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Figure 18-13. The variable Pressed will hold True or False. If the Left button is pressed immediately after the count-up from one to three, the NXT BUTTONS SENSOR block will detect the press and change the logic value to True. When the program starts, the initial value will be False, but it will change to True if the Left button is bumped at the end of the count-up. Next, I want SPOT to count down from three to one. I ll drop another three SOUND blocks into the program. Figure 18-14 shows these three SOUND blocks and the configuration panel for the last SOUND block.
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Figure 18-14. SPOT will count down from three to one. After the countdown ends, we need to display Pressed or Not Pressed on the LCD screen. To do this, we ll drop in another VARIABLE block. This time I m going to choose my Pressed variable again, but I m changing its Action section option to Read (see Figure 18-15).
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Figure 18-15. A SWITCH block will read the Pressed variable s value. Now, I drop in a SWITCH block and configure it to read a Logic value (see Figure 18-16), and I ve turned off Flat view so the SWITCH block is visible in its tabbed format. I ve also dragged a data wire from the VARIABLE block to the SWITCH block that contains the value True or False. I ve also dropped a DISPLAY block in the True tab (of the SWITCH block) and configured it to display Pressed (this isn t shown in the configuration panel in Figure 18-16, but it is easy enough to figure out).
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Figure 18-16. This DISPLAY block will display Pressed if the variable has a value of True.
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On the False tab, I place another DISPLAY block that clears the LCD screen and puts the words Not Pressed on the screen (see Figure 18-17).
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Figure 18-17. The words Not Pressed will display if the variable has a value of False. Finally, I drop in an NXT BUTTON WAIT block to wait until the Right button is pressed; this will give me time to view the results on the screen (see Figure 18-18).
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Figure 18-18. If I press the Right button, the program will end.
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CHAPTER 18 TITLE = ANYTHING YOU LIKE
Now run the program. Try it a few times press the Left button immediately after the count-up ends, or don t press it. Your decision to press or not press the button will be converted to a True or False value that is written to the Pressed variable. After the countdown, the Pressed variable is read by the SWITCH block, and the proper text is written on the LCD screen. Variables are a powerful way for your robot to store away information and to use that data later. Once the variable has been created and data written to it, that data will be available anytime you need it well, at least until the program ends. If you want to store data for use after the program ends and/or after the power has been turned off, you ll need a different type of block. And that s where the CONSTANT block becomes useful.
The CONSTANT Block
When you use the VARIABLE block in a program, any variable value (Number, Text, or Logic) will be saved only while the program is running. Once the program ends, your variable data is gone. Normally this isn t a big deal; you typically program your robots to use variables that are useful only at the time the program is running. But what if there s a value that often changes but is used by multiple programs uploaded to your robot For example, let s imagine a program (Program 1) that stops the robot 12 inches from a FINISH LINE and has the robot do a little dance before finishing. Let s say that the initial distance between the STARTING LINE and the FINISH LINE is 24 rotations. Program 1 could instruct the robot with a MOVE block to roll 20 rotations (four rotations short of the FINISH LINE) to stop just before it crosses the FINISH LINE and do a little dance before finishing. Let s also imagine another program (Program 2) that should have the robot roll half the distance between the STARTING LINE and FINISH LINE before doing its dance; you d program it with a MOVE block configured to roll the robot forwards 12 rotations. When the robot reaches this point, it stops, does a dance, and then continues to the finish line. Finally, let s have a third program (Program 3) that rolls the robot forward 1/4 of the total distance or six rotations, has the robot do a little dance, and then continues rolling it to the FINISH LINE. Once again, you could just configure the MOVE block to roll forward six rotations before beginning the dance routine. You should be able to program these three programs easily. But what happens if I change the distance between the STARTING LINE and FINISH LINE to 48 rotations Now you have to go into all three programs (Program 1, Program 2, and Program 3) and change a bunch of MOVE blocks configuration panels. There should be an easier way to save important values between programs and there is! It s called the CONSTANT block. Like the VARIABLE block, the CONSTANT block can store a single value a number, a logic value, or a string of text. This value is then stored on the NXT brick and is referred to as a global constant. As long as you don t delete the program, this value will stay on the brick even when the power is turned off! Because it stays on the brick, you can use the CONSTANT block in other programs to read the value stored as a constant on the brick. That s the global part it s available everywhere (well, at least by any program that s stored on your brick). To show you how it works, I m going to create new program (with the name Distance) that creates a constant called StartToFinish that holds a value equal to the distance (in inches) between the STARTING LINE and the FINISH LINE. Figure 18-19 shows the single block contained in this program.
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