.NET/Java PDF, Tiff, Barcode SDK Library

The break keyword you can see at the end of each case is present mainly for consistency with other C-like languages. In C and C++, if you leave off the break, the code will fall out of one case through to the next. So if we left off the break in the YellowFlag case, we d end up telling drivers not to overtake and then warning them about the safety car. This would be a bug and in general, you almost always don t want fallthrough. It s unfortunate that in C and C++ fall-through was the default. C# changes this: if you want fall-through you must ask for it explicitly by writing goto case "SafetyCar". But despite fall-through no longer being the implicit default, you still need to write the same break statement as you would in other C-family languages when you don t want fall-through if you leave it out you ll get an error.

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The next test case, which is shown in Listing 16-24, checks that the image storage and retrieval mechanisms work and are implemented in the testImages slot. The test procedure is very simple: Add an image to the database (tests addImage), make sure it is there (tests getIds), retrieve it (tests getImage), and compare it with the original image. One final test, which has been commented out, attempts to retrieve an image using an invalid id. This results in a call to qFatal in the ImageCollection class, and the application will end even if you call QTest::ignoreMessage(QString). The ignoreMessage can otherwise be handy to avoid showing expected warning messages emitted using qDebug or qWarning. Listing 16-24. Testing storing and retrieving images void ImageCollectionTest::testImages() { ImageCollection c; QCOMPARE( c.getIds( QStringList() ).count(), 0 ); QImage image( "test.png" ); c.addImage( image, QStringList() ); // Verify that we have one image and get the id for it QList<int> ids = c.getIds( QStringList() ); QCOMPARE( ids.count(), 1 ); int id = ids[0]; QImage fromDb = c.getImage( id ); QVERIFY( pixelCompareImages( image, fromDb ) );

You might be wondering what is the point this does exactly the same as Example 2-10, so why do we need a different syntax As it happens, we don t there s nothing you can do with switch and case that you can t do with if and else. But switch and

case offer one useful advantage: they make it clear what we re doing we re looking at a single expression (raceStatus) and we re choosing one of a number of options based

on the value of that expression. A developer familiar with C# can look at this code and understand the structure of the decision-making process at a glance. With the previous example, you would need to look at each else if statement in turn to make sure it wasn t doing something more complex chained else if statements are more flexible than switch statements, because each new link in the chain is allowed to test a completely different expression, but that flexibility comes at the cost of making it harder to understand the code. Sometimes a self-imposed constraint can make code easier to read and maintain, and a switch statement is a good example of that. Selection statements make programs considerably more useful than they would otherwise be they enable programs to make decisions. But our examples are still rather straightforward they run just once, from start to finish, with the odd variation in the execution flow. The amount of work that is done is pretty trivial. So there s another kind of statement that plays to a computer s greatest strength: the ability to perform simple repetitive tasks many times over.

// Will call qFatal and end the application // QTest::ignoreMessage( QtFatalMsg, "Failed to get image id" ); // fromDb = c.getImage( id+1 ); // QVERIFY( fromDb.isNull() ); // The ImageConnection adds a database that we close here QSqlDatabase::removeDatabase( QLatin1String( QSqlDatabase::defaultConnection ) ); }

An iteration statement allows a sequence of other statements to be executed several times (Repeated execution is also often known as a loop because, like the race car, the code goes round and round again) This seems like it could be useful in our race data analysis race cars usually complete many laps, so we will probably have multiple sets of data to process It would be annoying to have to write the same code 60 times just to process all the data for a 60-lap race Fortunately, we don t have to we can use one of C# s iteration statements Imagine that instead of passing in timing or fuel information as command-line arguments, the data was in files We might have a text file containing one line per lap, with the elapsed time at the end of each lap.