Archive Magazine 1996/ARCHIVE_96.iso/discs/gerald/7_12/ReadMe

Text File | 1993-12-27 | 15KB | 288 lines

%OP%VS4.13 (28-Apr-92), Gerald L Fitton, R4000 5966 9904 9938
%OP%DP0
%OP%IRY
%OP%PL0
%OP%HM0
%OP%FM0
%OP%BM0
%OP%LM4
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%OP%PDPipeLine
%OP%WC1026,2262,188,1748,0,0,0,0
%CO:A,72,72%
%C%Chained Dependent Documents
%C%by Gerald L Fitton
Keywords:
accounts set_value chain dependent Names Fitton

Introduction

In some applications you can generate what I am going to refer to as "a
long chain of unwanted dependent documents".  As the application grows
the chain grows unwieldy because all the files in the chain are loaded
one after the other when, perhaps, you want to work on only the most
recent document.

The set_value(,) function can be used to 'unlink' the chain without
destroying the (occasionally wanted) dependency.  Using Named variables
further simplifies many applications which use dependent documents.

For my example I've chosen a simple accounts application where each
month's transactions are kept in a separate document.  First I'll
explain how the basic application works, then I'll go on to to
demonstrate how the chain of dependent documents becomes unwieldy and,
then we'll see how set_value(,) can help by 'unlinking' the chain.
Finally we shall see how the use of Names for variables further
simplifies the creation of new documents which have to be added to the
'unlinked' chain.

Monthly transactions

Double click on the file [Acc_9301] to load it.  You will see that it
represents a set of transactions for January 1993.  Position the cursor
in slot [Acc_9301]D5 and you will find the formula D4+B5-C5.  The value
brought forward from the previous month is held in slot [Acc_9301]D4,
credits are added to that amount and debits are subtracted from it
(line by line) to find the current balance and, finally, the amount to
be carried forward to next month's accounts.

This formula has been replicated down the column from D5 to D8 by
marking D5D8 and using the command <CtrláBRD> (Block Replicate Down) so
that, for example, the formula in [Acc_9301]D7 is D6+B7-C7.

In the form in which I've included it on this disc, the file [Acc_9301]
contains only three transactions, two credits and one debit but you can
extend it quite simply.  Place the caret anywhere in the last row and
tap <F7> to insert a new row, make your credit or debit entry and,
finally, replicate the formula in the "Balance" column through to the
line marked "Carried Forward".

Usually you would Save the file at this point but, in this case, don't
do that or my next exercise won't work properly!

Brought Forward

Delete the modified file [Acc_9301] from your screen (decline to Save
it by clicking on the 'No' box) and then double click on the file
[Acc_9302] (the transactions for Februaryá1992).  You will find that
[Acc_9302] and [Acc_9301] are both loaded.  The reason why [Acc_9301]
is loaded is because slot [Acc_9302]D4 contains the formula
[Acc_9301]D8.  Provided that you have not modified my version of
[Acc_9301] then this formula will bring forward the balance from
Januaryá1993 into the Februaryá1993 document.

There are at least two difficulties which arise when you use this
strategy of entering the slot reference of the previous "Carried
Forward" file, [Acc_9301]D8, into the "Brought Forward" slot,
[Acc_9302]D4, of the current month's document.

The first difficulty arises if you don't have both [Acc_9301] and
[Acc_9302] loaded and you want to add to or delete rows from the
earlier document.  If you have [Acc_9302] loaded and add a row to
[Acc_9301] then you will find that the slot reference in [Acc_9302]D4
will change to match the extended [Acc_9301].  However, it is all too
easy to forget that you need both files loaded.  If you don't have them
both loaded when you add a row to the earlier document then, when you
do load the later document, it will look for the amount to bring
forward in the slot where it used to be instead of where it now is.

Yes!  Before you start writing to me, I do know that a method of
avoiding the 'loss' (some would say 'corruption') of slot references in
this fashion is to use Names in the earlier document for the slots used
by the later document.  Some of the examples I have received from you
use this method of 'non-local' Names - I don't recommend it.  I shall
explain how I use Names (and give my explanation of why 'non-local'
Names are not a good idea) later in this tutorial!

