Exercise - Use the string's IndexOf() and Substring() helper methods
- 25 minutes
In this exercise, you use the IndexOf() method to locate the position of one or more characters string inside a larger string. You use the Substring() method to return the part of the larger string that follows the character positions you specify.
You'll also use an overloaded version of the Substring() method to set the length of characters to return after a specified position in a string.
Prepare your coding environment
This module includes hands-on activities that guide you through the process of building and running demonstration code. You're encouraged to complete these activities using Visual Studio Code as your development environment. Using Visual Studio Code for these activities helps you to become comfortable writing and running code in a developer environment used by professionals worldwide.
Note
If you have completed other Microsoft Learn modules in this C# series, you may have already created a project folder for code samples. If that's the case, you can skip the following section of steps, and delete the code in the Project.cs file used for a previous exercise.
Open Visual Studio Code.
You can use the Windows Start menu (or equivalent resource for another OS) to open Visual Studio Code.
On the Visual Studio Code File menu, select Open Folder.
In the Open Folder dialog, navigate to the Windows Desktop folder.
If you have different folder location where you keep code projects, you can use that folder location instead. For this training, the important thing is to have a location that’s easy locate and remember.
In the Open Folder dialog, select Select Folder.
If you see a security dialog asking if you trust the authors, select Yes.
On the Visual Studio Code Terminal menu, select New Terminal.
Notice that a command prompt in the Terminal panel displays the folder path for the current folder. For example:
C:\Users\someuser\Desktop>At the Terminal command prompt, to create a new console application in a specified folder, type dotnet new console -o ./CsharpProjects/TestProject and then press Enter.
This .NET CLI command uses a .NET program template to create a new C# console application project in the specified folder location. The command creates the CsharpProjects and TestProject folders for you, and uses TestProject as the name of the
.csprojfile.In the EXPLORER panel, expand the CsharpProjects folder.
You should see the TestProject folder and two files, a C# program file named Program.cs and a C# project file named TestProject.csproj.
In the EXPLORER panel, to view your code file in the Editor panel, select Program.cs.
Delete the existing code lines.
You're using this C# console project to create, build, and run code samples during this module.
Close the Terminal panel.
Write code to find parenthesis pairs embedded in a string
Ensure that you have Visual Studio Code open and Program.cs displayed in the Editor panel.
Note
Program.cs should be empty. If if isn't, select and delete all code lines.
Type the following code into the Visual Studio Code Editor:
string message = "Find what is (inside the parentheses)"; int openingPosition = message.IndexOf('('); int closingPosition = message.IndexOf(')'); Console.WriteLine(openingPosition); Console.WriteLine(closingPosition);On the Visual Studio Code File menu, select Save.
The Program.cs file must be saved before building or running the code.
In the EXPLORER panel, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.
A Terminal panel should open, and should include a command prompt showing that the Terminal is open to your TestProject folder location.
At the Terminal command prompt, to run your code, type dotnet run and then press Enter.
Note
If you see a message saying "Couldn't find a project to run", ensure that the Terminal command prompt displays the expected TestProject folder location. For example:
C:\Users\someuser\Desktop\csharpprojects\TestProject>You should see the following output:
13 36In this case, the index of the
(character is 13. Remember, these values are zero-based, so it's the 14th character in the string. The index of the)character is36.Now that you have the two indexes, you can use them as the boundaries to retrieve the value between them.
Add code to retrieve the value between parenthesis
Update your code in the Visual Studio Code Editor as follows:
string message = "Find what is (inside the parentheses)"; int openingPosition = message.IndexOf('('); int closingPosition = message.IndexOf(')'); // Console.WriteLine(openingPosition); // Console.WriteLine(closingPosition); int length = closingPosition - openingPosition; Console.WriteLine(message.Substring(openingPosition, length));Save your code file, and then use Visual Studio Code to run your code. You should see the following output:
(inside the parenthesesThe
Substring()method needs the starting position and the number of characters, or length, to retrieve. So, you calculate the length in a temporary variable calledlength, and pass it with theopeningPositionvalue to retrieve the string inside of the parenthesis.The result is close, however the output includes the opening parenthesis. In this exercise, the inclusion of the parenthesis isn't desired. To remove the parenthesis from output, you have to update the code to skip the index of the parenthesis itself.
Modify the starting position of the sub string
Update your code in the Visual Studio Code Editor as follows:
string message = "Find what is (inside the parentheses)"; int openingPosition = message.IndexOf('('); int closingPosition = message.IndexOf(')'); openingPosition += 1; int length = closingPosition - openingPosition; Console.WriteLine(message.Substring(openingPosition, length));Save your code file, and then use Visual Studio Code to run your code. You should see the following output:
inside the parenthesesTake a moment to review the previous code and the line
openingPosition += 1;.By increasing the
openingPositionby1, you skip over the opening parenthesis character.The reason you're using the value
1is because that is the length of the character. If you attempt to locate a value starting after a longer string, for example,<div>or---, you would use the length of that string instead.Update your code in the Visual Studio Code Editor as follows:
string message = "What is the value <span>between the tags</span>?"; int openingPosition = message.IndexOf("<span>"); int closingPosition = message.IndexOf("</span>"); openingPosition += 6; int length = closingPosition - openingPosition; Console.WriteLine(message.Substring(openingPosition, length));Take a moment to review the previous code and the line
openingPosition += 6;.The preceding snippet of code shows how to find the value inside an opening and closing
<span>tag.In this case, you're adding
6to theopeningPositionas the offset to calculate the length of the sub string.
Avoid magic values
Hardcoded strings like "<span>" in the previous code listing are known as "magic strings" and hardcoded numeric values like 6 are known as "magic numbers". These "Magic" values are undesirable for many reasons and you should try to avoid them if possible.
Review the previous code to consider how the code might break if you hardcoded the string
"<span>"multiple times in your code, but misspelled one instance of it as"<sapn>".The compiler doesn't catch
"<sapn>"at compile time because the value is in a string. The misspelling leads to problems at run time, and depending on the complexity of your code, it might be difficult to track down.Furthermore, if you change the string
"<span>"to the shorter"<div>", but forget to change the number6to5, then your code produces undesirable results.Update your code in the Visual Studio Code Editor as follows:
string message = "What is the value <span>between the tags</span>?"; const string openSpan = "<span>"; const string closeSpan = "</span>"; int openingPosition = message.IndexOf(openSpan); int closingPosition = message.IndexOf(closeSpan); openingPosition += openSpan.Length; int length = closingPosition - openingPosition; Console.WriteLine(message.Substring(openingPosition, length));Take a minute to examine the updated code and the use of the keyword
constas used inconst string openSpan = "<span>";.The code uses a constant with the
constkeyword. A constant allows you to define and initialize a variable whose value can never be changed. You would then use that constant in the rest of the code whenever you needed that value. This ensures that the value is only defined once and misspelling theconstvariable is caught by the compiler.The previous code listing is a safer way to write the same code you examined in the previous section. Now, if the value of
openSpanchanges to<div>, the line of code that uses theLengthproperty continues to be valid.
Recap
This unit covered much material. Here's the most important things to remember:
IndexOf()gives you the first position of a character or string inside of another string.IndexOf()returns-1if it can't find a match.Substring()returns just the specified portion of a string, using a starting position and optional length.- There's often more than one way to solve a problem. You used two separate techniques to find all instances of a given character or string.
- Avoid hardcoded magic values. Instead, define a
constvariable. A constant variable's value can't be changed after initialization.