So-called "toggle" buttons and "radio lights" are among the most common components of graphical user interfaces. This exercise shows how they differ from other forms of buttons and labels, and provides more practice with resource settings.

You should have read all of Chapter 2 as well as Chapter 5.1 in Nye and O'Reilly, Vol. 4, before starting this exercise.

1. Make a copy of the labels application you used in Exercise 1, calling it toggles (and making all changes to resources and filenames needed to support that application name).

2. Now modify the program so that the short labels ("label_N") are Toggles instead of Labels (see vol. 5 of the O'Reilly series, or the man pages on the widgets, for information on Toggles). Compile and run your program.

   How, if at all, does this change the behavior of the application?

3. One feature of the Toggle widget is that it can be grouped with other Toggles to form a "radio group," where just one of the Toggles can be selected at a time. Change your code so that the Toggles inside each box form a radio group. To do this, you will need to create a variable to store the first Toggle's widget identifier (rather than discarding it). Then, change the loop that creates the remaining Toggles so that it also sets the XtNradioGroup resource to that widget identifier. Compile and run the program. Observe the effects of the radio group.

4. It is also possible to "pre-select" one of the Toggles in a radio group. Which resource setting accomplishes this?

   Change the code to pre-select the first toggle.

5. Change fill_box() so that you no longer discard the widget identifier from the Toggles created inside the loop. Now add a callback immediately after each Toggle is created; note that all Toggles should share a single callback function. At first, just have the callback print a message indicating that it was invoked.

   Now make the callback receive a value from the application. Try passing the "number" of the toggle (the same N that is printed in "label_N") so that the callback can print the number of the Toggle which was selected. Don't forget to cast the int to XtPointer and then re-cast it before printing.

   How many times is the callback actually invoked? What does this tell you about how radio groups work?