VB.NET - Loop Over String Chars

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.

String Length

String Chars

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.

For

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.

Detail The characters in the input string are accessed separately and printed the screen.

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

Char

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 Module

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.