visual basic barcode program 3: Implementing Language Interpreters in Java in Java

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3: Implementing Language Interpreters in Java
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All keywords must be entered lowercase. */ Keyword kwTable[] = { new Keyword("print", PRINT), // in this table. new Keyword("input", INPUT), new Keyword("if", IF), new Keyword("then", THEN), new Keyword("goto", GOTO), new Keyword("for", FOR), new Keyword("next", NEXT), new Keyword("to", TO), new Keyword("gosub", GOSUB), new Keyword("return", RETURN), new Keyword("end", END) };
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The lookUp( ) method, shown next, uses kwTable to convert a token to its internal representation. If no match is found, UNKNCOM is returned.
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/* Look up a token's internal representation in the token table. */ private int lookUp(String s) { int i; // Convert to lowercase. s = s.toLowerCase(); // See if token is in table. for(i=0; i < kwTable.length; i++) if(kwTable[i].keyword.equals(s)) return kwTable[i].keywordTok; return UNKNCOM; // unknown keyword }
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The run( ) Method
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After an SBasic object has been created, the program that it encapsulates is executed by calling run( ), shown here:
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// Execute the program. public void run() throws InterpreterException { // Initialize for new program run. vars = new double[26]; fStack = new Stack(); labelTable = new TreeMap();
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gStack = new Stack(); progIdx = 0; scanLabels(); // find the labels in the program sbInterp(); // execute }
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The run( ) method begins by allocating the array that holds the values of the variables, a stack for FOR loops, a tree map for labels, and a stack for GOSUBs. Next, progIdx, which holds the location of the program that is currently being interpreted, is set to 0. These fields are set each time run( ) is called, thus enabling repeated execution of the same program. Next, scanLabels( ) is called, which scans the program, looking for labels. When one is found, the label and its location are stored in the labelTable map. By finding all labels prior to execution, the execution speed of the program is improved. Finally, sbInterp( ) is called to begin execution of the program.
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The sbInterp( ) Method
The sbInterp( ) method begins and controls the execution of a Small BASIC program. This method is shown here:
// Entry point for the Small BASIC interpreter. private void sbInterp() throws InterpreterException { // This is the interpreter's main loop. do { getToken(); // Check for assignment statement. if(tokType==VARIABLE) { putBack(); // return the var to the input stream assignment(); // handle assignment statement } else // is keyword switch(kwToken) { case PRINT: print(); break; case GOTO: execGoto(); break; case IF: execIf(); break;
3: Implementing Language Interpreters in Java
case FOR: execFor(); break; case NEXT: next(); break; case INPUT: input(); break; case GOSUB: gosub(); break; case RETURN: greturn(); break; case END: return; } } while (!token.equals(EOP)); }
All interpreters are driven by a top-level loop that reads the next token from the program and selects the appropriate action to process it. The Small BASIC interpreter is no exception. This main loop is contained in sbInterp( ). It works like this. First, a token is read from the program. Assuming no syntax errors are found, if the token is a variable, then an assignment is taking place. Otherwise, the token must be either a line number (which is ignored) or a keyword. If it is a keyword, the appropriate case statement is selected based on the value of kwToken, which contains the internal representation of the keyword. Each keyword is handled by its own method and is described in turn by the sections that follow.
Assignment
In traditional BASIC, assignment is a statement, not an operation, and this is the way it is treated by Small BASIC, too. The general form of a BASIC assignment statement is var-name = expression The assignment statement is interpreted using the assignment( ) method shown here:
// Assign a variable a value. private void assignment() throws InterpreterException { int var; double value; char vname;
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