vb.net - VB .net - 在字符串中找到第 N 次出现的字符的最短和最快方法？

Question

实现这一目标的专业方法是什么？

谢谢。

Answer 1

我无耻地从这个问题中扯掉了这个例子，并将它从 C# 转换为 VB.net。

Public Function GetNthIndex(s As String, t As Char, n As Integer) As Integer
    Dim count As Integer = 0
    For i As Integer = 0 To s.Length - 1
        If s(i) = t Then
            count += 1
            If count = n Then
                Return i
            End If
        End If
    Next
    Return -1
End Function

Answer 2

这是使用 Linq 的一种方法。

Public Function GetNthIndex(searchString As String, charToFind As Char, n As Integer) As Integer
    Dim charIndexPair = searchString.Select(Function(c,i) new with {.Character = c, .Index = i}) _
                                    .Where(Function(x) x.Character = charToFind) _
                                    .ElementAtOrDefault(n-1)
    Return If(charIndexPair IsNot Nothing, charIndexPair.Index, -1)
End Function

用法：

Dim searchString As String = "Assessment"
Dim index As Integer = GetNthIndex(searchString, "s", 4) 'Returns 5

Answer 3

如果你想要更快：

Public Function NthIndexOf(s As String, c As Char, n As Integer) As Integer
    Dim i As Integer = -1
    Dim count As Integer = 0

    While count < n AndAlso i >= 0
        i = s.IndexOf(c, i + 1)
        count += 1
    End While

    Return i

End Function

尽管如果您要在一长串“a”中寻找第 n 个“a”（例如），它比 Mike C 的回答要慢一些。

编辑：根据 spacemonkeys 的评论进行了调整。

Answer 4

我的 Andew 版本，但我相信这会考虑到第一个字符是否是您正在寻找的字符

 Public Function GetNthIndexStringFunc(s As String, t As String, n As Integer) As Integer
        Dim newFound As Integer = -1
        For i As Integer = 1 To n
            newFound = s.IndexOf(t, newFound + 1)
            If newFound = -1 Then
                Return newFound
            End If
        Next
        Return newFound
    End Function