Variable Block

Think of a variable as a place to store a value in the NXTs built-in memory. Other programming blocks can read the variable’s current value (and even change it) by connecting to the block with data wires.

Creating a Variable

1. To create a variable, you must first choose the Define Variable command in the Edit menu of the software. In the dialog window, give your new variable a short, easy-to-understand name and set the type of data that the variable will hold (i.e., text, number, or logic).
2. Then drag a Variable block into your program and select the variable’s name from the list.
3. Choose to either "read" from or "write" to the variable. Choosing to write to a variable will let you change the variable’s value over time by overwriting its current value with new information. To do this, you must wire an input data wire to the block’s data hub that will deliver the new information.
4. You can give the variable a starting constant value by typing in the input box (or by using the radio buttons if your variable’s data type is set to logic).

Using a Variable

A variable’s value can change over time. You might create a variable called "Recorded Light Level" and give it an initial value of 50. Then as your robot (fitted with a light sensor) drives to a certain location, you can use a Variable block (set to "write") to record the current light level at the location. To do this, you would connect an output data wire from the Light Sensor block to the input plug of the Variable block.

Later in your program you can use a Variable block (set to "read") to read the light value recorded at the location. Other blocks connected to the block’s output plug by data wire would be able to access the information saved in the variable (i.e., the recorded light level).

1. The block’s data hub will open automatically when the block is placed in the work area. A data wire must be connected to this block’s output plug for other blocks to read the variable’s current value. To write new information to the variable, you must also connect an input data wire to the block’s data hub. (See the Data Hub section below for more information.)

Configuring the Variable Block

1. In the list, select the variable that you created using the Define Variable command of the Edit menu.
2. Choose to either read the variable’s current value or write new information to the variable.
3. To give your variable a constant value, either type a number or text into the input box, or use the radio buttons if the variable is of the logic data type. Remember that if an input data wire is connected to the block and the Variable block is set to "read," new information coming in dynamically through the data wire will take precedence over any constant value.

Sharing Information between a Program and a My Block

In order to share data between your main program and a My Block (i.e., between two .rbt files), do these two things:

1. Define a variable with the same name and data type in both the main program and in the My Block.
2. Use that variable in a Variable block in the main program and in the My Block.

Any place that the variable is used will read/write to the same memory location on the NXT.

Configuring the Variable block’s Data Hub

You can control the Variable block dynamically by connecting data wires (from other blocks’ data hubs) to the Variable block’s data hub.

Open a block’s data hub by clicking the tab at the lower left edge of the block after it has been placed on the work area.

Data wires carrying input information to a block are connected to the plugs on the left side of its data hub. Data wires carrying output information are connected to the plugs on the right side.

[A] Input plug
[B] Output plug
[C] Number data wire (yellow)
[D] Logic data wire (green)
[E] Text data wire (orange)
[F] Broken data wire (gray)

Passing data from the input plug to the output plug

If an input plug has a corresponding output plug (see A above), the input data will pass through from the input plug to the output plug without being changed. In this case, you can only use the output plug if the input plug is connected to an input data wire; connecting an output data wire to such an output plug without a connected input data wire will cause the output data wire to be "broken" (and colored gray).

Data wires carry specific types of data

Each data wire carries a specific type of data between blocks. For example, if a data wire is dragged from a logic plug on a block’s data hub, it can only be connected to a logic plug on another block’s data hub. The chart below shows what kind of data each plug can accept or send out.

Data wire colors

Data wires are identified with specific colors: wires carrying number data are colored yellow, wires carrying logic data are colored green, and wires carrying text data are colored orange.

"Broken" data wires

If you try to connect a data wire to a plug of the wrong data type, the data wire will be broken (and colored gray). You will not be able to download your program if a data wire is broken.

If you click a broken wire you can read why it is broken in the small help window in the lower right corner of the work area.

Data must be within the possible range of the plug

If an input data wire transmits a value outside the possible range of the plug it is connected to, the block will either ignore the value or change it to a value within its range. For plugs that allow just a few input values (example: just 0, 1, or 2), the plug will ignore the input if a value arrives outside its range.

For plugs that accept larger input ranges (example: 0 – 100), the plug will force any input outside its range to fit. For example, if a Move block’s Power plug receives an input value of 150, the block will change the input value to 100 (i.e., a number within the Power plug’s range).

This chart shows the different characteristics of the plugs on the Variable block’s data hub:

	Plug	Data Type	Possible Range	What the Values Mean	This Plug is Ignored When...
	Value*	Any		Value to read or write

*This data plug will change appearance based on the variable's data type.