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4 | 4 |
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5 | 5 | The programs we have seen in the previous two chapters have been a little predictable in how they run as they have a *linear execution path* through the `main()` function. Such simple programs have very little practical use. More complex programs, which alter their *control flow* based on *user input* fall into two types. *Batch programs* take all of their input at the beginning of their execution, usually from any or all of: program parameters, an environment variable, or an input file. *Interactive programs* enact a dialog with the *user* (the computer operator) while the program is executing. This dialog is often two-way as the user is not necessarily expected to know what input is required without being prompted. Interactive programs often use either a console or a *GUI* (Graphical User Interface, historically found on desktop computers, but more often found these days on tablets and smartphones). Interactive console programs often produce output to the console *interleaved* with user input, while batch programs ususally know all of their input at the beginning of their execution and produce all of their output following this with no further user involvement or action. As an example of a modern alternative, a purely voice-activated device (possibly without a screen) has an interface which interestingly has more in common with an interactive console program than with a GUI application. |
6 | 6 |
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7 | | -As a compliment to the stream output object `cout`, the stream input object `cin` (an abbreviation of "Character Input") overloads `>>` (the *stream extraction operator*) to allow variables to be set from user input. When a `cin` input expression is reached, the program waits (indefinitely) for the user to type some input and press Enter. The following program outputs a message inviting the user to enter a number, and then prints this number out again on the console. Before `cin` is used, the variable to be used to accept the input into must have already been defined so that the type of the required input can be deduced. Providing an initial value is preferred (empty braces give it a default value) in case the read by `cin` fails due to either invalid input, such as the user typing letters where digits were required, or end-of-input (Ctrl-D or Ctrl-Z): |
| 7 | +As a complement to the stream output object `cout`, the stream input object `cin` (an abbreviation of "Character Input") overloads `>>` (the *stream extraction operator*) to allow variables to be set from user input. When a `cin` input expression is reached, the program waits (indefinitely) for the user to type some input and press Enter. The following program outputs a message inviting the user to enter a number, and then prints this number out again on the console. Before `cin` is used, the variable to be used to accept the input into must have already been defined so that the type of the required input can be deduced. Providing an initial value is preferred (empty braces give it a default value) in case the read by `cin` fails due to either invalid input, such as the user typing letters where digits were required, or end-of-input (Ctrl-D or Ctrl-Z): |
8 | 8 |
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9 | 9 | ```cpp |
10 | 10 | // 03-age1.cpp : get and then display an integer |
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