visual basic barcode program Adding Variables to the Parser in Java

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Adding Variables to the Parser
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All programming languages, many calculators, and spreadsheets use variables to store values for later use. Before the parser can be used for such applications, it needs to be expanded to include variables. To accomplish this, you need to add several things to the parser. First, of course, are the variables themselves. As stated earlier, we will use the letters A through Z for variables. The variables are stored in an array inside the Parser class. Each variable uses one array location in a 26-element array of doubles. Therefore, add the following field to the Parser class:
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// Array for variables. private double vars[] = new double[26];
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Each element in the array is automatically initialized to zero when a Parser object is instantiated. You will also need a method to look up the value of a given variable. Because the variables are named A through Z, they can easily be used to index the array vars by subtracting the ASCII value for A from the variable name. The method findVar( ), shown here, accomplishes this:
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// Return the value of a variable. private double findVar(String vname) throws ParserException { if(!Character.isLetter(vname.charAt(0))){ handleErr(SYNTAX);
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return 0.0; } return vars[Character.toUpperCase(vname.charAt(0))-'A']; }
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As this method is written, it will actually accept long variable names, such as A12 or test, but only the first letter is significant. You can change this feature to fit your needs. You must also modify the atom( ) method to handle both numbers and variables. The new version is shown here:
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// Get the value of a number or variable. private double atom() throws ParserException { double result = 0.0; switch(tokType) { case NUMBER: try { result = Double.parseDouble(token); } catch (NumberFormatException exc) { handleErr(SYNTAX); } getToken(); break; case VARIABLE: result = findVar(token); getToken(); break; default: handleErr(SYNTAX); break; } return result; }
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Technically, these additions are all that is needed for the parser to use variables correctly; however, there is no way for these variables to be assigned a value. To enable a variable to be given a value, the parser needs to be able to handle the assignment operator, which is =. To implement assignment, we will add another method, called evalExp1( ), to the Parser class. This method will now begin the recursive-descent chain. This means that it, not evalExp2( ), must be called by evaluate( ) to begin parsing the expression. The evalExp1( ) method is shown here:
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// Process an assignment. private double evalExp1() throws ParserException {
2: A Recursive-Descent Expression Parser
double result; int varIdx; int ttokType; String temptoken; if(tokType == VARIABLE) { // save old token temptoken = new String(token); ttokType = tokType; // Compute the index of the variable. varIdx = Character.toUpperCase(token.charAt(0)) - 'A'; getToken(); if(!token.equals("=")) { putBack(); // return current token // restore old token -- not an assignment token = new String(temptoken); tokType = ttokType; } else { getToken(); // get next part of exp result = evalExp2(); vars[varIdx] = result; return result; } } return evalExp2(); }
The evalExp1( ) method needs to look ahead to determine whether an assignment is actually being made. This is because a variable name always precedes an assignment, but a variable name alone does not guarantee that an assignment expression follows. That is, the parser knows that A = 100 is an assignment, but it is also smart enough to know that A/10 is not. To accomplish this, evalExp1( ) reads the next token from the input stream. If it is not an equal sign, the token is returned to the input stream for later use by calling putBack( ), shown here:
// Return a token to the input stream. private void putBack() { if(token == EOE) return; for(int i=0; i < token.length(); i++) expIdx--; }
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