SEARCH FOR SUBSTRINGS

When working with filenames that are embedded in a fully qualified file path, it is helpful to have substring searching capabilities. Different parts of

that fully qualified path can be useful to your program. For example, you may want to check for the file extension or maybe for a certain directory path. The String class provides several methods that assist you in this process.

One way to search for substrings is to use comparision methods. Comparison methods that work with substrings are StartsWith and EndsWith, which essentially do what the method name indicates (that is, find substrings that start a string and finish off a string). These methods yield a Boolean response that indicates if the substring was found.

If you are just looking for a specific character, you can use the IndexOf and LastIndexOf method of the String class to see what index position contains that character.

Another useful way to find substrings is to use regular expressions. This is a more sophisticated way to determine if a substring exists. With regular expressions, you can go further than substrings and look for patterns that exist in the string.

Another handy string-comparison method is the EndsWith method. You can use EndsWith to identify the extension of a file and determine if code should run or not.

TYPE THIS:

using System;

using System.Text.RegularExpressions; namespace StringSample

{

class Search

{

static void Main()

{

string[] sFileNames = new string[3] { "allphotos.aspx", "lri_familyreunion_jan2001_001.jpg", "hri_familyreunion_jan2001_001.jpg"};

Regex rePictureFile = new Regex(".jpg");

foreach (string sFileName in sFileNames)

{

if (rePictureFile.Match(sFileName).Success) Console.WriteLine("{0} is a photo file.",

sFileName);

}

}

}

}

RESULT:

C:\>csc SearchSubStrings_ai.cs

C:\> SearchSubStrings_ai.exe

lri_familyreunion_jan2001_001.jpg is a photo file.

hri_familyreunion_jan2001_001.jpg is a photo file.

C:\>

‡ Use a foreach statement to loop through all the elements in the array.

° In the foreach loop, use the EndsWith function to check the element for a .jpg extension and write the filename to the console if it is a JPEG file.

· Set a debug stop.

‚ Press F5 to save, build, and run the console application.

■ A message appears displaying the two JPEG filenames.

167

If you need to create a string from replacing characters in an existing string, you can use either the String or StringBuilder classes to perform this operation. For example, you may want to take a comma-separated file and

turn it into a tab-separated file.

On the String class, the Replace method lets you replace a character in one string with another character. When you use the String.Replace method, it will search for all instances of the character in the affected string and replace it with the character you specify. If you do not intend to remove more than one of the same character from your string, your best course of action is to replace your original

string using the StringBuilder class discussed earlier in this chapter.

The StringBuilder.Replace method is much more flexible with replacing characters within a string. The

StringBuilder.Replace method gives you four methods for replacing characters: replacing a character string with a new one, replacing all instances of a specified character with another one (just like the String.Replace method), replacing all instances of a string within a specified range, and replacing all instances of a character in a specified range with a new character.

The String.Replace method is rather straightforward, but it is also quite limited just like the String class it references. If you have a StringBuilder class, then you can use the

StringBuilder.Replace method for changing your original String class.

The String.Replace and StringBuilder. Replace methods are both case sensitive, so if you try to replace a character with the lowercase letter t, then Replace function will leave all uppercase T characters alone. If you want to search for all uppercase T characters, then you have to include another String.Replace or String.Builder.Replace method that searches for an uppercase T.

Visual C# returns an ArgumentNullException exception if the character that you are trying to replace is a null value — the character you are trying to find does not exist or if the string has no characters in it at all.

‡ Use the Replace function to replace the commas with tabs and write the result to the console.

° Set a debug stop.

· Press F5 to save, build, and run the console application.

■ A message appears with tabs separating the image filenames.