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This chapter is from the book

This chapter is from the book

Recursion

Calling a method recursively or implementing the method using recursion refers to use of a method that calls itself. Recursion is sometimes the simplest way to implement a particular algorithm. Listing 5.18 counts the lines of all the C# source files (*.cs) in a directory and its subdirectory.

Listing 5.18: Counting the Lines within *.cs Files, Given a Directory

using System.IO;

public static class LineCounter
{
  // Use the first argument as the directory
  // to search, or default to the current directory
  public static void Main(string[] args)
  {
      int totalLineCount = 0;
      string directory;
      if (args.Length > 0)
      {
          directory = args[0];
      }
      else
      {
          directory = Directory.GetCurrentDirectory();
      }
      totalLineCount = DirectoryCountLines(directory);
      System.Console.WriteLine(totalLineCount);
    }

  static int DirectoryCountLines(string directory)       
  {
      int lineCount = 0;
      foreach (string file in
          Directory.GetFiles(directory, "*.cs"))
      {
            lineCount += CountLines(file);
        }

      foreach (string subdirectory in
          Directory.GetDirectories(directory))
      {
          lineCount += DirectoryCountLines(subdirectory);
      }

      return lineCount;
  }

  private static int CountLines(string file)
  {
      string line;
      int lineCount = 0;
      FileStream stream =
          new FileStream(file, FileMode.Open);4
      StreamReader reader = new StreamReader(stream);
      line = reader.ReadLine();

       while(line != null)
       {
           if (line.Trim() != "")
           {
               lineCount++;
           }
           line = reader.ReadLine();
       }

      reader.Close();  // Automatically closes the stream
      return lineCount;
  }
}

Output 5.9 shows the results of Listing 5.18.

Output 5.9

104

The program begins by passing the first command-line argument to DirectoryCountLines() or by using the current directory if no argument is provided. This method first iterates through all the files in the current directory and totals the source code lines for each file. After processing each file in the directory, the code processes each subdirectory by passing the subdirectory back into the DirectoryCountLines() method, rerunning the method using the subdirectory. The same process is repeated recursively through each subdirectory until no more directories remain to process.

Readers unfamiliar with recursion may find it confusing at first. Regardless, it is often the simplest pattern to code, especially with hierarchical type data such as the filesystem. However, although it may be the most readable approach, it is generally not the fastest implementation. If performance becomes an issue, developers should seek an alternative solution to a recursive implementation. The choice generally hinges on balancing readability with performance.

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