What is a cursor
In FoxPro a cursor is a temporary table, the term is short for "CURrent Set Of Records"
and is nothing to do with the arrow that the mouse is moving around the screen. You use
an SQL query to create a cursor. Open the Data Session window and run this code in the
Command Window:
Select
Cust_ID ;
From
customer ;
Into Cursor
csrCustID ;
Order By
cust_ID
You will see that csrCustID has appeared in the Data Session window and that it looks
very similar to the Customer table beneath it. If you were to click on the Customer
table then the only difference that you'd see is that the status line at the bottom of
the window would show the work area and the size of the table. In general, a cursor
behaves very like a table and the next paragraph will explain why.
Where a cursor is stored
The cursor looks like a FoxPro table and that's because it's stored as a FoxPro
table. Select the cursor and type ?dbf() in the Command
Window to see the name of the dbf file for the current work area. You'll get something
like E:\Temp\000052Q1006I.Tmp as the answer. The cursor is being held in dbf format in
your Windows Temp folder.
There is no need to do anything about these temporary files, Fox is supposed to delete
them when you close the cursor or close FoxPro. The process does sometimes fail but no
harm is done. Fox has no record of the temporary file and will not try to reuse it.
You can safely delete any files that FoxPro misses.
NOFILTER - reusing a table
Sometimes you do not get a temporary file at all. Sometimes Fox realises that it can
satisfy your SQL request by just filtering records from the current table. This
optimisation generally gives you a faster response but it can also crash your code if you
are expecting to be able to alter the contents of the cursor without affecting its
parent table. Use the Nofilter keyword to force Fox into
creating a temporary file:
Select
Cust_ID ;
From
customer ;
Into Cursor
csrCustID
Nofilter ;
Order By
cust_ID
The Nofilter keyword is a recent addition to Visual FoxPro.
In earlier versions we had to add a calculated field to the SQL so that Fox was forced
to create a new file. Adding a dummy field:
Select
Cust_ID,
"Dummy" As Dummy ;
From
customer ;
Into Cursor
csrCustID
Nofilter ;
Order By
cust_ID
meant that Fox could not get all the data it needed for the cursor by filtering the
source table and forced it to create a new file to hold the extra field.
READWRITE - Modifying a cursor
A standard cursor is read-only and this can be an annoyance because it means that
you cannot index the table. FoxPro uses byte 28 of a dbf file to say whether or
not there is an index file associated with this data table. This byte cannot be set in
a read-only cursor so the cursor cannot be indexed. Use the
Readwrite option to create a read/write cursor.
Select
Cust_ID,
"Dummy" As Dummy ;
From
customer ;
Into Cursor
csrCustID
Readwrite ;
Order By
cust_ID
As with the Nofilter keyword mentioned above,
Readwrite is a new addition to Visual FoxPro and you
might see a workaround in older code. The technique was to create the cursor and then
to open the cursor's temporary dbf file as though it were a regular FoxPro table:
Select
Cust_ID ;
From
customer ;
Into Cursor
csrTemp ;
Order By
cust_ID
Select 0
Use Dbf('csrTemp')
Again Alias csrCustID
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