visual basic barcode program PRINT "A Simple Small BASIC Program" FOR X = 1 TO 10 GOSUB 100 NEXT END 100 PRINT X RETURN in Java

Drawer PDF-417 2d barcode in Java PRINT "A Simple Small BASIC Program" FOR X = 1 TO 10 GOSUB 100 NEXT END 100 PRINT X RETURN

PRINT "A Simple Small BASIC Program" FOR X = 1 TO 10 GOSUB 100 NEXT END 100 PRINT X RETURN
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The program produces the following output:
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A Simple Small BASIC Program 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
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Although the meanings of the Small BASIC keywords are nearly intuitive, each is also fully explained later in this chapter.
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One last point: Small BASIC is patterned after the original version of BASIC from several years ago, not Visual Basic. Visual Basic has very little in common with the original BASIC language. Of course, once you understand how the interpreter works, you can change it to interpret whatever language or variant you desire.
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An Overview of the Interpreter
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At the outset it is necessary to reemphasize that the interpreter developed in this chapter is a source code interpreter. This means that it executes a program by reading its source code one statement at a time, performing each specified operation as it goes. This differs from a pseudo-code interpreter, such as the original Java runtime system that interprets bytecode. The difference is that a source code interpreter operates directly on the source code of the program. A pseudo-code interpreter executes a program after it has been converted by a compiler into a machine-independent, intermediate code. Source code interpreters are easier to create and don't require a separate compilation stage. The Small BASIC interpreter contains two major subsystems: the expression parser, which handles numeric expressions, and the interpreter, which actually executes the program. The expression parser is adapted from the one shown in 2. As it is used here, it parses a numeric expression that is contained within a larger program, rather than just parsing a selfcontained expression as it did in 2. Both the interpreter and the parser subsystems are contained within a single interpreter class, called SBasic. Although it would in theory have been possible to use two separate classes, one for the interpreter and one for the expression parser, it was more efficient to combine both into a single class. The reason for this is that the expression parser and the interpreter code are highly intertwined. For example, both operate on the same character array that holds the program. Separating them into two classes would have added considerable overhead, a loss of performance, and duplication of functionality. Furthermore, because parsing an expression is simply a part of the larger task of interpreting a program, it makes sense to have the entire mechanism contained within a single class. The interpreter works by reading the source code of a program one token at a time. When it encounters a keyword, it does whatever that keyword requests. For example, when the interpreter reads PRINT, it prints the value of the expression that follows. When it reads GOSUB, it executes the specified subroutine. This process continues until the end of the program is reached. Thus, the interpreter simply does what the program tells it to do!
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The Small BASIC Interpreter
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The code for the Small BASIC interpreter is fairly long longer than one would normally put in a chapter of a book. However, don't be intimidated by its size. Despite its length, the interpreter is conceptually simple, and once you grasp its general mode of operation, each part is easy to understand. The entire code for the Small BASIC interpreter is shown next. The remainder of this chapter will explain how it works, and how to use it.
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3: Implementing Language Interpreters in Java
// A Small BASIC Interpreter. import java.io.*; import java.util.*; // Exception class for interpreter errors. class InterpreterException extends Exception { String errStr; // describes the error public InterpreterException(String str) { errStr = str; } public String toString() { return errStr; } } // The Small BASIC interpreter. class SBasic { final int PROG_SIZE = 10000; // maximum program size // These are the token types. final int NONE = 0; final int DELIMITER = 1; final int VARIABLE = 2; final int NUMBER = 3; final int COMMAND = 4; final int QUOTEDSTR = 5; // These are the types of errors. final int SYNTAX = 0; final int UNBALPARENS = 1; final int NOEXP = 2; final int DIVBYZERO = 3; final int EQUALEXPECTED = 4; final int NOTVAR = 5; final int LABELTABLEFULL = 6; final int DUPLABEL = 7; final int UNDEFLABEL = 8; final int THENEXPECTED = 9; final int TOEXPECTED = 10; final int NEXTWITHOUTFOR = 11; final int RETURNWITHOUTGOSUB = 12; final int MISSINGQUOTE = 13;
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