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:Dangimport redef 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 formuladef 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 formuladef 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 formuladef 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 formuladef 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 resdef 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))