Python正则表达计算器

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)