Tip 68: Removing Duplicate Items from List Box Controls

* VB-CODE (2)

Abstract
The Visual Basic® StrComp function can be used in conjunction with
the RemoveItem method to delete entries that are duplicated in List
Box controls. This article explains how you can compare the items in
two separate List Box controls and delete the duplicate entries from
one of the controls.

Comparing the Contents of Two List Box Controls
The AddItem and RemoveItem methods allow you to add or delete items
from a List Box control in Visual Basic®. If the Sorted property of a
List Box is set to True, the items are automatically sorted in
alphabetical order as each new item is added or an old item is
removed.

If you have two List Box controls in your Visual Basic application
and you want to remove the items from the second control that are
already in the first List Box, you can use the StrComp function,
which allows you to compare two strings to see if they are identical.
When using StrComp to determine if two strings are identical, you can
tell the function to ignore uppercase and lowercase differences.
In other words, the function can be told to treat the string "this is
a test" to be the same as or different from the string "THIS IS A
TEST".

If you want StrComp to ignore the case of the strings you are
comparing, use the 1 argument. To make StrComp include the case of
the strings in the comparison, use the 0 argument (that is,
X = StrComp(String1, String2, 0) or X=StrComp(String1, String2, 1).)
In addition, the Option Compare Text command, which you would place
in the General Declarations section of a form or module, tells
StrComp (and other string functions) that all string comparisons are
to ignore the upper- and lowercase differences.

After you call the StrComp function, it returns the status of the
string comparison. StrComp returns one of the following four possible
values:

-1    The first string is less than the second string.
0     The first string is identical to the second string.
1     The first string is greater than the second string.
NULL  Either String1 or String2 is a NULL (empty) string.

Example Program
The program below shows how to remove duplicate items from List Box
controls. When the program is first executed, both List Box controls
contain two entries that are identical. Clicking the "Remove
Duplicates" command button removes the identical items from the
second List Box control.

1. Create a new project in Visual Basic. Form1 is created by default.
2. Add the following code to the Form_Load event for Form1:

Sub Form_Load()
 List1.AddItem "test1"
 List1.AddItem "test2"
 List1.AddItem "test3"
 List1.AddItem "test4"

 List2.AddItem "test1"
 List2.AddItem "test2"
 List2.AddItem "test5"

End Sub

3. Add a List Box control to Form1. List1 is created by default. Set
   its Sorted property to True.
4. Add a second List Box control to Form1. List2 is created by
   default. Set its Sorted property to True.
5. Add a Command Button control to Form1. Command1 is created by
   default.
6. Add the following code to the Click event for Command1:

Sub Command1_Click()
   Call EliminateDupEntries(List1, List2)
End Sub

7. Create a new function called EliminateDupEntries. Add the
   following code to this function:

Sub EliminateDupEntries(First As Control, Sec As Control)
 Dim Findx As Integer
 Dim Sindx As Integer
 Dim Ret As Integer

 Findx = 0
 For Sindx = 0 To Sec.ListCount - 1
   For Findx = Sindx To First.ListCount - 1
       Ret = StrComp(First.List(Findx), Sec.List(Sindx))
      Select Case Ret
          Case 0:
           Sec.RemoveItem Sindx
           Sindx = Sindx - 1
           Exit For
          Case 1:
           Exit For
       End Select
   Next Findx
 Next Sindx
End Sub