python实现逆波兰计算表达式的代码

如下资料是关于python实现逆波兰计算表达式的代码。

symbol_priority = {}
symbol_priority[0] = ['#']
symbol_priority[1] = ['(']
symbol_priority[2] = ['+', '-']
symbol_priority[4] = [')']

def comparePriority(symbol, RPN_stack, symbol_stack):
'''Compare priority between two symbols'''

global symbol_priority
if len(symbol_stack) > 0:
symbol_pop = symbol_stack.pop()
else:
return

for list in symbol_priority.values():
if (symbol in list) and (symbol_pop in list):
'''same priority'''
symbol_stack.append(symbol_pop)
symbol_stack.append(symbol)
return
elif symbol in list:
'''symbol is smaller'''
RPN_stack.append(symbol_pop)
#recusion call
comparePriority(symbol, RPN_stack, symbol_stack)
return
elif symbol_pop in list:
'''symbol is bigger'''
symbol_stack.append(symbol_pop)
symbol_stack.append(symbol)
return
else:
continue

symbol_stack.append(symbol_pop)
return

def scanEveryone(input_string, RPN_stack, symbol_stack):
for ch in input_string:
if ch.isdigit():
RPN_stack.append(ch)
else:
if len(symbol_stack) > 0:
if ch == '(':
symbol_stack.append(ch)
elif ch == ')':
while True:
symbol_pop = symbol_stack.pop()
if symbol_pop == '(':
break
else:
RPN_stack.append(symbol_pop)
else:
comparePriority(ch, RPN_stack, symbol_stack)
else:
symbol_stack.append(ch)

def scanInput(RPN_stack, symbol_stack):
input_string = raw_input()
input_string += '#'
scanEveryone(input_string, RPN_stack, symbol_stack)

def calRPN(RPN_stack):
value_stack = []
RPN_stack.append('#')

for value in RPN_stack:
if value == '#':
return value_stack.pop()
break
if value.isdigit():
value_stack.append(value)
else:
right_value = value_stack.pop()
left_value = value_stack.pop()
cal_string = left_value + value + right_value
value_stack.append(str(eval(cal_string)))

def main():
RPN_stack = []
symbol_stack = []

scanInput(RPN_stack, symbol_stack)
print calRPN(RPN_stack)

if name == 'main':
main()

calRPN.py