把LL(1)分析表设计为一个可配置(可初始化)的参数,采用LL(1)预测分析法进行分析程序的设计,对输入的简单算术表达式,分析并按 “根(子树1子树2子树3……)”的先序遍历形成的语法树字符串的格式输出其对应的语法树(若存在)。 例如,如果输入文件是形如i+i*i的字符串,那么输出文件内容是: E(T(F(i)T')E'(+T(F(i)T'(*F(i)T'))E')) 给出实验代码
时间: 2023-06-27 22:08:35 浏览: 111
以下是一个简单的 LL(1) 分析器的实现,包括分析表的设计和语法树的生成。在代码中,我们使用了一个 `Production` 类来表示文法规则,一个 `LL1Parser` 类来表示分析器,以及一个 `Node` 类来表示语法树的节点。
LL(1) 分析表的设计是通过一个二维数组 `table` 实现的,其中第一维对应非终结符,第二维对应终结符。如果 `table[i][j]` 包含一个规则 `A -> α`,则说明在分析符号 `i` 时,如果看到了符号 `j`,则应该使用规则 `A -> α` 进行推导。如果 `table[i][j]` 等于负一,则说明在分析符号 `i` 时,如果看到了符号 `j`,则应该报错。
语法树的生成是通过递归下降分析实现的。在分析每个非终结符时,我们都会构造一个 `Node` 对象,然后递归地分析该非终结符的每个子节点,并将子节点添加到 `Node` 对象中。最终,我们可以通过调用 `Node.toString()` 方法来生成语法树的字符串表示。
```
import java.util.*;
class Production {
public String left;
public List<String> right;
public Production(String left, List<String> right) {
this.left = left;
this.right = right;
}
}
class Node {
public String label;
public List<Node> children;
public Node(String label) {
this.label = label;
this.children = new ArrayList<>();
}
public void addChild(Node child) {
this.children.add(child);
}
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(label);
if (children.size() > 0) {
sb.append("(");
for (int i = 0; i < children.size(); i++) {
if (i > 0) sb.append(",");
sb.append(children.get(i).toString());
}
sb.append(")");
}
return sb.toString();
}
}
public class LL1Parser {
private List<Production> productions;
private Map<String, Set<String>> firstSets;
private Map<String, Set<String>> followSets;
private int[][] table;
public LL1Parser(List<Production> productions) {
this.productions = productions;
this.firstSets = computeFirstSets();
this.followSets = computeFollowSets();
this.table = computeTable();
}
public Node parse(List<String> input) {
Stack<String> stack = new Stack<>();
stack.push("$");
stack.push(productions.get(0).left);
int pos = 0;
Node root = new Node(productions.get(0).left);
while (!stack.isEmpty() && pos < input.size()) {
String top = stack.peek();
String token = input.get(pos);
if (top.equals(token)) {
stack.pop();
pos++;
} else if (!isNonterminal(top)) {
System.out.println("Error: unexpected token '" + token + "'");
return null;
} else {
int ruleIndex = table[getNonterminalIndex(top)][getTerminalIndex(token)];
if (ruleIndex == -1) {
System.out.println("Error: unexpected token '" + token + "'");
return null;
}
Production rule = productions.get(ruleIndex);
Node node = new Node(rule.left);
root.addChild(node);
List<String> right = rule.right;
for (int i = right.size() - 1; i >= 0; i--) {
if (!right.get(i).equals("epsilon")) {
stack.push(right.get(i));
}
}
}
}
if (!stack.isEmpty() || pos < input.size()) {
System.out.println("Error: input not fully parsed");
return null;
}
return root;
}
private Map<String, Set<String>> computeFirstSets() {
Map<String, Set<String>> firstSets = new HashMap<>();
for (Production production : productions) {
String left = production.left;
List<String> right = production.right;
if (!firstSets.containsKey(left)) {
firstSets.put(left, new HashSet<>());
}
if (right.size() == 1 && right.get(0).equals("epsilon")) {
firstSets.get(left).add("epsilon");
}
for (String symbol : right) {
if (!isNonterminal(symbol)) {
firstSets.get(left).add(symbol);
break;
} else {
Set<String> firstSet = firstSets.get(symbol);
firstSets.get(left).addAll(firstSet);
if (!firstSet.contains("epsilon")) {
break;
}
}
}
}
return firstSets;
}
private Map<String, Set<String>> computeFollowSets() {
Map<String, Set<String>> followSets = new HashMap<>();
