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))