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An expression s type matters. The examples we just looked at involve numbers or numeric variables, and are of type double or int. Expressions can be of any type, though. For example, ("Hello, " + "world") is an expression of type string. If you wrote an assignment statement that tried to assign that expression into a variable of type double, the compiler would complain it insists that expressions are either of the same type as the variable, or of a type that is implicitly convertible to the variable s type. Implicit conversions exist for numeric types when the conversion won t lose information for example, a double can represent any value that an int can, so you re allowed to assign an integer expression into a double variable. But attempting the opposite would cause a compiler error, because doubles can be larger than the highest int, and they can also contain fractional parts that would be lost. If you don t mind the loss of information, you can put a cast in front of the expression:

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int approxKmPerHour = (int) kmPerHour;

his book has relied on QMake to build the example applications by using standard project files without using any advanced features. However, QMake can also be used to manage advanced projects and to handle projects resulting in multiple executables, libraries, and plugins. This chapter introduces some of the most common features you ll need when you want to create more sophisticated Qt projects. You ll also learn about a great alternative to Qmake: Kitware s CMake (http://www.cmake. org/). Like QMake, CMake is an open-source, cross-platform build system. It s worth discussing CMake because of its adoption as the build tool for one of Qt s most prominent users, the KDE project (http://www.kde.org/).

This casts the kmPerHour (which we declared earlier as a double) to an int, meaning it ll force the value to fit in an integer, possibly losing information in the process.

A variable doesn t have to be stuck with its initial value for its whole life. We can assign new values at any time.

<asp:Label ID="lblQuote" runat="server" Text="Label" Width="144px"> </asp:Label> <asp:Button ID="btnMore" runat="server" Text="..." OnClick="btnMore_Click" CausesValidation="False" /> </ContentTemplate> </atlas:UpdatePanel> <atlas:UpdateProgress runat="server" ID="prog1"> <ProgressTemplate> Loading... </ProgressTemplate> </atlas:UpdateProgress> <atlas:TimerControl runat="server" Interval="60000" ID="quotetimer" OnTick="quoteTimer_tick" /> </strong> </td> <td class="style1" style="width: 147px"> <atlas:UpdatePanel ID="UpdatePanel4" runat="server"> <ContentTemplate> <asp:Label ID="lblMoreQuote" runat="server"> </asp:Label> </ContentTemplate> </atlas:UpdatePanel> <atlas:UpdateProgress runat="server" ID="prog2"> <ProgressTemplate> Loading... </ProgressTemplate> </atlas:UpdateProgress> </td> </tr> </table> The first UpdatePanel control (called UpdatePanel3) contains a Label control and a Button control. The Label control will contain the company name and current stock price, as you saw in Figure 11-6 and Figure 11-7. This gets updated whenever you enter a new stock ticker in the text box, and you ll see from the markup that this fires an event, called TextBox1_TextChanged, whenever the text changes. The code for this event is simple it calls the DoUpdate() function: protected void TextBox1_TextChanged(object sender, EventArgs e) { DoUpdate(); }

The previous section showed how to assign an expression s value into a newly declared variable:

QMake is the build tool that is shipped with Qt. It is versatile and can be used to build most projects on all the platforms supported by Qt. It is used to generate a build environment from a project file. It can also create Makefiles and project files for Visual Studio and Xcode.

double kmPerHour = kmTravelled / (elapsedSeconds / (60 * 60));

If at some later stage in the program s execution new information becomes available, we could assign a new value into the kmPerHour variable assignment statements aren t required to declare new variables, and can assign into existing ones:

kmPerHour = updateKmTravelled / (updatedElapsedSeconds / (60 * 60));

This overwrites the existing value in the kmPerHour variable. C# offers some specialized assignment statements that can make for slightly more succinct code. For example, suppose you wanted to add the car s latest lap time to the variable holding the total elapsed time. You could write this:

To start using QMake, let it create a project file for itself by executing the following command: qmake -project QMake will look for files that it recognizes in the current directory and subdirectories and then add them to a standardized project for building an application.

elapsedSeconds = elapsedSeconds + latestLapTime;

This evaluates the expression on the righthand side, and assigns the result to the variable specified on the lefthand side. However, this process of adding a value to a variable is so common that there s a special syntax for it: