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python实现逆波兰计算表达式的代码

2021-11-12 13:31:31 作者：互联网

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