# 开发一个简单的python计算器
# 实现加减乘除及括号优先级解析
# 用户输入 1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*99/4*2998 +10 * 568/14 )) - (-4*3)/ (16-3*2) )
# 等类似公式后，必须自己解析里面的(),+,-,*,/符号和公式(不能调用eval等类似功能偷懒实现)，运算后得出结果，结果必须与真实的计算器所得出的结果一致 一.对公式进行简单处理（去掉空格及公式化简）
import re def format_string(string):
string = string.replace(' ','')
string = string.replace('++','+')
string = string.replace('+-','-')
string = string.replace('-+','-')
string = string.replace('--','+')
string = string.replace('*+','*')
string = string.replace('/+','/')
return string
二.对于不符合要求的公式进行异常处理，给出错误提示信息
def check_expression(string):
check_result = True
if not string.count("(") == string.count(")"):
print('括号不对称')
check_result = False
if re.findall('[a-z]',string.lower()):
print('表达式有错，包含非法字符')
check_result = False
return check_result
三.进行乘除运算，并将计算的结果替换掉原公式中的乘除法运算
def mul_div(string):
#正则表达式匹配（x乘以y或x除以y）
regular = '\d+\.?\d*([*/]|\*\*)[\-]?\d+\.?\d*'
#一直到公式中没有除法或乘法运算时，退出循环
while re.findall(regular,string):
expression = re.search(regular,string).group()
#计算乘法，并将计算的结果替换原来的式子
if expression.count("*") == 1:
x,y = expression.split('*')
mul_result = str(float(x) * float(y))
string = string.replace(expression,mul_result)
string = format_string(string)
#计算除法，并将计算的结果替换原来的式子
if expression.count("/") :
x,y = expression.split('/')
div_result = str(float(x) / float(y))
string = string.replace(expression,div_result)
string = format_string(string)
return string
四.进行加减法运算
def add_sub(string):
sub_regu = '[\-]?\d+\.?\d*\-[\-]?\d+\.?\d*'
add_regu = '[\-]?\d+\.?\d*\+[\-]?\d+\.?\d*'
#计算加法
while re.findall(add_regu,string):
add_list = re.findall(add_regu,string)
for add_str in add_list:
x,y = add_str.split("+")
add_result = "+" + str(float(x) + float(y))
string = string.replace(add_str, add_result)
#计算减法
while re.findall(sub_regu,string):
sub_list = re.findall(sub_regu,string)
for sub_str in sub_list:
numbers = sub_str.split('-')
if len(numbers) == 3:
result = 0
#计算-1-2这种情况
for v in numbers:
if v:
result -= float(v)
else:
x,y = numbers
result = float(x)-float(y)
string = string.replace(sub_str,"+" + str(result))
string = format_string(string)
return string
五.*造好了，主逻辑程序
if __name__ =="__main__":
if check_expression(source):
strs = format_string(source)
while source.count('(') > 0:
#匹配最里层括号的内容
strs = re.search('\([^()]*\)',source).group()
replace_str = mul_div(strs)
replace_str = add_sub(replace_str)
#每次计算的结果去掉括号
source = format_string(source.replace(strs,replace_str[1:-1]))
# print(source)
#else下边的逻辑是计算不带括号的式子
else:
replace_str = mul_div(source)
replace_str = add_sub(replace_str)
source = source.replace(source,replace_str)
print(source)
六.总结-难点
1.匹配最里层的式子：'\([^()]*\)'
2.匹配乘除法及加减法的正则表达式
3.主逻辑的设计
七.总程序
import re def format_string(string):
string = string.replace(' ','')
string = string.replace('++','+')
string = string.replace('+-','-')
string = string.replace('-+','-')
string = string.replace('--','+')
string = string.replace('*+','*')
string = string.replace('/+','/')
return string def mul_div(string):
"计算乘除运算，所得的结果替换原先的表达式返回"
regular = '\d+\.?\d*([*/]|\*\*)[\-]?\d+\.?\d*'
while re.findall(regular,string):
expression = re.search(regular,string).group()
if expression.count("*") == 1:
x,y = expression.split('*')
mul_result = str(float(x) * float(y))
string = string.replace(expression,mul_result)
string = format_string(string)
if expression.count("/") :
x,y = expression.split('/')
div_result = str(float(x) / float(y))
string = string.replace(expression,div_result)
string = format_string(string)
return string def add_sub(string):
sub_regu = '[\-]?\d+\.?\d*\-[\-]?\d+\.?\d*'
add_regu = '[\-]?\d+\.?\d*\+[\-]?\d+\.?\d*'
#计算加法
while re.findall(add_regu,string):
add_list = re.findall(add_regu,string)
for add_str in add_list:
x,y = add_str.split("+")
add_result = "+" + str(float(x) + float(y))
string = string.replace(add_str, add_result)
#计算减法
while re.findall(sub_regu,string):
sub_list = re.findall(sub_regu,string)
for sub_str in sub_list:
numbers = sub_str.split('-')
if len(numbers) == 3:
result = 0
#计算-1-2这种情况
for v in numbers:
if v:
result -= float(v)
else:
x,y = numbers
result = float(x)-float(y)
string = string.replace(sub_str,"+" + str(result))
string = format_string(string)
return string def check_expression(string):
check_result = True
if not string.count("(") == string.count(")"):
print('表达式有错，括号不对')
check_result = False
if re.findall('[a-z]',string.lower()):
print('表达式有错，包含非法字符')
check_result = False
return check_result if __name__ =="__main__":
source = '1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*99/4*2998 +10 * 568/14 )) - (-4*3)/ (16-3*2) )'
if check_expression(source):
strs = format_string(source)
while source.count('(') > 0:
strs = re.search('\([^()]*\)',source).group()
replace_str = mul_div(strs)
replace_str = add_sub(replace_str)
source = format_string(source.replace(strs,replace_str[1:-1]))
else:
replace_str = mul_div(source)
replace_str = add_sub(replace_str)
source = source.replace(source,replace_str)
print(source)