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{ tokType = NONE; token = ""; // Check for end of expression. if(expIdx == exp.length()) { token = EOE; return; } // Skip over white space. while(expIdx < exp.length() && Character.isWhitespace(exp.charAt(expIdx))) ++expIdx; // Trailing whitespace ends expression. if(expIdx == exp.length()) { token = EOE; return; } if(isDelim(exp.charAt(expIdx))) { // is operator token += exp.charAt(expIdx); expIdx++; tokType = DELIMITER; } else if(Character.isLetter(exp.charAt(expIdx))) { // is variable while(!isDelim(exp.charAt(expIdx))) { token += exp.charAt(expIdx); expIdx++; if(expIdx >= exp.length()) break; } tokType = VARIABLE; } else if(Character.isDigit(exp.charAt(expIdx))) { // is number while(!isDelim(exp.charAt(expIdx))) { token += exp.charAt(expIdx); expIdx++; if(expIdx >= exp.length()) break; } tokType = NUMBER; } else { // unknown character terminates expression token = EOE; return;
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2: A Recursive-Descent Expression Parser
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} } // Return true if c is a delimiter. private boolean isDelim(char c) { if((" +-/*%^=()".indexOf(c) != -1)) return true; return false; } }
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To try the enhanced parser, you can use the same program that you used for the simple parser. With the enhanced parser, you can now enter expressions like A = 10/4 A B C = A * (F 21)
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In expression parsing, a syntax error is simply a situation in which the input expression does not conform to the strict rules required by the parser. Most of the time, this is caused by human error, usually typing mistakes. For example, the following expressions are not valid for the parsers in this chapter: 10 ** 8 ((10 5) * 9 /8 The first contains two operators in a row, the second has unbalanced parentheses, and the last has a division sign at the start of an expression. None of these conditions is allowed by the parser. Because syntax errors can cause the parser to give erroneous results, you need to guard against them. In the parser, the handleErr( ) method is called when an error is detected. Unlike some other types of parsers, the recursive-descent method makes syntax checking easy because, for the most part, it occurs in atom( ), findVar( ), or evalExp6( ), where parentheses are checked. When handleErr( ) is called, it throws a ParserException that contains a description of the error. This exception is not caught by Parser, but thrown to the calling code. Thus, the parser immediately stops when an error is encountered. You can, of course, change this behavior to suit your own needs. One thing you might want to do is expand the information contained in a ParserException object. As it is currently written, this class stores only a string describing the error. You might want to add the error code itself, the index in the expression string at which point the error occurred, or other information.
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A Calculator Applet
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The parser is extremely simple to use and can be added to nearly any application. To understand just how easy it is to utilize the parser consider the following example. In only a few lines of code it creates a fully functional calculator applet. The calculator uses two text fields. The first contains the expression to be evaluated. The second displays the result. The result text field is read-only. Error messages are displayed on the status line. Sample output (using the Applet Viewer) is shown in Figure 2-1.
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// A simple calculator applet. import java.awt.*; import java.awt.event.*; import java.applet.*; /* <applet code="Calc" width=200 height=150> </applet> */ public class Calc extends Applet implements ActionListener { TextField expText, resText; Parser p; public void init() { Label heading = new Label("Expression Calculator ", Label.CENTER); Label explab = new Label("Expression ", Label.CENTER); Label reslab = new Label("Result ", Label.CENTER); expText = new TextField(24); resText = new TextField(24); resText.setEditable(false); // result field for display only add(heading); add(explab); add(expText); add(reslab); add(resText); /* Register expression text field to receive action events. */ expText.addActionListener(this); // create parser
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