📅  最后修改于: 2023-12-03 15:20:09.744000             🧑  作者: Mango
SLR(1)是一种自底向上的语法分析方法,它是 LR(0) 的扩展,具有较强的处理能力和比较高的自动分析效率。SLR(1)的全称是 Simple LR(1),解析器产生状态图,自底向上匹配单词和生产式。若能成功地在最终状态中匹配和识别整个程序,就认为程序语法正确。如果有语法错误则解析将失败。
SLR(1)解析的主要运作流程包括以下四个步骤:
分析表的构建包括以下几步:
在词法分析中,将输入字符串分解成单词,输出相应的符号,编写一个管理输入字符串符号序列的分析器。
在语法分析中,根据SLR分析表,按照当前状态和下一个输入单词进行规约或移进操作,以确定下一个状态。每次操作后,更新状态,并调整输入符号序列,直到匹配整个输入符号串。如果分析失败,则会报告语法错误。
在语法错误处理中,需要根据错误的位置和错误类型进行处理,报告错误并尝试恢复同步。常见的错误处理方法包括丢弃或插入一些单词,跳过到下一个语法符号,或者直接停止解析。
下面是一个使用SLR(1)解析进行语法分析的示例代码,例如识别简单的赋值语句:
# 所有终结符
TERMINALS = {'ID', 'NUM', '=', '+'}
# 所有文法产生式
PRODUCTIONS = {
'E': {'E + T', 'T'},
'T': {'T * F', 'F'},
'F': {'( E )', 'ID', 'NUM'},
'S': {'ID = E'}
}
# 词法分析
def lexer(expression):
tokens = re.findall('[A-Za-z]+\\d*|[0-9]+|[=+()]', expression)
return [(t, t) if t in TERMINALS else ('ID', t) for t in tokens]
# 构造文法产生式的闭包
def closure(i, productions):
items = productions.items()
closures = {i}
while True:
new_items = set()
for lhs, rhss in items:
for rhs in rhss:
for j, t in enumerate(rhs):
if (j, t) == (0, lhs):
new_items |= {(lhs, rhs[1:])}
if rhs and rhs[0] == i:
new_items |= {(lhs, rhs[1:])}
if not new_items - closures:
break
closures |= new_items
return frozenset(closures)
# 在两种状态间的转换
def goto(closures, symbol):
new = set()
for lhs, rhs in closures:
if rhs and rhs[0] == symbol:
new |= {(lhs, rhs[1:])}
return closure(frozenset(new))
# 构造SLR分析表
def parse_table(terminals, productions):
states = []
i = closure({('S_', ('S',))}, productions)
state_map = {i: 0}
states.append(i)
i = 0
while i < len(states):
state = states[i]
i += 1
for terminal in terminals:
closure_ = goto(state, terminal)
if closure_:
if closure_ not in state_map:
states.append(closure_)
state_map[closure_] = len(states) - 1
print('ACTION[%d, %s] = S%d' % (state_map[state], terminal, state_map[closure_]))
for lhs, rhs in closure_:
if not rhs:
print('ACTION[%d, %s] = R%s' % (state_map[state], lhs, lhs))
# 测试赋值语句的语法分析
def test():
expression = 'a = b + 300 * 1'
productions = {}
for k, v in PRODUCTIONS.items():
lhs, *rhs = k if k != 'S' else 'S_', *v
for t in rhs:
productions.setdefault((lhs, t), []).append((lhs, tuple(t.split())))
terminals = set(productions.keys()) - set(PRODUCTIONS.keys())
parse_table(terminals, productions)
tokens = lexer(expression + '$')
print(tokens)
i = 0
symbol_seq = [t[0] for t in tokens]
state_seq = [0]
stack = [None]
reductions = 0
while True:
current_state, symbol = state_seq[-1], symbol_seq[i]
if (current_state, symbol) not in parse_table:
print('Syntax error')
break
action = parse_table[(current_state, symbol)]
if action[0] == 'S':
i += 1
state_seq.append(action[1])
stack.append(symbol)
elif action[0] == 'R':
lhs, rhs = action[1], productions[action][0][1]
stack_tokens = stack[-len(rhs):]
if stack_tokens != list(rhs):
print('Syntax error')
break
stack = stack[:-len(rhs)]
state_seq = state_seq[:-len(rhs)]
symbol = lhs
stack.append(symbol)
state = state_seq[-1]
print('ACTION[%d, %s]->R%s' % (state, symbol, lhs))
reductions += 1
elif action == ('accept',):
print('Expression is syntactically correct')
break
print(reductions)
if __name__ == '__main__':
test()
以上是一个使用Python编写的 SLR(1)解析示例代码,实现了从开始构建分析表到语法分析全过程的演示,以及对错误分析和处理的说明。