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>a3:"                 LESSON7 -- Using Associated Files
>a6:"       Throughout these lessons we have seen how two separate files 
>a7:" can be displayed simultaneously to the screen.  This capability is made
>a8:" possible by Perfect Calc's unique system of 'multiple buffers', which
>a9:" allows more than one spreadsheet to be held in machine memory at one
>a10:" time.  As we have mentioned, up to seven associated spreadsheets can be
>a11:" active simultaneously, permitting us to switch back and forth between
>a12:" them with ease, copying and sharing data as needed (this feature is
>a13:" only available with Perfect Calc!).
>a15:"       In this lesson we are going to examine how spreadsheets can be
>a16:" linked together and made to share data--a feature we are sure you will
>a17:" agree represents one of the most remarkable advances ever to appear
>a18:" in personal computer software.  For the first time, a powerful set of
>a19:" programming tools are available to the typical personal computer user
>a20:" in a 'natural language'.  With this feature you will find yourself
>a21:" doing programming without ever knowing that you are!
>a24:"       When a spreadsheet is copied from disk to machine memory, it is
>a25:" placed in a temporary work space called a 'buffer', which serves to 
>a26:" isolate it from other spreadsheets and internal program functions.  
>a27:" Thus, changes and recalculations made to a spreadsheet in one buffer
>a28:" do not affect files held in other buffers, a necessary condition if 
>a29:" the integrity of spreadsheet files is to be maintained.
>a31:"       However, sometimes it is desireable or even necessary to have
>a32:" spreadsheets work together sharing both functions and data.  For
>a33:" example, if we are preparing a tax report, it might be convenient if 
>a34:" the 'tax' spreadsheet could directly reference positions on other
>a35:" spreadsheets that calculate our income, net worth, etc.,  thus elimin-
>a36:" ating the need for manual transfer of data.
>a38:"       Perfect Calc provides a means of doing this through a process
>a39:" called 'linking' or 'associating' files.
>a45:"  Consider the following formula:   b17 = b18 + incomeÆc6Å
>a46:"  
>a47:"       This formula instructs Perfect Calc to calculate the sum of two
>a48:" variables, one of which is held in the current buffer and one in a
>a49:" buffer called 'income'.  When calculating this formula,  Perfect Calc
>a50:" will go to the spreadsheet in the buffer 'income' and retrieve whatever
>a51:" value is located in position 'c6', afterwards returning to complete 
>a52:" the calculation in the current buffer.  (Note the syntax of the cross-
>a53:" reference:  the variable is enclosed in square brackets and follows
>a54:" immediately after the name of the buffer containing it.)
>a56:"       The referenced buffer must exist (i.e.  be active in memory) and
>a57:" must hold the correct spreadsheet.  If it does not, the formula will
>a58:" either compute to 'Error!' or produce a faulty value.
>a59:"       However, more is needed than just cross-references between the
>a60:" spreadsheet files.  Spreadsheets which reference each other must be
>a61:" 'linked' or 'associated' in such a way that a recalculation in one
>a62:" automatically causes a recalculation in the other.  Otherwise data in
>a63:" the non-current spreadsheet may become outdated, if it is not regularly
>a64:" updated by switching to it and forcing a recalculation.  Remember, a
>a65:" recalculation normally affects only the spreadsheet in the current 
>a66:" buffer, and no other.
>a68:"       Perfect Calc provides a means of doing this through its 
>a69:" ASSOCIATE FILES Command, which will link the files such that a recalcu-
>a70:" lation in one will cause a recalculation in all.
>a72:"       Linked or associated files exist in a special 'dependency' to
>a73:" each other.  The first file (considered the master file) is dependent
>a74:" upon the file, or files, which follow it.  Dependency means that the
>a75:" files are recalculated in a specific order:  that is, the 'depended-on'
>a76:" files are always recalculated BEFORE those files which depend on them
>a77:" are recalculated.
