.NET/ASP.NET/C#/VB.NET PDF Document SDK

blocking the neighbors. The best action when missing the second chopstick and returning the first one is to sleep for a while to let the neighboring philosophers finish their meals before trying to get two chopsticks again. This would roughly translate into the following algorithm: 1. Acquire the left chopstick. 2. Try to acquire the right chopstick. 3. If both sticks were acquired, continue to step 6. 4. Release the left chopstick. 5. Think for a while before continuing with step 1. 6. Eat. 7. Release the right chopstick. 8. Release the left chopstick. This eating algorithm can starve up to three philosophers in a worst-case scenario, but at least two of them will get food the deadlock is avoided. Because the probability for getting two chopsticks is equal for all five philosophers, in real life five philosophers will get something to eat now and then.

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The result of the preceding section was nice and neat; but what if our array of strings had come from a user Users have a tendency to whack the Return key a few times before they write anything at all, and add spurious spaces and tabs to the beginning and end of lines, particularly when copying and pasting between applications. They might also add commas or periods or something like that, again in the interest of tidiness. They might spell things incorrectly. There s no accounting for what users might do. Let s simulate that with a new function shown in Example 10-62.

private static string[] SoliloquizeLikeAUser() { return new string[] { "", null, " ", String.Empty, " To be, or not to be--that is the question: ", "Whether 'tis nobelr in the mind to suffer,", "\tThe slings and arrows of outrageous fortune ,", "", "\tOr to take arms against a sea of troubles, ", "And by opposing end them.", "", "", "", "", ""}; }

Notice their extensive use of the Return key, the tendency to put the odd comma at the end of the line, and the occasional whack of the Tab key at the beginning of lines. Sadly, if we use this function and then print the output using String.Concat like we did in Example 10-57, we end up with output like this:

focus() scrollIntoView() removeCssClass(String className)

One common threading scenario in which semaphores come in handy is when you have one or more threads producing data and one or more threads consuming that data. These threads are referred to as producers and consumers. Usually the producers and consumers share a buffer through which the information is sent. By letting one semaphore keep track of the free space in the buffer and another semaphore keep track of the available data in the buffer, it is possible to let the consumers work in parallel until the buffer is either full or empty (the producers or consumers must stop and wait until there is more free space or data available).

To be, or not to be--that is the question: Whether 'tis nobelr in the mind to suffer, The slings and arrows of outrageous fortune , Or to take arms against a sea of troubles, And by opposing end them.

We can write some code to tidy this up. We can build up our output string, concatenating the various strings, and cleaning it up as we go. This is going to involve iterating through our array of strings, inspecting them, perhaps transforming them, and then appending them to our resultant string. Example 10-63 shows how we could structure this, although it does not yet include any of the actual cleanup code.

string[] strings = SoliloquizeLikeAUser(); string output = String.Empty; // This is equivalent to "" foreach (string line in strings) { // Do something to look at the line... // then... output = output + line + Environment.NewLine; } Console.WriteLine(output);

This would work just fine; but look at what happens every time we go round the loop. We create a new string and store a reference to it in output, throwing away whatever was in output before. That s potentially very wasteful of resources, if we do this a lot. Fortunately, the .NET Framework provides us with another type we can use for precisely these circumstances: StringBuilder.

To show you how to work with semaphores, you ll create an application that consists of a producer and a consumer. The producer will pass a given text through a circular buffer to the consumer, which will print the received text to the debug console. Because there will be only one circular buffer, you have implemented it as a set of global variables, as shown in Listing 12-18. The obvious solution if you are planning on using several buffers is to declare these global variables in a class. Then refer each producer and consumer using a buffer to an instance of that class. The buffer consists of a size, bufferSize, and the actual buffer, buffer. Because you are planning on moving QChar objects, the buffer is of that type. The buffer also needs two semaphores: one for keeping track of the free space available and one for the number of available data items. Finally, there is a flag called atEnd that tells the consumer the producer will produce no more data.

Earlier we saw that the Array class offers a variety of helper methods for finding elements in arrays. If you try to use these directly on a List<T>, it won t work. The following code from Example 7-14 will not compile if events is a List<CalendarEvents>, for example:

DateTime dateOfInterest = new DateTime (2009, 7, 12); CalendarEvent[] itemsOnDateOfInterest = Array.FindAll(events, e => e.StartTime.Date == dateOfInterest);