Python利用正则表达式实现计算器算法思路解析

 #!/usr/bin/env python # -*- coding:utf-8 -*- # Author:Dang import re def update_formula(calc_list,calc_operator_list): # 通过拆分后的表达式列表与符号列表重新组合 for index,item in enumerate(calc_list): if index == 0: formula = item elif index != 0: formula += calc_operator_list[index-1] + item return formula def negative_start_issue(formula): #处理负数在括号内表达式开头的情形 calc_list = re.split("[+-]",formula)  #通过+-符号将各个乘除运算分隔出来 calc_operator_list = re.findall("[+-]",formula) for index,item in enumerate(calc_list): if index == 0 and item == '':  # 处理负号在开头的问题 calc_list[index] = '0' else: calc_list[index] = item.strip() formula = update_formula(calc_list,calc_operator_list) return formula def deal_unusual_issue(formula): # 双加减符号处理 formula = formula.replace(" ","") #去掉空格 formula = formula.replace("++","+") formula = formula.replace("+-", "-") formula = formula.replace("-+", "-") formula = formula.replace("--", "+") return formula def deal_negative_issue(formula): # 处理乘除运算中负数的计算问题（分前后位置两种情况） # 1.负数在后 m = re.search("[0-9]+[.]*[0-9]*[*|/][-][0-9]+[.]*[0-9]*",formula) # minus_pre = re.search("[0-9]+[.]*[0-9]*[*|/][-][0-9]+[.]*[0-9]*",formula).group() # 注意匹配的必要项与非必要项，如："[0-9]+[.][0-9]+[*|/][-][0-9]+[.][0-9]+"误把非必要项当做必要项。 if m: minus_pre = m.group() minus_pro = "-"+minus_pre.replace("-","") formula = formula.replace(minus_pre,minus_pro) if "*-" in formula or "/-" in formula: return deal_negative_issue(formula) # 2.负数在前 formula = deal_unusual_issue(formula) return formula def multiply_divide(formula): # print("[%s]"%formula,formula) # 乘除计算 calc_list = re.split("[*/]", formula) operator_list = re.findall("[*/]", formula) # 将乘号除号通过列表方式分隔出来 # print("sub_calc_list:", sub_calc_list) # print("sub_operator_list:", sub_operator_list) res = 0 for index2, i in enumerate(calc_list): if index2 == 0: res = float(i) else: if operator_list[index2 - 1] == '*': # 通过sub_operator_list中的index判断到底是加法还是减法， res *= float(i) elif operator_list[index2 - 1] == '/': res /= float(i) return res def add_abstract(formula): # 加减计算 # 1.开头位置负数处理 formula = negative_start_issue(formula) # 2.双加减符号处理 formula = deal_unusual_issue(formula) # 3.加减逻辑运算 calc_list = re.split("[+-]", formula) operator_list = re.findall("[+-]", formula) res = 0 for index, i in enumerate(calc_list): if index == 0: res = float(i) else: if operator_list[index-1] == '+': res += float(i) elif operator_list[index-1] == '-': res -= float(i) return res """

 """def elementary_arithmetic(formula): # 负数处理 formula = negative_start_issue(formula) formula = deal_negative_issue(formula) # 乘除运算 calc_list = re.split("[+-]",formula)  # 通过+-符号将各个乘除运算分隔出来 calc_operator_list = re.findall("[+-]",formula) for index1, item in enumerate(calc_list): calc_list[index1] = str(multiply_divide(item)) #数据类型的强制转换！！！ formula = update_formula(calc_list,calc_operator_list) # 加减运算 formula = add_abstract(formula) return formula """ 括号运算 """ def calculator(formula): #数据预处理 formula = formula.replace(" ","") m = re.search("\([^()]*\)",formula) # 判断是否需要进行括号运算 if m: # 括号运算 # 提取最小括号运算式，计算结果，并返回。 subformula = m.group().strip("()") # 把找出来的括号剥离 print("subformula:",subformula,type(subformula)) subres = elementary_arithmetic(subformula) # 调用四则混合运算主函数 print("subres:",subres) formula = formula.replace(m.group(), str(subres)) print("updated formula:",formula) if "(" in formula: return calculator(formula) else: print("formula result:",formula) # 除去所有括号后可能出现：1-2*-312.8 formula = elementary_arithmetic(formula) return formula else: return elementary_arithmetic(formula) # 以下为测试代码： formula = "1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*9/4*28 +10 * 56/14 )) - (-4*3)/ (16-3*2) )" print("%s = "%formula,calculator(formula))