>a79:" HOW TO LINK FILES
>a81:"       Given several files which reference each other, you would place
>a82:" the cursor in the file which would depend on all the other files, and
>a83:" type:
>a84:"                         Control--x  a
>a86:"       Perfect Calc would respond with the message:
>a88:"                         "Associate file:  <CR>  "
>a89:"       You would then type the name of the first file which the current
>a90:" file would 'depend on', and which would have to be recalculated before
>a91:" the current file is recalculated.
>a93:"       Perfect Calc would 'associate' the two files and switch you to
>a94:" the file that you have just named.  If this second file depends on yet
>a95:" a third file, you would give the ASSOCIATE FILES Command again, linking
>a96:" the third file to the second.  Up to seven spreadsheets may be linked
>a97:" consecutively in this way.
>a99:"       To preserve the 'associative' links you must save each file in
>a100:" turn, using the SAVE FILE Command  (Control--x  Control--s).
>a102:"      When reading such a chain of linked files into memory, either when
>a103:" beginning Perfect Calc or by using the FIND FILE Command, you have only
>a104:" to name the first file in the chain.  Perfect Calc will automatically
>a105:" read the remaining linked files without your having to specify them.
>a107:"       Associated spreadsheets, as we will see shortly, allow us to
>a108:" handle data in ways that previously were not possible.  In the remain-
>a109:" der of this lesson let us examine a set of linked spreadsheets to
>a110:" demonstrate exactly how this feature works.
>a112:"       At this time move the cursor to line  121  and give the CREATE
>a113:" TWO WINDOWS Command  (Control--x  2).
>a115:"       (Do not synchronize the windows.)
>a116:"       Move the cursor to the top window, using the OTHER WINDOW 
>a117:" Command  (Control--x  o ).
>a119:"       Type the FIND FILE Command  (Control--x  Control--f),  followed
>a120:" by the filename:  'Rental.pc'.                    
>a121:"       In the next few moments Perfect Calc will read into machine
>a122:" memory three separate, but 'linked', files:
>a124:"       1)  'Rental.pc'
>a125:"       2)  'Unit1.pc'
>a126:"       3)  'Unit2.pc'
>a128:"       When the read operation is complete, Perfect Calc will display
>a129:" the 'unit2.pc' file in the top window.  Use the SWITCH BUFFERS Command
>a130:" to go to the first buffer in the chain, 'rental'.
>a131:"       As you can see, 'Rental.pc' is a simple rental property manage-
>a132:" ment program, which calculates the total monthly income generated by
>a133:" various holdings of property, in this case 'Unit1' and 'Unit2' .
>a135:"       Move the cursor to position 'b5' and examine the formula contain-
>a136:" ed there:
>a137:"                     Formula:  unit1Æb19Å
>a139:"       This tells Perfect Calc to display in position 'b5' whatever
>a140:" value is held in position 'b19' of the spreadsheet buffer 'unit1' .
>a141:"       Let us switch to buffer 'unit1' to see what data is being refer-
>a142:" enced in its spreadsheet.  Use the SWITCH BUFFER Command  
>a143:" (Control--x  b),  followed by the buffer name: 'unit1' .
>a145:"       Perfect Calc switches us to buffer 'unit1' .  The spreadsheet
>a146:" here contains income and expense figures for a three bedroom house, 
>a147:" from which it will calculate a 'Cash Flow' for the property.  Move
>a148:" the cursor to position 'b19', and examine the formula which accomplish-
>a149:" es this:
>a151:"                     Formula:  b8 - (b16 + b18)
>a153:"       This formula instructs Perfect Calc to subtract 'Total Expenses'
>a154:" and 'Mortgage Payment' from 'Monthly Rent', producing a figure that
>a155:" represents either a positive or negative 'Cash Flow'  ('b19').  To see
>a156:" how this value is referenced by the previous spreadsheet, 'rental.pc',
>a157:" enter the following values for January:
>a158:"       Monthly rent:      $550.00          Maintenance:      $35.00
>a159:"       Repairs:            $45.00          Taxes:           $110.75
>a160:"       Insurance:          $12.50          Mortgage:        $350.00 
>a162:"       Perfect Calc automatically calculates 'Total Expenses' and 'Cash
>a163:" Flow'.   Since this file is 'linked' to the file 'Rental.pc' which
>a164:" depends on it, we can assume that a recalculation has already occurred
>a165:" in the other file.  Switch back to the buffer 'Rental' to see what 
>a166:" has happened.  (use Control--X  b     to SWITCH BUFFERS)
>a168:"       As you can see, 'Rental.pc' has automatically referenced a value
>a169:" for January 'Cash Flow' from 'Unit1' and, using this value, has calcu-
>a170:" lated figures for 'YEARLY TOTALS' and 'Monthly Cash Totals'.
