这是 LL(1) 解析器的代码。当我输入 I + I ;
不是按照语法进行解析，
生产如下
> E > TFE > VUTFE > IVUTFE
在最后的生产中，有一个问题...在匹配第一个 I 后，应消除 V。
但这并没有发生。
语法规则是
( docs )
下面是代码
( Docs )
请帮忙找出错误!
package llparser;
import java.io.*;
import java.util.logging.Level;
import java.util.logging.Logger;
class finiteAutomaton{
static String[] move = {"TF","+E","","VU","*T","I"};
static int[][] transition = { {0,-1,-1,-1},
{-1,1,-1,2},
{3,-1,-1,-1},
{-1,2,4,2},
{5,-1,-1,-1} };
}
public class LLParser {
public static void main(String[] args) {
try {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str=null;
System.out.println("Enter a String for Parsing [ end with [;] ] :");
str=br.readLine();
str=str.toUpperCase();
if(!str.endsWith(";"))
throw new Exception("String should end with [;]");
str=str.substring(0,str.length()-1);
String temp = str.replaceAll(" ", "");
System.out.println(temp);
String tokenizedString;
String left;
String right="";
str=temp;
String CSF="E";
boolean flag=true;
char symbol;
int index=0;
int SSM=0;
int row,col;
symbol = str.charAt(index);
while(flag){
if(CSF.charAt(SSM)=='E') row=0;
else if(CSF.charAt(SSM)=='F') row=1;
else if(CSF.charAt(SSM)=='T') row=2;
else if(CSF.charAt(SSM)=='U') row=3;
else if(CSF.charAt(SSM)=='V') row=4;
else row=-1;
if(symbol=='I') col=0;
else if(symbol=='+') col=1;
else if(symbol=='*') col=2;
else if(symbol==';') col=3;
else col=-1;
if(row==-1 || col==-1 || finiteAutomaton.transition[row][col]==-1){
System.out.println("Error in Expression!");
System.exit(0);
}
tokenizedString=finiteAutomaton.move[finiteAutomaton.transition[row][col]];
left = CSF.substring(0, SSM);
int len=CSF.length();
if(SSM<len)
right= CSF.substring(SSM);
else
right="";
left = left + tokenizedString + right;
CSF = left;
if(symbol==tokenizedString.charAt(0)){
SSM++;
symbol = str.charAt(++index);
System.out.println("SSM=" + SSM + " symbol="+ symbol);
}
if(str.equals(CSF))
{
flag=false;
}
}
if(!flag){
System.out.println("String Accepted by Parser!");
}
} catch (IOException ex) {
Logger.getLogger(LLParser.class.getName()).log(Level.SEVERE, null, ex);
} catch (Exception ex) {
Logger.getLogger(LLParser.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
/*
Output
run:
Enter a String for Parsing [ String should end with [;] ] :
i+i;
I+I
SSM=1 symbol=+
Error in Expression! CSF=IVUTFE Symbol=+
BUILD SUCCESSFUL (total time: 6 seconds)*/
I have performed debugg and problem is
CSF = E
E -> TF CSF = TFE [col=0 as CSF(0)==E and row=4 as symbol==I]
Here E should be replaced by TF but in my code
E remains in CSF and TF is appended before it.
Now T should be replaced by VU but in my code
T remains in CSF and VU is appended before it.
T -> VU CSF = VUTFE
in next iteration.
V -> I CSF = IVUTFE
希望它有助于理解我的错误。
最佳答案
/*The grammar recognised is :
E ::= TF
F ::= +TF | #
T ::= VU
U ::= *VU | #
V ::= I
where I is any valid Identifer
___________________________________________________
| | Source Symbol |
|Non terminal|-------------------------------------|
| | I | + | * | ; |
|------------|---------|---------|---------|-------|
| E | E => TF | | | |
|------------|---------|---------|---------|-------|
| F | | F => +TF| | E => #|
|------------|---------|---------|---------|-------|
| T | T => VU | | | |
|------------|---------|---------|---------|-------|
| U | | U => # | U => *VU| U => #|
|------------|---------|---------|---------|-------|
| V | V => I | | | |
|____________|_________|_________|_________|_______|
*/
package llparser;
import java.io.*;
import java.util.logging.Level;
import java.util.logging.Logger;
class finiteAutomaton{
static String[] move = {"TF","+E","","VU","*T","I"};
static int[][] transition = { {0,-1,-1,-1},
{-1,1,-1,2},
{3,-1,-1,-1},
{-1,2,4,2},
{5,-1,-1,-1} };
}
public class LLParser {
public static void main(String[] args) {
try {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str=null;
System.out.println("Enter a String for Parsing [ String should end with [;] ] :");
str=br.readLine();
str=str.toUpperCase();
if(!str.endsWith(";"))
throw new Exception("String should end with [;]");
str=str.substring(0,str.length()-1); /* removing ; from string */
String temp = str.replaceAll(" ", ""); /* removing spaces */
str=temp;
String tokenizedString;
String left;
String right="";
String CSF="E";
char symbol;
int index=0;
int SSM=0;
int row,col;
symbol = str.charAt(index);
while(!str.equals(CSF)){
System.out.println();
System.out.println("Input String="+str +"\tSSM="+SSM+" Symbol="+symbol + " CSF="+CSF );
if(CSF.charAt(SSM)=='E') row=0;
else if(CSF.charAt(SSM)=='F') row=1;
else if(CSF.charAt(SSM)=='T') row=2;
else if(CSF.charAt(SSM)=='U') row=3;
else if(CSF.charAt(SSM)=='V') row=4;
else row=-1;
if(symbol=='I') col=0;
else if(symbol=='+') col=1;
else if(symbol=='*') col=2;
else if(symbol==';') col=3;
else col=-1;
if(row==-1 || col==-1 || finiteAutomaton.transition[row][col]==-1){
System.out.println("Error in Expression!" + "CSF=" + CSF + " Symbol=" + symbol);
System.exit(0);
}
tokenizedString=finiteAutomaton.move[finiteAutomaton.transition[row][col]];
left = CSF.substring(0, SSM);
int len=CSF.length();
if(SSM<len-1)
right= CSF.substring(SSM+1,len);
else
right="";
left = left + tokenizedString + right;
CSF = left;
if( tokenizedString.equals("")){
continue;
}
if(symbol==tokenizedString.charAt(0)){
SSM++;
char prev=symbol;
if(index<str.length()-1)
symbol = str.charAt(++index);
else
symbol = ';';
System.out.println("\n\tMatch Found Previous Symbol="+ prev + " New Symbol="+ symbol);
}
}
System.out.println("String Accepted by Parser!");
} catch (IOException ex) {
Logger.getLogger(LLParser.class.getName()).log(Level.SEVERE, null, ex);
} catch (Exception ex) {
Logger.getLogger(LLParser.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
关于java - Java 中的解析错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/20862135/