The second difficulty is one about which I get much correspondence.  To
understand why I get so many letters on this topic I'd like you to
imagine the situation in Decemberá1993.  When you load [Acc_9312] then
it will call up [Acc_9311], which in turn will call up [Acc_9310], and
so on until either all the monthly sheets have been loaded into memory
or (as has happened to some of my correspondents) you don't have any
memory left to use!

Loading all these earlier files is what I mean when I refer to a "chain
of unwanted dependent documents".  As a document become older the
likelihood of you wanting to change it is reduced so one way of
'breaking the chain' is to 'snapshot' a "Brought Forward" value with
<CtrláBSS> (Block SnapShot).  This method of 'unlinking' the earlier
dependent documents from later ones seems to be the favourite solution
adopted by the majority of my correspondents.  Of course, it requires a
personal judgement about the likelihood of wanting to change an earlier
'unlinked' document against the desirability of keeping it (and all
earlier ones in the chain) 'alive'.  I'd like to suggest to you what I
believe to be a much better alternative.

Carried Forward

You might regard the use of the formula [Acc_9301]D8 in slot
[Acc_9302]D4 as a function which creates a 'backwards chain' of
dependency.

Let me try to convince you that chaining backwards is not consistent
with the nature of the application we're considering.  The application
is one in which amendments to an earlier document need to be 'chained'
in the forward direction whereas additions to the most recent document
have no effect on earlier documents.  Chaining backwards, loading
earlier documents into memory, is a waste of time and effort!  What we
need is a set of documents which 'chain forwards'.  To put it another
way, we want to 'push' the "Carry Forward" balance from the earlier
document into the "Brought Forward" slot of the later document rather
than drag the value from the earlier document into the "Brought
Forward" slot of the later document.  If we can find a way of doing
this then we don't need to have the earlier documents in memory (unless
they need amendment - in which case we want any amendment to 'ripple
forwards' through the 'chain').

This is where the function set_value(,) comes in useful.  It can be
used to 'push' the "Carried Forward" balance into the "Brought Forward"
slot of the next document in exactly the way we want.  This is the
strategy I have adopted in dealing with the [Acc_94##] series.

Load the file [Acc_9403] and look at the "Brought Forward" balance in
slot [Acc_9403]D4.  You will find that it does not contain a formula
but a value.  The document [Acc_9403] contains no reference to earlier
documents so it does not 'pull' on the next link in the "chain of
unwanted dependent documents" uselessly dragging them into memory in
the way I've described in the previous section.

So how do we know that the "Brought Forward" balance is correct?  How
did it get there?  One way would be to use <CtrláBSS> (Block SnapShot)
and another would be to copy the "Carry Forward" manually into the
"Brought Forward" slot.  Either of these methods are unsatisfactory if
there is a chance that you might want to make an amendment to an
earlier document.  Amendments to earlier documents won't ripple through
a 'snapshot' and it's too easy to make a mistake!

Load [Acc_9402] and look at slot [Acc_9402]D12.  You will find the
formula set_value([Acc_9403]D4,D8).  It is this set_value(,) function
which 'pushes' the "Carried Forward" value from [Acc_9302]D8 into the
"Brought Forward" slot of the Marchá1994 document.  Make an amendment
to the earlier document and you will find the the amendment is
'chained' through to the later document.  Try adding rows to the
Februaryá1994 document and you will see that the set_value(,) formula
in what was [Acc_9402]D12 adjusts to the enlarged sheet.

Now delete the earlier [Acc_9402] document from the screen.  You can
continue to work on the Marchá1994 document and ultimately Save it with
all the additions you have just made.

To see how the 'chain' works delete both the Februaryá1994 and
Marchá1994 documents from the screen and then double click on
[Acc_9401] (the Januaryá1994 file).  That file and all subsequent files
(February and Marchá1994) will be loaded.  Make an amendment to the
Januaryá1994 file, Save it (not on the master disc) and then delete it
from the screen.

Using the 1994 Chain of Documents

For most of your work you will not need the earlier documents loaded;
probably you will need only the most recent document.  Make your
changes to it and then Save it.

Now and again you will need to change an earlier document either to
amend a value which has been entered incorrectly or to add an item
which has been forgotten.  If you load that earlier document then all
subsequent documents in the chain will be loaded because the
set_value(,) function will drive the "Carried Forward" value into the
next sheet.  When you make your amendments to an early document the
consequences of those changes will 'ripple through' all later
documents.

