简单的计算程序

今天的题目是：

如何写出一个简单的4则运算的程序。如输入 ” 1 -2*3 + 6/3″ ，输出 -3。只用支持正整数，不用考虑有括号的情况。

思路：因为不用考虑括号，所以乘和除的优先级最高。可以先把输入字符串用“+”和“-”分隔成一个数组，数组的结构是一个数字和符号间隔，把乘除法的项当作一个数字处理，如题目中的输入会被分成[“1″,”-“,”2*3″,”+”,”6/3″]，然后再每三个数组项计算一次，把得到的结果加入到剩下的数组里，递归计算。如果计算过程中碰到还有乘除法的项，如”2*3″，则先把这个项算出来。

具体代码如下：

#!/usr/bin/python
# encoding:UTF-8

'''
Calculate the input string and output the result.
For input of '1-2*3+6/3', output -3
Only consider the situation of int, assume no "(" in the string
'''
add_sub = ['-','+']
mul_div = ['*','/']

def parse(input, pattern):
    '''
        parse the input string to a raw list
        every elemtns in the list is either an operand or operator
        treat add/substruct and multiply/division differently
        the operand of add/sub could be a expression of mult/div
    '''
    raw_list = []
    digits = ''
    index = 0
    for c in input.strip():
        index += 1
        if c == ' ':
            #check if space
            continue
        elif c not in pattern:
            #if not a operator store it
            digits += c
            if index == len(input):
                raw_list.append(digits)
        elif c in pattern:
            #if find a operator, append the formal found digits
            #to the output list
            raw_list.append(digits)
            raw_list.append(c)
            digits = ''
    return raw_list

def cal_unit(expression):
    oper1, sign, oper2 = expression
    oper1 = check_if_mult_div(oper1)
    oper2 = check_if_mult_div(oper2)
    if sign == '+':
        result = oper1+oper2
    if sign == '-':
        result = oper1-oper2
    if sign == '/':
        result = oper1*1.0/oper2
    if sign == '*':
        result = oper1*oper2
    return str(result)

def check_if_mult_div(operand):
    if operand.find('/') == -1 and operand.find('*') == -1:
        return float(operand.strip())
    else:
        #if the operand contains * or /, calculate it
        return float(cal_all(parse(operand, mul_div)))

def cal_all(list):
    #user recursion to calculate the whole list
    if len(list) == 3:
        return cal_unit(list)
    else:
        return cal_all([cal_unit(list[:3])]+list[3:])

if __name__=='__main__':
    a = raw_input('Enter:')
    print cal_all(parse(a, add_sub))