Can I create a Jump table in VBA for Excel? - vba

Question

I wrote a simple translator / parser to process an EDI (830) document using multiple Select Case statements to determine the code to be executed. I’m opening a file in binary mode and splitting the document into individual lines, then each line is split into the various elements where the first element of every line has a unique segment identifier.
My code works perfectly as written. However, Select Case requires checking every Case until a match is found or the Case Else is executed. I’ve sequenced the Case statements in such a manner that the segments that appear most frequently (as in the case of loops), are placed first to minimize the number of "checks before code is actually executed.
Rather than using multiple Select Cases, I would prefer to determine an index for the segment identifier and simply call the appropriate routine using that index. I’ve used jump tables in C and Assembler and anticipated similar functionality may be possible in VBA.

Answer 1

You can do jump tables in VBA by using the Application.Run method to call the appropriate routine by name. The following code demonstrates how it works:
Public Sub JumpTableDemo()
Dim avarIdentifiers() As Variant
avarIdentifiers = Array("Segment1", "Segment2")
Dim varIdentifier As Variant
For Each varIdentifier In avarIdentifiers
Run "Do_" & varIdentifier
Next varIdentifier
End Sub
Public Sub Do_Segment1()
Debug.Print "Segment1"
End Sub
Public Sub Do_Segment2()
Debug.Print "Segment2"
End Sub

Answer 2

You can do this in Excel VBA, following the example below:
The example assumes you have split your EDI document into two columns, one with the 'processing instruction' and one with the data that instruction will process.
The jump table is to the right i.e. a distinct list of the 'processing instructions' plus a name of a Sub-routine to run for each instruction.
The code is:
Option Explicit
Sub JumpTable()
Dim wsf As WorksheetFunction
Dim ws As Worksheet
Dim rngData As Range '<-- data from your file
Dim rngCell As Range '<-- current "instruction"
Dim rngJump As Range '<-- table of values and sub to run for value
Dim strJumpSub As String
Dim strJumpData As String
Set wsf = Application.WorksheetFunction '<-- just a coding shortcut
Set ws = ThisWorkbook.Worksheets("Sheet1") '<-- change to your worksheet
Set rngData = ws.Range("A2:A17") '<-- change to your range
Set rngJump = ws.Range("E2:F4") '<-- change to your circumstances
For Each rngCell In rngData
strJumpSub = wsf.VLookup(rngCell.Value, rngJump, 2, False) '<-- lookup the sub
strJumpData = rngCell.Offset(0, 1).Value '<-- get the data
Application.Run strJumpSub, strJumpData '<-- call the sub with the data
Next rngCell
End Sub
Sub do_foo(strData As String)
Debug.Print strData
End Sub
Sub do_bar(strData As String)
Debug.Print strData
End Sub
Sub do_baz(strData As String)
Debug.Print strData
End Sub
Make sure that you have written a Sub for each entry in the jump table.