Fastest way to conditionally strip off the right part of a string - string

Question

I need to remove the numeric part at the end of a string. Here are some examples:
"abcd1234" -> "abcd"
"a3bc45" -> "a3bc"
"kj3ih5" -> "kj3ih"
You get the idea.
I implemented a function which works well for this purpose.
Function VarStamm(name As String) As String
Dim i, a As Integer
a = 0
For i = Len(name) To 1 Step -1
If IsNumeric(Mid(name, i, 1)) = False Then
i = i + 1
Exit For
End If
Next i
If i <= Len(name) Then
VarStamm = name.Substring(0, i - 1)
Else
VarStamm = name
End If
End Function
The question is: is there any faster (more efficient in speed) way to do this? The problem is, I call this function within a loop with 3 million iterations and it would be nice to have it be more efficient.
I know about the String.LastIndexOf method, but I don't know how to use it when I need the index of the last connected number within a string.

Answer 1

You can use Array.FindLastIndex and then Substring:
Dim lastNonDigitIndex = Array.FindLastIndex(text.ToCharArray(), Function(c) Not char.IsDigit(c))
If lastNonDigitIndex >= 0
lastNonDigitIndex += 1
Dim part1 = text.Substring(0, lastNonDigitIndex)
Dim part2 = text.Substring(lastNonDigitIndex)
End If

Answer 2

I was skeptical that the Array.FindLastIndex method was actually faster, so I tested it myself. I borrowed the testing code posted by Amessihel, but added a third method:
Function VarStamm3(name As String) As String
Dim i As Integer
For i = name.Length - 1 To 0 Step -1
If Not Char.IsDigit(name(i)) Then
Exit For
End If
Next i
Return name.Substring(0, i + 1)
End Function
It uses your original algorithm, but just swaps out the old VB6-style string methods for newer .NET equivalent ones. Here's the results on my machine:
RunTime :
- VarStamm : 00:00:07.92
- VarStamm2 : 00:00:00.60
- VarStamm3 : 00:00:00.23
As you can see, your original algorithm was already quite well tuned. The problem wasn't the loop. The problem was Mid, IsNumeric, and Len. Since Tim's method didn't use those, it was much faster. But, if you stick with a manual for loop, it's twice as fast as using Array.FindLastIndex, all things being equal

Answer 3

Given your function VarStamm and Tim Schmelter's one named VarStamm2, here is a small test performance I wrote. I typed an arbitrary long String with a huge right part, and ran the functions one million times.
Module StackOverlow
Sub Main()
Dim testStr = "azekzoerjezoriezltjreoitueriou7657678678797897898997897978897898797989797"
Console.WriteLine("RunTime :" + vbNewLine +
" - VarStamm : " + getTimeSpent(AddressOf VarStamm, testStr) + vbNewLine +
" - VarStamm2 : " + getTimeSpent(AddressOf VarStamm2, testStr))
End Sub
Function getTimeSpent(f As Action(Of String), str As String) As String
Dim sw As Stopwatch = New Stopwatch()
Dim ts As TimeSpan
sw.Start()
For i = 1 To 1000000
f(str)
Next
sw.Stop()
ts = sw.Elapsed
Return String.Format("{0:00}:{1:00}:{2:00}.{3:00}",
ts.Hours, ts.Minutes, ts.Seconds,
ts.Milliseconds / 10)
End Function
Function VarStamm(name As String) As String
Dim i, a As Integer
a = 0
For i = Len(name) To 1 Step -1
If IsNumeric(Mid(name, i, 1)) = False Then
i = i + 1
Exit For
End If
Next i
If i <= Len(name) Then
VarStamm = name.Substring(0, i - 1)
Else
VarStamm = name
End If
End Function
Function VarStamm2(name As String) As String
Dim lastNonDigitIndex = Array.FindLastIndex(name.ToCharArray(), Function(c) Not Char.IsDigit(c))
If lastNonDigitIndex >= 0 Then
lastNonDigitIndex += 1
Return name.Substring(0, lastNonDigitIndex)
End If
Return name
End Function
End Module
Here is the output I got:
RunTime :
- VarStamm : 00:00:38.33
- VarStamm2 : 00:00:02.72
So yes, you should choose his answer, his code is both pretty and efficient.