Книга: C# 2008 Programmer

Exiting from a Loop

Exiting from a Loop

To break out of a loop prematurely (before the exit condition is met), you can use one of the following keywords:

? break

? return

? throw

? goto

The break keyword allows you to break out of a loop prematurely:

int counter = 0;
do {
 Console.WriteLine(counter++);
 //---exits the loop when counter is more than 100
 if (counter > 100) break;
} while (true);

In this example, you increment the value of counter in an infinite do-while loop. To break out of the loop, you use a if statement to check the value of counter. If the value exceeds 100, you use the break keyword to exit the do-while loop.

You can also use the break keyword in while, for, and foreach loops.

The return keyword allows you to terminate the execution of a method and return control to the calling method. When you use it within a loop, it will also exit from the loop. In the following example, the FindWord() function searches for a specified word ("car") inside a given array. As soon as a match is found, it exits from the loop and returns control to the calling method:

class Program {
 static string FindWord(string[] arr, string word) {
  foreach (string w in arr) {
   //--- if word is found, exit the loop and return back to the
   // calling function---
   if (w.StartsWith(word)) return w;
  }
  return string.Empty;
 }
 static void Main(string[] args) {
  string[] words = {
   "-online", "4u", "adipex", "advicer", "baccarrat", "blackjack",
   "bllogspot", "booker", "byob", "car-rental-e-site",
   "car-rentals-e-site", "carisoprodol", "casino", "casinos",
   "chatroom", "cialis", "coolcoolhu", "coolhu",
   "credit-card-debt", "credit-report-4u"
  };
  Console.WriteLine(FindWord(words, "car")); //---car-rental-e-site---
 }
}

The throw keyword is usually used with the try-catch-finally statements to throw an exception. However, you can also use it to exit a loop prematurely. Consider the following block of code that contains the Sums() function to perform some addition and division on an array:

class Program {
 static double Sums(int[] nums, int num) {
  double sum = 0;
  foreach (double n in nums) {
   if (n == 0)
    throw new Exception("Nums contains zero!");
   sum += num / n;
  }
  return sum;
 }
 static void Main(string[] args) {
  int[] nums = { 1, 2, 3, 4, 0, 6, 7, 8, 9 };
  try {
   Console.WriteLine(Sums(nums, 2));
  } catch (Exception e) {
   Console.WriteLine(e.Message);
  }
 }
}

When the foreach loop reaches the fifth element of the array (0), it throws an exception and exits the loop. The exception is then caught by the try-catch loop in the Main() method.

The goto keyword transfers program control directly to a labeled statement. Using goto is not considered a best practice because it makes your program hard to read. Still, you want to be aware of what it does, so the following example shows its use:

string[] words = {
 "-online", "4u", "adipex", "advicer", "baccarrat", "blackjack",
 "bllogspot", "booker", "byob", "car-rental-e-site",
 "car-rentals-e-site", "carisoprodol", "casino", "casinos",
 "chatroom", "cialis", "coolcoolhu", "coolhu",
 "credit-card-debt", "credit-report-4u"
};
foreach (string word in words) {
 if (word == "casino")

goto Found;

}
goto Resume;
Found:
Console.WriteLine("Word found!");
Resume:
//---other statements here---

In this example, if the word casino is found in the words array, control is transferred to the label named Found: and execution is continued from there. If the word is not found, control is transferred to the label named Resume:.

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