>a172:"       Move the cursor to 'b6' of 'Rental.pc' and examine the formula
>a173:" at this position:
>a174:"                        b6 = unit2Æb19Å
>a176:"       This formula is identical to the previous one, except that its
>a177:" value is taken from the spreadsheet in buffer 'Unit2' .  Switch to
>a178:" this buffer using the SWITCH BUFFER Command  (Control--x  b).  This
>a179:" spreadsheet is identical to the spreadsheet in buffer 'Unit1', except
>a180:" that it calculates rental income for a 'warehouse' .  Enter several
>a181:" sample values in this spreadsheet, afterwards switching back to the 
>a182:" master spreadsheet in buffer 'rental' .
>a184:"       As before, 'rental.pc' has already obtained a 'Cash Flow' value
>a185:" from 'unit2.pc' and has recalculated its formulas accordingly.  
>a187:"       As we mentioned, a particular order of dependency exists between
>a188:" the spreadsheet files, which determines their order of recalculation.
>a189:" This relationship is reflected in the Buffer Directory, which displays
>a190:" the 'links' existing between the spreadsheets.
>a192:"       Move the cursor to the bottom window, using the OTHER WINDOW
>a193:" Command  (Control--x  o ), afterwards typing the CREATE ONE WINDOW
>a194:" Command (Control--x  1).  Now, give the BUFFER DIRECTORY Command:
>a196:"                    Control--x  Control--b
>a198:"       The Buffer Directory will display at the bottom of the screen,
>a199:" overwriting some of this text.  If necessary, scroll the window to
>a200:" erase the directory, or give the REDRAW WINDOW Command (Escape...
>a201:" Control--L ).
>a203:"       In the Buffer Directory 'arrows' indicate the links between 
>a204:" spreadsheets.  'Rental' is linked (i.e. depends on)  'Unit1', which
>a205:" is linked to 'Unit2'.  You should note that we are currently using four
>a206:" different spreadsheets in memory.  Amazing!
>a207:"   æType  Escape...Control--Lå
>a209:"       As you perhaps realize 'Unit1' does not really depend on 'Unit2'.
>a210:" The only dependent spreadsheet in the chain is 'Rental.pc', which 
>a211:" depends on all the files which follow it.  The files 'Unit1.pc' 
>a212:" and 'Unit2.pc' could therefore have been linked in reverse order,
>a213:" and a correct recalculation would have resulted.  However,
>a214:" this will not always be the case, and we can easily conceive of
>a215:" instances in which the order of recalculation among a series of files
>a216:" IS important and must be strictly followed.
>a218:"       One can see how simple and convenient associated spreadsheets are
>a219:" to use.  One master spreadsheet, working in conjunction with two
>a220:" subsidiary spreadsheets (a total of six subsidiary spreadsheets would
>a221:" have been possible), results in a very orderly and intelligible format,
>a222:" for what might otherwise have been a hodgepodge of loosely connected
>a223:" financial data, involving awkward and inefficient manual transfers of
>a224:" information.  The associated spreadsheet files, while maintaining their
>a225:" functional integrity, operate in a unified and harmonious fashion.
>a227:"       This concludes our lesson on 'Using Associated Files' .  However,
>a228:" before we end this series of lessons on learning to use Perfect Calc, 
>a229:" we wish to conclude with a few important 'suggestions' regarding the
>a230:" use of Perfect Calc, which we are sure you will find helpful.
>a232:"       At this time delete entire machine memory, and proceed to the 
>a233:" concluding lesson, 'lesson8.pc' with the FIND FILE Command.          
>a235:"                           END OF LESSON7
>a1
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