex.



I've skipped a lot here but, hopefully, it should get you started.
Please ask again if you want anything clarified.


The best way to understand Smalltalk is

1) Use the ClassBrowser to look at the methods implemented by a class
- in this case Date.  You can learn a lot about how a class works by
just reading the comments.

2) Experiment.  Use the debugger to evaluate bits of code in a
workspace and see where it takes you.
--
Ian

The From address is valid - for the moment.

Ian & James,

Thank you so much for your input to my questions. Being that the only
type of programing that I have done up to this part is autoit, I am
still learning concepts and I am greatful for your detail explainations
as well as examples.

One of my good friends who is a smalltalk programmer has shown the
class comments in the class bowser, though certain concepts I am still
unclear of. I will post more questions as they come.

In addition, I have plugged in the code and it works great.

Initially, I use the workspace to evaluate the code then put it in an
instance method of my main program. So far, I am truly impressed with
the power of smalltalk with how little code writing you have to do
inorder to make great programs.

thanks agian,

Rich