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X = Y ' X is the newly assigned variable; ' Y contains some value you re copying
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(Careful with this one! In most languages, what gets copied is the contents of the variable, not the variable itself. If you later change the contents of Y, X stays the same. Some programming languages let you specify a pointer to a variable, so that if you change Y, X changes too. Not all languages for robot programming provide for this, however.)
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Many programming languages let you reference specific portions of physical memory. For example:
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X = Peek (1024) ' read value of data starting at memory ' location 1024
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A port reference is a value maintained by your robot s hardware. A good example is the parallel printer port on your PC, which can be used to control robot parts like motors. For example:
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X = Inp (889) ' read value at port 889
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Evaluated expression The evaluated expression variable stores a value that is the result of an expression. For example:
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X = 2 + 2 ' X holds 4
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VARIABLES, EXPRESSIONS, AND OPERATORS 91
COMBINING DATA TYPES IN A VARIABLE
Modern high-level languages such as Visual Basic or JavaScript are designed to take the drudgery out of mundane programming chores. Visual Basic and JavaScript both support (at least as an option) weak data typing, which basically means that the language lets you write programs without requiring that you manage the data types stored in variables. In Visual Basic, for example, you can just throw all data into a variant data type, which accepts numbers of different sizes, text strings, you name it. Visual Basic takes care of managing the underlying memory requirements for the data you provide. The typical robot control language is designed to be small and fast because it s meant to be run on a small single-board computer, or an even smaller microcontroller. The Visual Basic programming environment takes up megabytes of hard disk space; the average robot control program is under 1 kilobyte in size. Because of their simplicity, programming languages for robot control require strong data typing, where you the programmer do all the data-typing work yourself. It s not as hard as it seems. But if you re used to languages such as Visual Basic and JavaScript, learning the requirements of strong data typing may require a period of adjustment. One of the most difficult aspects of strong data typing is that you cannot directly mix two data types together in a single variable. The reason the result of the mixing probably won t fit in the memory space allocated for the variable. If you add a byte and a string together in an integer variable, the memory will only hold the one byte. When you try to mix data types the programming environment you re using will either display an error or the robot will not function correctly. In fact, it could function erratically, possibly damaging itself, so exercise care! Here s an example of two data types that cannot be mixed when used in a programming environment that requires strong data typing:
Dim X as Byte Dim Y as Word Dim Z as Byte X = 12 Y = 1025 Z = X + Y
Both X and Z are byte-length variables, each of which holds eight-bit values (e.g., 0 to 255). Y is a word-length variable, which can hold a 16-bit value (e.g., 0 to 65535). The statement Z X Y will fail because Z cannot hold the contents of Y. If the programming environment doesn t catch this error, it ll create a bug in your robot. (At best, Z will hold the value 13 and not 1037, as you d otherwise expect. The value 13 is what s left of 1037 when that number is stored in a space of only eight bits.) Just because strong data typing restricts you from mixing and matching different data types doesn t mean you can t do it. The trick is to use the data conversion statements provided by the programming language. The most common data conversion is between integer numbers and text. Assuming the programming language you re using supports them, use Val to convert a string to a numeric type and Str to convert a numeric into a string:
Dim X as String Dim Y As Integer X = "1" Y = Val (X)
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