IndexOf returns the index of a substring. It scans the String. If the substring is not found, it returns -1. IndexOf is part of the .NET Framework's base class library. It is often useful—but there are some tricks to using it correctly.
First, here we locate the first index of a String in a source String. The example has an input String, and we search for the location of "Vader" with the IndexOf function. If the string is not found, the result is -1.
Program that uses IndexOf: VB.NET Module Module1 Sub Main() ' The input string you are using Dim s As String = "Darth Vader has pulmonary problems." ' Does the string contain "Vader"? If (s.IndexOf("Vader") <> -1) Then Console.Write("string contains 'Vader'") End If ' Finished Console.ReadLine() End Sub End Module Output string contains 'Vader'
In the example, the string "Vader" is found in the input String, at index 6. The console program then prints the message inside the If-statement. The final statement ReadLine is there so you can run the program without it exiting.Console
Here we look at looping with IndexOf on Strings. Sometimes you need to locate the first index of a String, and then continue locating further instances. You can accomplish this with a Do While construct and the IndexOf method.
Here: The example first declares the Dim String, which contains the letter 'a' in three places. Next, we call IndexOf on the String.
Program that uses IndexOf and While: VB.NET Module Module1 Sub Main() ' The input String dim Dim s As String = "You have a cat" ' The interation dim Dim i As Integer = s.IndexOf("a"c) ' Loop over the found indexes Do While (i <> -1) ' Write the substring Console.WriteLine(s.Substring(i)) ' Get next index i = s.IndexOf("a"c, i + 1) Loop Console.ReadLine() End Sub End Module Output ave a cat a cat at
The Do While loop continues until IndexOf returns -1. If the letter never occurs, the loop will never be entered. The loop prints out the Substring located at the index found by IndexOf. The three lines printed start with the letter "a".
Here we look at an overview of these methods in the .NET Framework. There are four Functions in the IndexOf family available in VB.NET. Usually you will only need the first, IndexOf, but the others are occasionally valuable.
IndexOf: This finds the first index of a Char or String. You can specify start and ending indexes of where you want to search.
IndexOfAny: This function finds the first index of any of the characters in the Array you send it.
This starts searching from the end.
It is the same as IndexOf in all other ways.
LastIndexOfAny: This finds the last index of any of the characters in the Array it receives.
Here we see a simple example of using the Substring Function in VB.NET with the result of IndexOf. The IndexOf call below locates the first index of the uppercase letter 'B'. You can find more details on the Substring function.Substring
Program that uses Substring: VB.NET Module Module1 Sub Main() ' The string you are searching Dim s As String = "Visual Basic rocks" ' Find index of uppercase letter 'B' Dim i As Integer = s.IndexOf("B"c) ' This new string contains the substring starting at B Dim part As String = s.Substring(i) Console.WriteLine(part) Console.ReadLine() End Sub End Module Output Basic rocks
IndexOfAny receives an array of characters as the argument and then returns the index of the first one found. You could call IndexOf many times to duplicate the functionality of IndexOfAny. But the IndexOfAny function is simpler to call.IndexOfAny
In the .NET platform, the Contains Function on System.String simply calls IndexOf internally. Therefore, you should only use it if you want a different value to indicate failure—False instead of -1.Contains
Result when substring exists: True.
Result when substring does NOT exist: False.
Result when substring exists: >= 0.
Result when substring does NOT exist: -1.
The most important performance tip I know is to use a character instead of a one-character String. In some cases, the character overload is 85% faster. It will return an equivalent value.
Note: A detailed benchmark is available on the C# version of this article. The underlying implementations are the same.IndexOf
We covered several examples of using the IndexOf Function. We used IndexOf with Strings and characters. Next we called it in Do While-loops. And we checked its return value, which is negative one if the argument is not found.
Review: This is a powerful Function in Visual Basic .NET and can simplify your program. Use it instead of duplicating For-loops.