表达式求值

描述：思路很简单，优先级比较完的情况下，采用两个栈就可以完成了……

#include <iostream>
#include <cstdio>
#include <cstring>
#include "hanshu.h"  //头文件
using namespace std;
int flag;//下面会多次用到flag
int num_value(char *s,char *str,double *num,int len,int i,int j);//计算实数值存入num中
char get_precede(char a,char b);//比较两个字符优先级，获取优先级
int calculate_num(char c,double *num,int j);//优先级比较大小后计算结果
double solve_s(char *s ,char *str,double *num,int len);//处理函数，得出等式的结果
int main()
{
    freopen("a.txt","r",stdin);
    double num[10010];
    char c,s[10010],str[10010];
    memset(s,0,sizeof(s));
    int left,right,len;
    left=right=len=flag=0;
    while(scanf("%c",&c)!=EOF)//单字符输入
    {
        if((c>='0'&&c<='9')||c=='.'||c=='('||c==')'||c=='+'||c=='-'||c=='*'||c=='/')//选择合适的字符输入
        {
            s[len++]=c;
            if(c=='(')
            {
                if(!right)flag=1;
                left++;
            }
            else if(c==')')
            {
                right++;
                if(!left) flag=2;
            }
        }
        if(c=='\n')//讨论几种不符合情况的输入
        {
            if(flag==2) printf("输入错误，首先出现了右括号！\n");
            else if(left>right) printf("左括号比右括号多!\n");
            else if(left<right) printf("右括号比左括号多!\n");
            else if(len<=0) printf("没有数据!\n");
            else
            {
                s[len++]='#';
                flag=0;
                memset(str,0,sizeof(str));
                memset(num,0,sizeof(num));
                double x=solve_s(s,str,num,len);
                if(flag) printf("出现零为除数\n");
                else printf("solve_s=%g\n",x);
            }
            memset(s,0,sizeof(s));
            flag=len=left=right=0;
        }
    }
    return 0;
}

//头文件

#ifndef HANSHU_H_INCLUDED
#define HANSHU_H_INCLUDED
char str_operator[]= {'+','-','*','/','(',')','#'};
char str_precede[7][7]=
{
    {'>','>','<','<','<','>','>'},
    {'>','>','<','<','<','>','>'},
    {'>','>','>','>','<','>','>'},
    {'>','>','>','>','<','>','>'},
    {'<','<','<','<','<','=',' '},
    {'>','>','>','>',' ','>','>'},
    {'<','<','<','<','<',' ','='}
};
extern int flag;
int num_value(char *s,char *str,double *num,int len,int i,int j)
{
    int k,count(0),c(0);
    for( k=i; k<len; k++)
        if(s[k]>='0'&&s[k]<='9')
        {
            num[j]=num[j]*10+s[k]-'0';
            if(c) count++;
        }
        else if(s[k]=='.') c=1;
        else break;
    for( c=0; c<count; c++)
        num[j]/=10;
    if((i==1&&(s[i-1]=='+'||s[i-1]=='-'))||(i>=2&&(s[i-1]=='-'||s[i-1]=='+')&&s[i-2]=='('))
    {
        flag=1;
        if(s[i-1]=='-') num[j]=0-num[j];
    }
    return k;
}
char get_precede(char a,char b)
{
    int i,j;
    for(int  k=0; k<7; k++)
    {
        if(a==str_operator[k]) i=k;
        if(b==str_operator[k]) j=k;
    }
    return str_precede[i][j];
}
int calculate_num(char c,double *num,int j)
{
    j--;
    switch(c)
    {
    case  '+':
        num[j-1]=num[j-1]+num[j];
        break;
    case  '-':
        num[j-1]=num[j-1]-num[j];
        break;
    case  '/':
        num[j-1]=num[j-1]/num[j];
        break;
    case  '*':
        num[j-1]=num[j-1]*num[j];
        break;
    }
    if(c=='-'||c=='+'||c=='*'||c=='/')
    {
        num[j]=0;
        flag=1;
        return j;
    }
    else return j+1;
}
double solve_s(char *s ,char *str,double *num,int len)
{
    int i,j,k(1);
    str[0]='#';
    for(i=j=0; i<len; i++)
    {
        if(s[i]>='0'&&s[i]<='9')
        {
            i=num_value(s,str,num,len,i,j);
            if(flag)
            {
                str[--k]=0;
                flag=0;
            }
            j++;
        }
        char c=get_precede(str[k-1],s[i]);
        if(c=='<') str[k++]=s[i];
        else if(c=='=') str[--k]=0;
        else if(c=='>')
        {
            while(c=='>')
            {
                if(str[k-1]=='/'&&!num[j-1])
                {
                    flag=1;
                    return -1;
                }
                flag=0;
                j=calculate_num(str[k-1],num,j);
                if(flag)
                {
                    str[--k]=0;
                    c=get_precede(str[k-1],s[i]);
                    flag=0;
                    continue;
                }
                else break;
            }
            if(str[k-1]=='('&&s[i]==')') str[--k]=0;
            else str[k++]=s[i];
        }
    }
    return num[0];
}

#endif // HANSHU_H_INCLUDED