求大神！！表达式顺序求值

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>

struct type
{
    float num1;
    float num2;
    int sign;
};

int main()
{
    char str[60],tmp;
    struct type sub={0};
    int i=0,flag=0,flag_dot=0;

    printf("请输入表达式:");
    gets(str);
    while(1)
    {
        strncpy(&tmp,str+i,1);
        if(tmp=='\0')break;
        i++;
        switch(tmp)
        {
        case '+':
            {
                if(flag==1)
                {
                    switch(sub.sign)
                    {
                    case 1:{sub.num2=sub.num2+sub.num1;break;}
                    case 2:{sub.num2=sub.num2-sub.num1; break;}
                    case 3:{sub.num2=sub.num2*sub.num1;break;}
                    case 4:{sub.num2=sub.num2/sub.num1;break;}
                    default:{printf("输入错误!\n");exit(1);}
                    }

                }
                else
                {
                    flag=1;
                    sub.num2=sub.num1;
                }
                sub.num1=0;
                sub.sign=1;
                flag_dot=0;
                break;
            }
        case '-':
            {
                if(flag==1)
                {
                    switch(sub.sign)
                    {
                    case 1:{sub.num2=sub.num2+sub.num1;break;}
                    case 2:{sub.num2=sub.num2-sub.num1; break;}
                    case 3:{sub.num2=sub.num2*sub.num1;break;}
                    case 4:{sub.num2=sub.num2/sub.num1;break;}
                    default:{printf("输入错误!\n");exit(1);}
                    }

                }
                else
                {
                    flag=1;
                    sub.num2=sub.num1;
                }
                sub.num1=0;
                sub.sign=2;
                flag_dot=0;
                break;
            }
        case '*':
            {
                if(flag==1)
                {
                    switch(sub.sign)
                    {
                    case 1:{sub.num2=sub.num2+sub.num1;break;}
                    case 2:{sub.num2=sub.num2-sub.num1; break;}
                    case 3:{sub.num2=sub.num2*sub.num1;break;}
                    case 4:{sub.num2=sub.num2/sub.num1;break;}
                    default:{printf("输入错误!\n");exit(1);}
                    }

                }
                else
                {
                    flag=1;
                    sub.num2=sub.num1;
                }
                sub.num1=0;
                sub.sign=3;
                flag_dot=0;
                break;
            }
        case '/':
            {
                if(flag==1)
                {
                    switch(sub.sign)
                    {
                    case 1:{sub.num2=sub.num2+sub.num1;break;}
                    case 2:{sub.num2=sub.num2-sub.num1; break;}
                    case 3:{sub.num2=sub.num2*sub.num1;break;}
                    case 4:{sub.num2=sub.num2/sub.num1;break;}
                    default:{printf("输入错误!\n");exit(1);}
                    }

                }
                else
                {
                    flag=1;
                    sub.num2=sub.num1;
                }
                sub.num1=0;
                sub.sign=4;
                flag_dot=0;
                break;
            }
        case '.':{flag_dot=1;break;}
        default:
            {
                if(tmp>=48&&tmp<=57)
                {
                    if(flag_dot>0)
                    {
                        sub.num1=sub.num1+(float)(tmp-48)/pow(10,flag_dot);
                        //printf("num1=%.2f,tmp=%d,flag_dot=%d\n",sub.num1,tmp,flag_dot);
                        flag_dot++;
                    }
                    else
                        sub.num1=sub.num1*10+tmp-48;
                    break;
                }
                else
                {
                   printf("输入错误!\n");
                   exit(1);
                }
            }
        }
        //printf("num1=%.2f,num2=%.2f,sign=%d\n",sub.num1,sub.num2,sub.sign);
    };

    switch(sub.sign)
    {
    case 1:{sub.num2=sub.num2+sub.num1;break;}
    case 2:{sub.num2=sub.num2-sub.num1;break;}
    case 3:{sub.num2=sub.num2*sub.num1;break;}
    case 4:{sub.num2=sub.num2/sub.num1;break;}
    default:{printf("输入错误!\n");exit(1);}
    }
    printf("表达式的值为%.4f",sub.num2);

    return 0;
}