Branching and Looping in C#, Part 2 - break and continue statements

(Page 6 of 6 )

The break statement breaks the execution of the current statement block and executes the first line that follow the block. Usually you will find code like this:

static void Main(string[] args)
{
  int x = 0;
  for(;;)
  {
    x = Convert.ToInt32(Console.ReadLine());
    if(x == -1)
      break; 
    else
      Console.WriteLine("It's " + x);
  }
  Console.WriteLine("Program exist");
}

The code is simply an infinite for loop (infinite for loops constructed using empty expressions for all the parts as you can see), and inside the loop we use an if statement to test if the input number is -1; if so,  the for loop breaks, and execution begins at the first line after the for loop. You can check the message "Program exist" using CTRL + F5. The above code can be written using the while statement as follows:

static void Main(string[] args)
{
  int x = 0;
  while(true)
  {
    x = Convert.ToInt32(Console.ReadLine());
    if(x == -1)
      break;
    else
      Console.WriteLine("It's " + x);
  }
  Console.WriteLine("Program exist");
}

As you can see, the for(;;) loop is the same as while(true), but you need to use the break statement, or maybe your algorithm needs the goto statement in order to terminate the execution of the infinite loop, and continue on the first statement after the loop. As we saw before,  the break statement's primary use is with the switch statement, so I will not discuss it again. Before we end our discussion about the break statement, I want to distinguish between break and goto through code. Consider the following code:

static void Main(string[] args)
{
  for(;;)
  {
    Console.WriteLine("Enter a word");
    string temp = Console.ReadLine();
    Char[] tempChar = temp.ToCharArray();
    foreach(Char x in tempChar)
    {
      if(x == 'h')
        break;
      else
        Console.WriteLine(x);
    }
  }
}

This code takes a word from the user and writes every character in the word in a separate line. If the character is 'h', it breaks using the break statement. Now enter the following words and see what will happen: "Michael" then "Mic" then "h". Some of you will think that whenever the program encounters the 'h' it will terminate because of the break statement; the result is the following:

As you can see, in the first word "Michael" the program prints only the characters before the character 'h'; in the second word "Mic" it prints all the characters; and in the third word (which is only the letter h) it will print nothing. Guess what? The loop is still here and the program didn't terminate. All of this happens because the break statement breaks only the current block, which in our case is the inner foreach block. So when it breaks the foreach loop, the infinite for loop iterates again and again, and it will never end. One perfect solution for this problem involves using the goto statement as follows:

static void Main(string[] args)
{
  for(;;) 
  {
    Console.WriteLine("Enter a word");
    string temp = Console.ReadLine();
    Char[] tempChar = temp.ToCharArray();
    foreach(Char x in tempChar)
    {
      if(x == 'h') 
        goto exist;
      else
        Console.WriteLine(x);
    }
  }

  exist:
    Console.WriteLine("the Program is terminating");
}

You will get the following result, as expected:

This is one perfect use for the goto statement, where the break statement is not appropriate at all.

C# features another keyword that must be discussed with the break statement; it's the continue statement. As you know, the break statement breaks the execution of the current loop and begins execution on the first line after the current loop. The continue statement differs from the break statement because it will not break the execution of the current loop; instead it will terminate the current iteration and begin a new iteration in the loop. In other words, the continue statement, when encountered in the loop, will escape the next statements in the iteration and go to the top of the loop to CONTINUE looping. Here is the same example as I used above, this time with continue instead of break and goto:

static void Main(string[] args)
{
  for(;;)
  {
    Console.WriteLine("Enter a word");
    string temp = Console.ReadLine();
    Char[] tempChar = temp.ToCharArray();
    foreach(Char x in tempChar)
    {
      if(x == 'h')
        continue;
      else
        Console.WriteLine(x);
    }
  }
}

Enter the word "Michael" and the see the result:

What happened is that the program actually escaped the character 'h' and continued printing all the other characters without terminating the loop. As I said before, each statement fits inside a specific scenario, so use the continue statement when you need to escape a number of statements without the loop termination.

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