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This chapter is from the book

This chapter is from the book

Jump Statements

It is possible to alter the execution path of a loop. In fact, with jump statements, it is possible to escape out of the loop or to skip the remaining portion of an iteration and begin with the next iteration, even when the loop condition remains true. This section considers some of the ways to jump the execution path from one location to another.

The break Statement

To escape out of a loop or a switch statement, C# uses a break statement. Whenever the break statement is encountered, control immediately leaves the loop or switch. Listing 3.48 examines the foreach loop from the tic-tac-toe program.

Listing 3.48. Using break to Escape Once a Winner Is Found

class TicTacToe      // Declares the TicTacToe class.
{
  static void Main() // Declares the entry point of the program.
  {
      int winner=0;
      // Stores locations each player has moved.
      int[] playerPositions = {0,0};

      // Hardcoded board position
      //  X | 2 | O
      // ---+---+---
      //  O | O | 6
      // ---+---+---
      //  X | X | X
      playerPositions[0] = 449;
      playerPositions[1] = 28;

      // Determine if there is a winner
      int[] winningMasks = {
            7, 56, 448, 73, 146, 292, 84, 273 };

      // Iterate through each winning mask to determine
      // if there is a winner.
      foreach (int mask in winningMasks)                                                                   
      {                                                                                                    
          if ((mask & playerPositions[0]) == mask)
          {
              winner = 1;
              break;                                                                                       
          }
          else if ((mask & playerPositions[1]) == mask)
          {
              winner = 2;
              break;                                                                                       
          }
      }                                                                                                    

      System.Console.WriteLine(
          "Player {0} was the winner", winner);
  }
}

Output 3.24 shows the results of Listing 3.48.

Output 3.24.

Player 1 was the winner

Listing 3.48 uses a break statement when a player holds a winning position. The break statement forces its enclosing loop (or a switch statement) to cease execution, and control moves to the next line outside the loop. For this listing, if the bit comparison returns true (if the board holds a winning position), the break statement causes control to jump and display the winner.

Later in the program, you can iterate over each mask corresponding to winning positions on the board to determine whether the current player has a winning position, as shown in Listing 3.48.

The continue Statement

You might have a block containing a series of statements within a loop. If you determine that some conditions warrant executing only a portion of these statements for some iterations, you can use the continue statement to jump to the end of the current iteration and begin the next iteration. The continue statement exits the current iteration (regardless of whether additional statements remain) and jumps to the loop condition. At that point, if the loop conditional is still true, the loop will continue execution.

Listing 3.50 uses the continue statement so that only the letters of the domain portion of an email are displayed. Output 3.25 shows the results of Listing 3.50.

Listing 3.50. Determining the Domain of an Email Address

class EmailDomain
{
  static void Main()
  {
      string email;
      bool insideDomain = false;
      System.Console.WriteLine("Enter an email address: ");

      email = System.Console.ReadLine();

      System.Console.Write("The email domain is: ");

      // Iterate through each letter in the email address.
      foreach (char letter in email)
      {
          if (!insideDomain)
          {
              if (letter == '@')
              {
                  insideDomain = true;
              }
              continue;
          }

          System.Console.Write(letter);
      }
  }
}

Output 3.25.

Enter an email address:
mark@dotnetprogramming.com
The email domain is: dotnetprogramming.com

In Listing 3.50, if you are not yet inside the domain portion of the email address, you can use a continue statement to move control to the end of the loop, and process the next character in the email address.

You can almost always use an if statement in place of a continue statement, and this is usually more readable. The problem with the continue statement is that it provides multiple flows of control within a single iteration, and this compromises readability. In Listing 3.51, the sample has been rewritten, replacing the continue statement with the if/else construct to demonstrate a more readable version that does not use the continue statement.

Listing 3.51. Replacing a continue with an if Statement

foreach (char letter in email)
{
  if (insideDomain)
  {
      System.Console.Write(letter);
  }
  else
  {
      if (letter == '@')
      {
          insideDomain = true;
      }
  }
}

The goto Statement

Early programming languages lacked the relatively sophisticated “structured” control flows that modern languages such as C# have as a matter of course, and instead relied upon simple conditional branching (if) and unconditional branching (goto) statements for most of their control flow needs. The resultant programs were often hard to understand. The continued existence of a goto statement within C# seems like an anachronism to many experienced programmers. However, C# supports goto, and it is the only method for supporting fall-through within a switch statement. In Listing 3.52, if the /out option is set, code execution jumps to the default case using the goto statement; similarly for /f.

Listing 3.52. Demonstrating a switch with goto Statements

// ...
static void Main(string[] args)
{
  bool isOutputSet = false;
  bool isFiltered = false;

  foreach (string option in args)
  {
      switch (option)
      {
          case "/out":
              isOutputSet = true;
              isFiltered = false;
              goto default;                                                                               
          case "/f":
              isFiltered = true;
              isRecursive = false;
              goto default;                                                                               
          default:
              if (isRecursive)
              {
                  // Recurse down the hierarchy
                  // ...

              }
              else if (isFiltered)
              {
                  // Add option to list of filters.
                  // ...
              }
              break;
      }

  }

  // ...

}

Output 3.26 shows how to execute the code shown in Listing 3.52.

Output 3.26.

C:\SAMPLES>Generate /out fizbottle.bin /f "*.xml" "*.wsdl"

To branch to a switch section label other than the default label, you can use the syntax goto case constant; where constant is the constant associated with the case label you wish to branch to. To branch to a statement that is not associated with a switch section, precede the target statement with any identifier followed by a colon; you can then use that identifier with the goto statement. For example, you could have a labeled statement myLabel : Console.WriteLine(); and then the statement goto myLabel; would branch to the labeled statement. Fortunately, C# prevents using goto to branch into a code block; it may only be used to branch within a code block or to an enclosing code block. By making these restrictions, C# avoids most of the serious goto abuses possible in other languages.

In spite of the improvements, using goto is generally considered to be inelegant, difficult to understand, and symptomatic of poorly structured code. If you need to execute a section of code multiple times or under different circumstances, either use a loop or extract code to a method of its own.

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