java递归下降分析_JAVA中的递归下降

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public class Parser {

// These are the token type

final int NONE = 0;

final int DELIMITER = 1;

final int VARIABLE = 2;

final int NUMBER = 3;

// These are the type of syntax errors

final int SYNTAX = 0;

final int UNBALPARENS = 1;

final int NOEXP = 2;

final int DIVBYZERO = 3;

// These token indicates end-of-expression

final String EOF = “\0”;

private String exp;

private int expIdx;

private String token;

private int tokType;

public double evaluate(String expstr) throws ParserException {

double result;

exp = expstr;

expIdx = 0;

getToken();

// no expression present

if (token.equals(EOF))

handleErr(NOEXP);

result = evalExp2();

if (!token.equals(EOF)) {

handleErr(SYNTAX);

}

return result;

}

private double evalExp2() throws ParserException {

char op;

double result;

double partialResult;

result = evalExp3();

while ((op = token.charAt(0)) == ‘+’ || op == ‘-‘) {

getToken();

partialResult = evalExp3();

switch (op) {

case ‘-‘:

result = result – partialResult;

break;

case ‘+’:

result = result + partialResult;

break;

}

}

return result;

}

private double evalExp3() throws ParserException {

char op;

double result;

double partialResult;

result = evalExp4();

while ((op = token.charAt(0)) == ‘/’ || op == ‘*’ || op == ‘%’) {

getToken();

partialResult = evalExp4();

switch (op) {

case ‘*’:

result = result * partialResult;

break;

case ‘/’:

if (partialResult == 0.0)

handleErr(DIVBYZERO);

result = result / partialResult;

break;

case ‘%’:

if (partialResult == 0.0)

handleErr(DIVBYZERO);

result = result / partialResult;

break;

}

}

return result;

}

private double evalExp4() throws ParserException {

double result;

double partialResult;

double ex;

int t;

result = evalExp5();

if (token.equals(“^”)) {

getToken();

partialResult = evalExp4();

ex = result;

if (partialResult == 0.0) {

result = 1.0;

} else

for (t = (int) partialResult – 1; t > 0; t–)

result = result * ex;

}

return result;

}

private double evalExp5() throws ParserException {

double result;

String op;

op = “”;

if ((tokType == DELIMITER) && token.equals(“+”) || token.equals(“-“)) {

op = token;

getToken();

}

result = evalExp6();

if (op.equals(“-“))

result = -result;

return result;

}

private double evalExp6()throws ParserException{

double result;

if(token.equals(“(“)){

getToken();

result = evalExp2();

if(!token.equals(“)”))

handleErr(UNBALPARENS);

getToken();

}

else result = atom();

return result;

}

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;

default:

handleErr(SYNTAX);

break;

}

return result;

}

private void handleErr(int error) throws ParserException {

String[] err = { “Syntax error”, “Unbalanced parentheses”,

“No expression Present”, “Division by zero” };

throw new ParserException(err[error]);

}

//下面的函数就是获得独立元素的值，并且用tokType来保存类型

private void getToken() {

// obtain the next token

tokType = NONE;

token = “”;

if (expIdx == exp.length()) {

token = EOF;

return;

}

while (expIdx < exp.length()

&& Character.isWhitespace(exp.charAt(expIdx)))

++expIdx;

if (expIdx == exp.length()) {

token = EOF;

return;

}

if (isDelim(exp.charAt(expIdx))) {// is operator

token += exp.charAt(expIdx);

expIdx++;

tokType = DELIMITER;

} else if (Character.isLetter(exp.charAt(expIdx))) {

while (!isDelim(exp.charAt(expIdx))) {

token += exp.charAt(expIdx);

expIdx++;

if (expIdx >= exp.length())

break;

}

tokType = VARIABLE;

} else if (Character.isDigit(exp.charAt(expIdx))) {

while (!isDelim(exp.charAt(expIdx))) {

token += exp.charAt(expIdx);

expIdx++;

if (expIdx >= exp.length())

break;

}

tokType = NUMBER;

} else {

token = EOF;

return;

}

}

private boolean isDelim(char c) {

if ((“+-*/%^=()”.indexOf(c)) != -1)

return true;

return false;

}

}

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