刚发帖问的程序贴出来大家看看，欢迎讨论

#include<iostream>
#include<stdio.h>
#include<string>
#include<stdlib.h>
using namespace std;

string calculate(string str)
{
    double num[100];//把str里由符号分隔的数存入num数组中
    int nCount=0;//num数组的下标
    char opr[100];
    string temp;
    int pos=0;
    int lastopr=0;
    int i;
    for(i=0;i<str.length();i++)
    {
        if(str[i]=='+'||str[i]=='-'||str[i]=='*'||str[i]=='/')
        {
            temp=str.substr(pos,i-pos);
            pos=i+1;
            num[nCount]=atof(temp.c_str());
            opr[nCount]=str[i];
            nCount++;
            lastopr=i;
        }   //数字和符号的存储格式为 数   num:num[0],num[1]...num[nCount-1],num[nCount].
    }                             // 符号 opr:opr[0],opr[1]...opr[nCount-1].
    //把最后一个数字放入数组中
    temp=str.substr(lastopr+1,str.length()-lastopr-1);
    num[nCount]=atof(temp.c_str());

    //先算乘除法
    i=0;
    while(i<nCount)
    {
        if(opr[i]=='*'||opr[i]=='/')
        {
            switch(opr[i])
            {
                case'*':num[i]*=num[i+1];break;
                case'/':num[i]/=num[i+1];break;
            }
            int j=i+1;
            while(j<nCount)
            {
                num[j]=num[j+1];
                opr[j-1]=opr[j];
                j++;
            }
            nCount--;
            i--;
        }
        i++;
    }

    //再算加减法
    i=0;
    while(i<nCount)
    {
        if(opr[i]=='+'||opr[i]=='-')
        {
            switch(opr[i])
            {
                case'+':num[i]+=num[i+1];break;
                case'-':num[i]-=num[i+1];break;
            }
            int j=i+1;
            while(j<nCount)
            {
                num[j]=num[j+1];
                opr[j-1]=opr[j];
                j++;
            }
            nCount--;
            i--;
        }
        i++;
    }
    char tempChar[100];
    sprintf(tempChar,"%.6f",num[0]);
    temp=tempChar;
    return temp;
}

int main()
{
    string str;
    cin>>str;
    cout<<calculate(str)<<endl;
    return 0;
}