When you have finished with the early document you can Save it and then
delete it from the screen; in fact you can work through all the chained
documents in chronological order Saving the new versions and then
deleting them from the screen.

If you run out of memory you can 'unlink' the chain by tricking the
package into believing that a particular file can't be found.  For
example, if you change the name of the file [Acc_9404] to, say,
[Acc_9404a] (use the RISCáOS feature Rename) and then double click on
[Acc_9401] you will find that the chain is broken when [Acc_9404] can't
be found.  You can make your amendments to the earliest file and then
go through the process of Saving the chain of files and deleting them
from the screen.

Restore the renamed file to its original name and then load the file
whose position in the chain is just prior to that one.  The new values
will ripple through the rest of the files in the chain.

Using Names

The process of creating a new document is made easier if you use Names.

I have used Names in the [Acc_95##] series.  Before we have a look at
the detail, double click on [Acc_9501] and you will find that the 1995
series will load and can be used in exactly the same way as you have
seen when you used the [Acc_94##] series.

Load all three files by clicking on [Acc_9501] and then have a look at
the contents of slot [Acc_9502]D12.  You will see that the formula,
instead of set_value([Acc_9503]D4,D8) has been replaced by
set_value([Acc_9503]bf,cf).  Two names, bf and cf (Brought Forward &
Carried Forward) are used in the formula instead.  The Name cf is
'local' to the [Acc_9502] sheet whereas the Name [Acc_9503]bf is what I
am calling 'non-local'.

Now let's have a look at the Names which have been defined in
[Acc_9502], what I am calling 'local' Names.  Place the pointer on the
italic f which is just to the left of the cross and tick boxes (in the
same line as the formula line) and click once.  At the bottom of the
pop-up menu you will find the option Edit name.  Run the pointer
through Edit name and you will see that within [Acc_9502] only two
names have been defined, bf and cf which are both 'local'.  By looking
at the definitions (as if you were going to edit them) you will see
that the name bf refers to D4 and the name cf refers to D8 both slots
within the current sheet.  There are no definitions which refer to
slots in other sheets.

Now let's see how many names we have in the whole chain of three
documents.  Once again click on the italic f but this time run the
pointer through Names.  You will find that six names have been defined,
two we know about (bf and cf) but the other four are names which have
been defined (locally) in the other two documents.

Defining the two names bf and cf in all documents makes it easier to
create the next document.  Let's create [Acc_9504] using the blank file
[Acc_95xx] as the template.  Double click on the file [Acc_95xx] to
load it, change the formula in the slot [Acc_95xx]D12 from its present
value to set_value([Acc_9505]bf,cf) and then Save the file as
[Acc_9504].

Next you will need to 'push' the "Carried Forward" value from the
previous document, [Acc_9503] into your new [Acc_9504] file.  To do
this you double click on [Acc_9503] to load it - then delete it from
the screen!  You will find that the "Brought Forward" value in
[Acc_9504] has taken the correct value 'pushed' into it from the
[Acc_9503] file you loaded and then removed from the screen and memory.

To summarise

When you have a long 'chain' of dependent documents then you should
consider whether you want to 'drag' or 'push' values from one member of
the 'chain' to another.  I suggest that for an application such as that
of this tutorial you need to 'push' rather than 'drag' slot values from
one sheet to the next in the 'chain'.  When you have decided that the
correct strategy is to 'push' you need to use the set_value(,) function
in the (often unwanted) 'dependent' document to 'push' the value (or
values) you want to carry forward to the next document.

Names can be used for single slots or for ranges of slots.  Generally
it is better to define names in such a way that the definition is
'local'.  That is to say the definition does not refer to (and thus
call into memory) another document.  If you define a Name in one
document you can always use it in another document - we have done this
in the 1995 'chain' of documents - but if you define a name in one
document which refers to a slot in a different document (what I call a
'non-local' definition) then you have a 'link' that you may not want.

Finally

If you have an application which you think may be simplified by the use
of set_value(,) or Names but can't quite work out how to do it, then
please ask me.  I will be able to work on your problem and send you the
solution much more easily if you provide a disc based example of what
you are doing together with some note (in a [ReadMe] file) indicating
exactly what you are trying to achieve.