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VB.NET Loop Over String: For, For Each

Use the For and For-Each loops on a String to loop over its characters.
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.StringsFor Each, For
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.

Note: The value after To (input.Length - 1) has one subtracted from the max. This means the maximum index is one less than the length.

Characters: The characters in the input string were accessed separately and printed the screen.

Get char: In the For-loop, we get the Char with an element indexing expression (input(i)).

Char
VB.NET program that loops over strings Module Module1 Sub Main() ' Input string. Dim input As String = "perls" 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 Console.ReadLine() End Sub End Module Output -- For Each -- p e r l s -- For -- p e r l s
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.

Benchmarks
VB.NET program that times Length locals, loop bounds 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 Module Output 294.88 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.
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