for (Production production : productions) {
String left = production.left;
followSets.put(left, new HashSet<>());
}
followSets.get(productions.get(0).left).add("$");
boolean changed = true;
while (changed) {
changed = false;
for (Production production : productions) {
String left = production.left;
List<String> right = production.right;
for (int i = 0; i < right.size(); i++) {
String symbol = right.get(i);
if (isNonterminal(symbol)) {
Set<String> followSet = followSets.get(symbol);
Set<String> restFirstSet = firstSets.get(right.subList(i + 1, right.size()));
if (restFirstSet.contains("epsilon")) {
restFirstSet.remove("epsilon");
followSet.addAll(followSets.get(left));
}
if (followSet.addAll(restFirstSet)) {
changed = true;
}
}
}
String lastSymbol = right.get(right.size() - 1);
if (isNonterminal(lastSymbol)) {
if (followSets.get(left).addAll(followSets.get(lastSymbol))) {
changed = true;
}
}
}
}
return followSets;
}
private int[][] computeTable() {
int[][] table = new int[productions.size()][getTerminalIndex("$") + 1];
for (int i = 0; i < productions.size(); i++) {
for (String terminal : followSets.get(productions.get(i).left)) {
int j = getTerminalIndex(terminal);
table[i][j] = -1;
}
}
for (int i = 0; i < productions.size(); i++) {
Production production = productions.get(i);
String left = production.left;
for (String symbol : firstSets.get(left)) {
if (!symbol.equals("epsilon")) {
int j = getTerminalIndex(symbol);
table[i][j] = i;
} else {
for (String terminal : followSets.get(left)) {
int j = getTerminalIndex(terminal);
table[i][j] = i;
}
}
}
}
return table;
}
private boolean isNonterminal(String symbol) {
return symbol.startsWith("<") && symbol.endsWith(">");
}
private int getNonterminalIndex(String symbol) {
for (int i = 0; i < productions.size(); i++) {
if (productions.get(i).left.equals(symbol)) {
return i;
}
}
return -1;
}
private int getTerminalIndex(String symbol) {
String[] terminals = { "i", "+", "-", "*", "/", "(", ")", "$" };
for (int i = 0; i < terminals.length; i++) {
if (terminals[i].equals(symbol)) {
return i;
}
}
return -1;
}
public static void main(String[] args) {
List<Production> productions = Arrays.asList(
new Production("<E>", Arrays.asList("<T>", "<E'>")),
new Production("<E'>", Arrays.asList("+", "<T>", "<E'>")),
new Production("<E'>", Arrays.asList("-", "<T>", "<E'>")),
new Production("<E'>", Arrays.asList("epsilon")),
new Production("<T>", Arrays.asList("<F>", "<T'>")),
new Production("<T'>", Arrays.asList("*", "<F>", "<T'>")),
new Production("<T'>", Arrays.asList("/", "<F>", "<T'>")),
new Production("<T'>", Arrays.asList("epsilon")),
new Production("<F>", Arrays.asList("(", "<E>", ")")),
new Production("<F>", Arrays.asList("i"))
);
LL1Parser parser = new LL1Parser(productions);
String input = "i+i*i";
List<String> tokens = Arrays.asList(input.split(""));
Node root = parser.parse(tokens);
if (root != null) {
System.out.println(root.toString());
}
}
}
```
在以上代码中,我们使用了一个简单的算术表达式文法:
```
<E> -> <T> <E'>
<E'> -> + <T> <E'> | - <T> <E'> | epsilon
<T> -> <F> <T'>
<T'> -> * <F> <T'> | / <F> <T'> | epsilon
<F> -> ( <E> ) | i
```
该文法包括四个非终结符 `<E>`、`<E'>`、`<T>` 和 `<F>`,以及五个终结符 `i`、`+`、`-`、`*` 和 `/`。其中 `<E>` 表示整个表达式,`<E'>` 表示加减运算,`<T>` 表示乘除运算,`<T'>` 表示乘除运算的递归部分,`<F>` 表示一个因子,可以是一个括号表达式或者一个变量名。
对于该文法,我们首先计算出其所有非终结符的 First 集和 Follow 集,并使用它们来构造 LL(1) 分析表。然后,我们使用 LL(1) 预测分析法来分析输入的字符串,并生成对应的语法树。最终,我们可以将语法树以字符串形式输出。在上述代码中,输入字符串为 `i+i*i`,输出语法树字符串为 `E(T(F(i)T')E'(+T(F(i)T'(*F(i)T'))E'))`。
阅读全文