50行代码实现python计算器主要功能
实现功能:计算带有括号和四则运算的式子 3*( 4+ 50 )-(( 100 + 40 )*5/2- 3*2* 2/4+9)*((( 3 + 4)-4)-4)
基本思路:使用正则表达式提取出每一层小括号式子,计算出结果后替换该括号,直至最后一层。
0、计算前的准备,进行式子的整理,如果非法则退出。这里我只对空格和符号进行了处理,真实场景中应该做更多的判断:
def check(s): s = s.replace(" ", '') s = re.sub(r'(\+-)|(-\+)', '-', s) s = re.sub(r'(\+\+)|(--)', '+', s) return s
1、只有加减运算的式子:findall匹配后直接使用sum函数对list求和,此时的减号看作负号,加号看作了正号,所以代码极其简洁:
import re def plus_all(s): #实现加减运算 patter_plus = r'[+-]?\d+\.?\d*' ret = re.findall(patter_plus, s) return sum([float(x) for x in ret])
2、只有乘除运算的式子:和以上思路一致,主要想把除法(/)转换为乘法后用reduce计算。其先匹配出带有乘除运算符的list,然后通过change_mul_div()函数将除法转换成倒数计算
from functools import reduce def mul_all(s): patter_mul = r'[*/]?[-]?\d+\.?\d*' ret = re.findall(patter_mul, s) return reduce(lambda m, n: m*n, [change_mul_div(x) for x in ret]) def change_mul_div(s): if "/" in s: return 1 / float(s.replace('/', "")) else: return float(s.replace("*", ''))
3、含有加减乘除但不含有括号的式子:先用mul_all(s)函数将式子中的所有乘除法计算完毕,然后回到第1步。cal_all_mul_div函数实现计算式子中所有乘除法:
def cal_all_mul_div(s): patter_all_mul_div = r'([+-]?\d+\.?\d*[*/])+[+-]?\d+\.?\d*' ret = re.search(patter_all_mul_div, s) while ret: mul_div = mul_all(ret.group()) s = s.replace(ret.group(), ''.join(['+', str(mul_div)])) ret = re.search(patter_all_mul_div, s) return s
4、不断提取括号进行计算:
def bracket(s): patter_kuohao = r'\([^()]+\)' ret = re.search(patter_kuohao, s) while ret: cal_result = plus_all(cal_all_mul_div(ret.group())) s=s.replace(ret.group(), ''.join(['+', str(cal_result)])) ret = re.search(patter_kuohao, s) return s
所有代码合并如下:
import re from functools import reduce s3 = "3*( 4+ 50 )-(( 100 + 40 )*5/2- 3*2* 2/4+9)*((( 3 + 4)-4)-4)" def check(s): s = s.replace(" ", '') #对空格进行处理 s = re.sub(r'(\+-)|(-\+)', '-', s) #对符号进行处理 s = re.sub(r'(\+\+)|(--)', '+', s) return s def plus_all(s): patter_plus = r'[+-]?\d+\.?\d*' ret = re.findall(patter_plus, s) #找出所有的加数 return sum([float(x) for x in ret]) #生成器生成列表并求和 def mul_all(s): patter_mul = r'[*/]?[-+]?\d+\.?\d*' ret = re.findall(patter_mul, s) return reduce(lambda m, n: m * n, [change_mul_div(x) for x in ret]) #reduce连乘 def change_mul_div(s): #把除法改成乘以其倒数 if "/" in s: return 1 / float(s.replace('/', "")) else: return float(s.replace("*", '')) def cal_all_mul_div(s): patter_all_mul_div = r'([+-]?\d+\.?\d*[*/])+[+-]?\d+\.?\d*' ret = re.search(patter_all_mul_div, s) while ret: mul_div = mul_all(ret.group()) s = s.replace(ret.group(), ''.join(['+', str(mul_div)])) ret = re.search(patter_all_mul_div, s) return s def bracket(s): patter_kuohao = r'\([^()]+\)' s = check(s) ret = re.search(patter_kuohao, s) while ret: cal_result = plus_all(cal_all_mul_div(ret.group())) s = check(s.replace(ret.group(), ''.join(['+', str(cal_result)]))) #替换时需要在前面加入正号,并对s进行check操作 ret = re.search(patter_kuohao, s) return s s3 = ''.join(["(", s3, ")"]) print(float(bracket(s3)))
以上即用不超过50行代码实现了计算器的主要功能。