Loop over string. For and For-Each loops pass over Chars in Strings. With these loops we access each individual character. We then can make a decision based on its value.
We demonstrate String loops in VB.NET. Further, we test the performance of accessing the Length and storing it in a local variable outside of the loop.
This program declares a String that contains some characters. Next, the For-Each loop is used. It declares a variable for the loop (element) and uses the For Each, As, In syntax.
And The For-loop is next—it uses the For, As, To syntax. This is the standard loop syntax in VB.NET.
Module Module1
Sub Main()
' Input string.
Dim input As String = "abc"
Console.WriteLine("-- For Each --")
' Use For Each loop on string.
For Each element As Char In input
Console.WriteLine(element)
Next
Console.WriteLine("-- For --")
' Use For loop on string.
For i As Integer = 0 To input.Length - 1
Dim c As Char = input(i)
Console.WriteLine(c)
Next
End Sub
End Module-- For Each --
a
b
c
-- For --
a
b
c
Benchmark, length cache. Should we store the length of the string in local, and access that, for the fastest string loop? Here we test 2 versions of a method that loops over a string.
Version 1 The Length of the string is accessed directly in the loop bounds (we use a For-loop here).
Version 2 A local variable (length) is used to store the length of the string. We then access that in the loop expression.
Result We do not get any benefit from storing Length is a local variable in VB.NET.
Module Module1
Sub Main()
Dim m As Integer = 10000000
Dim s1 As Stopwatch = Stopwatch.StartNew
For i As Integer = 0 To m - 1
' Version 1: do not store length in local variable.
A()
Next
s1.Stop()
Dim s2 As Stopwatch = Stopwatch.StartNew
For i As Integer = 0 To m - 1
' Version 2: store length in local.
B()
Next
s2.Stop()
Dim u As Integer = 1000000
Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
End Sub
Sub A()
Dim value As String = "birds of a feather"
Dim sum As Integer = 0
For i As Integer = 0 To value.Length - 1
If value(i) = "a" Then
sum += 1
End If
Next
End Sub
Sub B()
Dim value As String = "birds of a feather"
Dim sum As Integer = 0
Dim lengthCache As Integer = value.Length - 1
For i As Integer = 0 To lengthCache
If value(i) = "a" Then
sum += 1
End If
Next
End Sub
End Module294.88 ns Access length in For-statement (Faster)
297.07 ns Use local variable for length
A summary. Looping over the characters in a String is common. We can test for invalid characters, search for a certain sequence of characters, or build up a separate data structure.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
