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How to loop a table in MapBasic

UPDATED: March 26, 2018


How to loop a table in MapBasic

One of the often used things in MapBasic is to loop thru a table to read values from the table and then react to or modify these values.

MapBasic has a number of looping methods:

  • Do Until ... Loop
  • Do While ... Loop
  • While ... Wend
  • For ... Next

However one seems better than the others when it comes to looping a table: Do Until ... Loop. At least that's the one I have been using for years. You can also use Do While ... Loop and While ... Wend.

But I will recommend that you stay away from For ... Next when looping thru a table. I'll get back to why.

Now you need a few other statements as well when looping a table:

  • Fetch First From some_table_name
  • Fetch Next From some_table_name
  • EOT( some_table_name )

Let's try to put all this together:
 

Set CoordSys Table MY_TABLE

Fetch First From MY_TABLE
Do Until EOT(MY_TABLE)

   nRowID = MY_TABLE.ROWID
   oFeature = MY_TABLE.OBJ

   Fetch Next From MY_TABLE
Loop

In the example above I loop the table called MY_TABLE. For each record I read the RowID, the hidden dynamic column referring to a specific post in a table or query, and the spatial object from the column called OBJ.

Oh, and I set my application to use the same coordinate system as the table before I start looping.

Now this loop doesn't really do anything clever yet. I would need to add my logic just before the Fetch Next From statement.

I also promised to explain why you shouldn't use the For ... Next loop when working with a table. The problem isn't as much the loop, but more that people tend to combine this loop with a different Fetch statement; Fetch Rec nRec From.
 

Dim nRec, nRowID As Integer,
    oFeature As Object

Set CoordSys Table MY_TABLE

For nRec = 1 To TableInfo(MY_TABLE, TAB_INFO_NROWS)
   Fetch Rec nRec From MY_TABLE

   nRowID = MY_TABLE.ROWID
   oFeature = MY_TABLE.OBJ
Next

Don't use this structure!!

This Fetch statement will put the table cursor at a specific row in the table, which in most case is fine. But it has a drawback; it will fail if the specific row has been deleted. And if you haven't built-in error handling in your application, the application will crash.

Fetch Next, on the other side, will skip the deleted records during the loop.


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