[RESOLVED] Fast Letters Only Validation

Hi,

i am wanting to detect whether a short string has numeral or operation characters within it, so that i may reject this string, 1000s of these will be sifted through, therefore i am wanting this to be as fast as possible.

Oviously checking the ascii codes of each character will not be efficient.
Using multiple Instr or Replace functions would not be efficient either, would it?

Please note, a string may contain letters and numbers, but is still rejected because of the numbers.

What do you recommend?

The only way I know of to do it is to check each letter in the string... if they are short strings then the efficiency of it shouldn't be a huge issue..

Now if you only want letters a to z... and anything else would be unacceptable then you can do this..

VB Code:

Function AllLetters(tString As String) As Boolean
     Dim strLetters As String
     Dim i As Integer
     strLetters = "abcdefghijklmnopqrstuvwxyz"
 
     For i = 1 To Len(tString)
          If Instr(strLetters, LCase(Mid(tString, i, 1))) = 0 Then
                AllLetters = False
                Exit Function
          End If
     Next
 
     AllLetters = True
End Function

If you know for sure it's only going to be numbers that make it unacceptable you can do it slightly different.

VB Code:

Function AllLetters(tString As String) As Boolean
     Dim i As Integer
 
     For i = 1 To Len(tString)
          If IsNumeric(Mid(tString, i, 1)) Then
                  AllLetters = False
                  Exit Function
          End If
     Next
  
     AllLetters = True
End Function

steven,

thanks alot for the idea's and samples.

although still requiring comparison for each char, your first sample (where i wouldve used ascii) allows for an easy method to also allow - and ' without a seperate comparison to the alphabet, unlike the way i wouldve done it with ascii.

seems like the best way to do it so far doesn't it, and peformance wise, i tested it out, and no noticable lag with it.

I only made one slight change, which was to scrap lcase and mid
lcase because i forgot to specify this has already been performed, and mid to the .net method .substring

VB Code:

Function AllLetters(ByVal tString As String) As Boolean
        Dim strLetters As String
        Dim i As Integer
        strLetters = "abcdefghijklmnopqrstuvwxyz-'"
 
        Try
            For i = 0 To Len(tString) - 1
                If InStr(strLetters, tString.Substring(i, 1)) = 0 Then
                    AllLetters = False
                    Exit Function
                End If
            Next
        Catch
            MsgBox(Err.Description)
        End Try
 
        AllLetters = True
    End Function

I have a couple of points for you Phill. Normally you wouldn't notice a difference but if you are calling this function many times, it is actaully more efficient to use a Return statement than it is to use an Exit Function statement or allow execution to fall to the End Function statement.

Also, you might like to test these variations to see if there is any appreciable difference in execution time:

VB Code:

Function IsValidString(ByVal text As String) As Boolean
        If text Is Nothing Then
            Return True
        End If
 
        Dim validChars As Char() = "abcdefghijklmnopqrstuvwxyz-'".ToCharArray()
 
        For i As Integer = 0 To text.Length - 1
            If Array.IndexOf(validChars, text.Chars(i)) = -1 Then
                Return False
            End If
        Next i
 
        Return True
    End Function

VB Code:

Function IsValidString(ByVal text As String) As Boolean
        If text Is Nothing Then
            Return True
        End If
 
        Dim validChars As Char() = "abcdefghijklmnopqrstuvwxyz-'".ToCharArray()
        Dim textChars As Char() = text.ToCharArray()
 
        For Each ch As Char In textChars
            If Array.IndexOf(validChars, ch) = -1 Then
                Return False
            End If
        Next ch
 
        Return True
    End Function

I'm quite sure that you could use regular expressions to accomplish the same task, but I don't know whether it would be faster or not. I don't know regular expressions well enough to give you the correct syntax but you may like to lok into it.

Thanks Jm,

i have used Return now, as for char arrays, i believe the only advantage is memory here, when i am heavy into this project again i will test actual times and see if this is the case or not.

The advantage of char arrays is actually the fact that you don't have to repeatedly call methods or properties, which take longer than array access via an index. Whether that difference is significant is the question.

Using jmcilhinney's idea of using regex:

VB Code:

Dim str As String = System.Text.RegularExpressions.Regex.Match(sInputstring, "[\w\-']+").ToString
        If str.Length <> sInputstring.Length Then Return False