how to connect barcode scanner to visual basic 2010 PROGRAMMING CONCEPTS: THE FUNDAMENTALS in Software

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82 PROGRAMMING CONCEPTS: THE FUNDAMENTALS
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back up and get out of the way, routine 1 would take over again. Multitasking programming of the kind shown in Fig. 7.2b is used in the LEGO Mindstorms Robotic Invention System (as just one example), which enables the robot s operating system to support up to 10 simultaneous routines (called tasks) at one time.
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VARIABLES
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A variable is a special holding area for information, a kind of storage box for data. In most programming languages you can make up the name of the variables as you write the program. The contents of the variable is either specified by you or filled in from some source when the program is run. Since the information is in a variable, it can be used someplace else in the program as many times as you need. Variables can hold different kinds of data, depending on the number of bits or bytes that are needed to store that data. A numeric value from 0 to 255, for example, requires eight bits, that is, one byte, of storage space. Therefore, the variable for such a value needs to be at least one byte large, or else the number won t be stored properly. If you define a variable type that stores fewer than eight bits, its contents will be invalid. If you define a variable type that stores more than eight bits, you ll waste memory space that might otherwise be used for additional programming code.
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EXPRESSIONS
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An expression is a mathematical or logical problem that a program must solve before it can continue. Expressions are often simple math statements, such as 2 2. When you ask a program to perform some type of calculation or thinking process, you re asking it to evaluate an expression.
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All routines running simultaneously
Routine_3 Procedural routines
Routine_3 Multitasking routines
FIGURE 7.2 Procedural programming and multitasking programming use subroutines in different ways.
IMPORTANT PROGRAMMING CONCEPTS 83
One of the most common expressions is evaluating if a statement is True or False (we ve capitalized the words true or false to show that you re dealing with logical functions). Here s a good example of a True/False expression that must be evaluated by a program:
If Number=10 Then End
This expression reads: If the contents of the Number variable is equal to 10, then end the program. Before proceeding, your robot must pause, take a peek inside the Number variable, and apply it to the logical expression. If the result is True, then the program ends. If it s False (Number has a value other than 10), then something else happens.
STRINGS
A common term encountered in programming circles is the string. A string is simply a sequence of alphabetic or numeric characters. These characters are stored within the computer s memory one right after the other, like beads on a string. In the context of programming languages, strings are most often used in variables. Once the string is stored in a variable, it can be acted upon by the program. For example, if the string is text, you can compare it with another string to see if the two are the same. Strings are also often used to allow the robot to communicate with you via a liquid crystal display (LCD) panel.
NUMERICAL VALUES
Computers, and the programs that run on them, are designed from the ground up to work with numerical values (numbers). As with strings, numerical values are often used in variables. Numbers differ from strings in one important way, however: the program can perform calculations on two numbers and provide you with the result. Math calculations are not possible with strings because you can t multiply or divide strings since they usually consist of text. (Actually, you can perform calculations with the numeric equivalent of a string character, but this is not typically done.) Note that many programming languages can store digits as either numbers or as strings. The way the value is stored will determine what kind of operations you can perform on it. If the digits 12345 are stored as a number, you can use them in a math calculation. But if the digits 12345 are stored as a string, you can t use them in a calculation.
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