Python学习笔记（十二）：

1. 计算器

• 利用Python的正则表达式写的简易计算器

# author : Ryoma

# time   : 17:39

import re

def add(string):

    equ = string

    flag = True

    while flag:

        is_right = re.search('[\-]?\d+\.?\d*\+[\-]?\d+\.?\d*', equ)

        if is_right:

            old = is_right.group()

            # 计算加法

            nums = old.split('+')

            sum = float(nums[0]) + float(nums[1])

            new = str(sum)

            equ = equ.replace(old,new)

        else:

            flag = False

    return equ

def reduce(string):

    equ = string

    flag = True

    while flag:

        is_right = re.search('[\-]?\d+\.?\d*-[\-]?\d+\.?\d*', equ)

        if is_right:

            old = is_right.group()

            # 计算减法

            count = len(re.findall('-',old))

            if count==1:

                nums = old.split('-')

                sum = float(nums[0]) - float(nums[1])

                new = str(sum)

                equ = equ.replace(old, new)

            elif count==2:

                nums = old.split('-')

                sum = - float(nums[1]) - float(nums[2])

                new = str(sum)

                equ = equ.replace(old, new)

        else:

            flag = False

    return equ

def ride(string):

    equ = string

    flag = True

    while flag:

        is_right = re.search('[\-]?\d+\.?\d*\*[\-]?\d+\.?\d*', equ)

        if is_right:

            old = is_right.group()

            # 计算乘法

            nums = old.split('*')

            sum = float(nums[0]) * float(nums[1])

            new = str(sum)

            equ = equ.replace(old,new)

        else:

            flag = False

    return equ

def divide(string):

    equ = string

    flag = True

    while flag:

        is_right = re.search('[\-]?\d+\.?\d*/[\-]?\d+\.?\d*', equ)

        if is_right:

            old = is_right.group()

            # 计算除法

            nums = old.split('/')

            if nums[1] == '0':

                exit('0不能做被除数！')

            sum = float(nums[0]) / float(nums[1])

            new = str(sum)

            equ = equ.replace(old,new)

        else:

            flag = False

    return equ

# 判断优先级 按照优先级进行计算：括号>乘除法>加减法

def pri(equ):

    flag = True

    while flag:

        # 查找最里层括号

        ret = re.search('\([^()]+\)', equ)

        if ret:

            # 记录查找的式子

            old = ret.group()

            # 去除括号

            new = old.replace('(', '')

            new = new.replace(')', '')

            # 计算-先乘除后加减

            new = ride(new)

            new = divide(new)

            new = add(new)

            new = reduce(new)

            # 替换方程式

            equ = equ.replace(old, new)

        else:

            flag = False

    else:

        equ = ride(equ)

        equ = divide(equ)

        equ = add(equ)

        equ = reduce(equ)

    return equ

# 格式处理

def check(equ):

    equ = equ.replace(' ','')

    if len(re.findall('[^0-9\-+/*\(\).]', equ)):

        print("请输入正确的表达式！")

    elif not equ.count('(') == equ.count(')'):

        print("存在未闭合的括号！")

    else:

        equ = pri(equ)

        return equ

if __name__ == '__main__':

    print("欢迎使用计算器".center(50,'-'))

    while 1:

        equ = input("输入你要计算的式子：")

        equ = check(equ)

        print(equ)