python实现lr0分析法及其实现
时间: 2023-07-07 17:39:09 浏览: 61
LR(0)语法分析法是一种自底向上的语法分析方法,它可以通过构建LR(0)自动机来实现对给定文法的语法分析。以下是Python实现LR(0)分析法的步骤:
1. 定义文法符号类
首先,我们需要定义一个文法符号类,用来表示终结符和非终结符。
```python
class Symbol:
def __init__(self, name, is_terminal):
self.name = name
self.is_terminal = is_terminal
def __str__(self):
return self.name
```
2. 定义产生式类
接下来,我们需要定义一个产生式类,用来表示文法的产生式。
```python
class Production:
def __init__(self, lhs, rhs):
self.lhs = lhs
self.rhs = rhs
def __str__(self):
return f"{self.lhs} -> {' '.join(str(s) for s in self.rhs)}"
```
3. 定义文法类
接着,我们需要定义一个文法类,用来表示整个文法。
```python
class Grammar:
def __init__(self, productions, start_symbol):
self.productions = productions
self.start_symbol = start_symbol
def __str__(self):
return '\n'.join(str(p) for p in self.productions)
```
4. 定义LR(0)自动机状态类
现在,我们需要定义一个LR(0)自动机状态类,用来表示LR(0)自动机的状态。
```python
class LR0State:
def __init__(self, items):
self.items = items
def __str__(self):
return '\n'.join(str(item) for item in self.items)
```
其中,items是一个包含LR(0)项的列表,每个LR(0)项都是一个元组,包含产生式和一个位置。
```python
class LR0Item:
def __init__(self, production, dot):
self.production = production
self.dot = dot
def __str__(self):
rhs = list(map(str, self.production.rhs))
rhs.insert(self.dot, '·')
return f"{self.production.lhs} -> {' '.join(rhs)}"
```
5. 定义LR(0)分析器类
最后,我们需要定义一个LR(0)分析器类,用来实现LR(0)分析法。
```python
class LR0Parser:
def __init__(self, grammar):
self.grammar = grammar
self.start_state = LR0State([LR0Item(grammar.productions[0], 0)])
self.states = []
self.transitions = {}
self.build()
def build(self):
self.states.append(self.start_state)
unprocessed_states = [self.start_state]
while unprocessed_states:
state = unprocessed_states.pop(0)
for symbol in self.get_next_symbols(state):
items = self.get_items_for_symbol(state, symbol)
if items:
new_state = LR0State(items)
if new_state not in self.states:
self.states.append(new_state)
unprocessed_states.append(new_state)
self.transitions[(state, symbol)] = new_state
def get_next_symbols(self, state):
symbols = set()
for item in state.items:
if item.dot < len(item.production.rhs):
symbols.add(item.production.rhs[item.dot])
return symbols
def get_items_for_symbol(self, state, symbol):
items = []
for item in state.items:
if item.dot < len(item.production.rhs) and item.production.rhs[item.dot] == symbol:
items.append(LR0Item(item.production, item.dot + 1))
return items
def parse(self, input_string):
stack = [self.start_state]
input_tokens = input_string.split()
input_tokens.append('$')
i = 0
while True:
state = stack[-1]
if state == LR0State([]):
return False
elif i == len(input_tokens):
return True
elif (state, input_tokens[i]) in self.transitions:
stack.append(self.transitions[(state, input_tokens[i])])
i += 1
else:
return False
```
以上就是Python实现LR(0)分析法的全部步骤。通过定义文法符号类、产生式类、文法类、LR(0)自动机状态类和LR(0)分析器类,我们可以很容易地实现LR(0)分析法。