Optimal String Array Search

Category:

C#, VB.NET, ASP.NET

Type:

Snippets

Difficulty:

Intermediate

Author:

Anonymous

Version Compatibility: Visual Basic 6, Visual Basic.NET

More information:

If you have a string array sorted in ascending order, this code returns the index of the string found with the array. It returns -1 if the string is not found in the array. For large arrays, it uses a recursive function to locate the index by dividing the array in two. For small arrays it does a sequential lookup.

Instructions: Copy the declarations and code below and paste directly into your VB project.

Declarations:
Dim MySortedStringArray() as string, FoundIndex as long, SearchStr as string

Code:

FoundIndex = GetIndex(SearchStr, MySortedStringArray)

Function GetIndex(sch As String, ByRef StrArray() As String) As Long
   '  This function chooses the optimum search method for a sorted string array
   Dim FilterResults() As String, idx As Long
   FilterResults = Filter(StrArray, sch)
   If FilterResults(0) <> sch Then
      GetIndex = -1
   Else
      GetIndex = Int((UBound(StrArray) - LBound(StrArray)) / 2)
      If (UBound(StrArray) - LBound(StrArray)) < 84 Or _
         StrArray(GetIndex) > sch Then ' Do sequential search
         For GetIndex = LBound(StrArray) To UBound(StrArray)
             If StrArray(GetIndex) = sch Then Exit For
         Next
      Else
         GetIndex = RecursiveArraySearch(LBound(StrArray), UBound(StrArray), sch, StrArray)
      End If
   End If

End Function

Function RecursiveArraySearch(LB As Integer, UB As Integer, sch As String, ByRef SA() As String) As Integer
      RecursiveArraySearch = LB + Int((UB - LB) / 2)
      If SA(RecursiveArraySearch) = sch Then Exit Function
      If SA(RecursiveArraySearch) > sch Then RecursiveArraySearch = RecursiveArraySearch(LB, RecursiveArraySearch - 1, sch, SA)
      If SA(RecursiveArraySearch) < sch Then RecursiveArraySearch = RecursiveArraySearch(RecursiveArraySearch + 1, UB, sch, SA)
End Function