c/c++语言开发共享C++之四则运算表达式求值

#include<stdio.h> #include<stdlib.h> #include<string.h> #include<math.h>  #define inf float(0x3f3f3f3f) #define maxsize 100  char priority[7] = {'+', '-', '*', '/', '(', ')', '#'};  char priority_relationship[7][7] = {     {'>', '>', '<', '<', '<', '>', '>'},      {'>', '>', '<', '<', '<', '>', '>'},      {'>', '>', '>', '>', '<', '>', '>'},      {'>', '>', '>', '>', '<', '>', '>'},      {'<', '<', '<', '<', '<', '=', ' '},      {'>', '>', '>', '>', ' ', '>', '>'},      {'<', '<', '<', '<', '<', ' ', '='}  };   typedef struct stacknode {     char data[maxsize];   // 压入栈里面的数据都是字符型，在进行运行时，记得将字符型数字转换为浮点型数字     struct stacknode *next; }stacknode, *linkstack;  void initstack(linkstack &s) {// 构造一个空栈s，栈顶指针置空     s = null; }  void push(linkstack &s, char data[]) {// 在栈顶插入元素data     stacknode *p;      p = (stacknode *)malloc(sizeof(stacknode));  // 生成新的结点     strcpy(p->data, data);  // 将新结点的数据域置为data     p->next = s;    // 将新结点插入栈顶     s = p;    // 修改栈顶指针为p }  char *pop(linkstack &s) {// 删除s的栈顶元素， 用data返回其值     char data[maxsize];     if(s == null) printf("错误！！！n栈为空， 无法执行删除命令...");     else     {         stacknode *p;          strcpy(data, s->data);  // 将栈顶元素赋给data         p = s;   // 用p临时保存栈顶元素的空间，以备释放         s = s->next;   //修改栈顶指针          free(p);    // 释放原栈顶元素的空间         return data;     } }  char *gettop(linkstack &s) {     if(s != null)         return s->data;     else     {         printf("错误！！！n栈顶为空");         return "0";     } }  float str_to_float(char *str) {     float num = 0;     int state_1 = 0;     int state_2 = 0;     while(( *str != '' && *str >= '0' && *str <= '9') || *str == '.' || (*str == '-' && *(str + 1) != ''))     {// 注意判断小数点和负号         if(*str == '.') state_1 = 1;         else if(*str == '-') state_2 = 1;         else         {             if(state_1 == 0) num = num * 10 + (*str - '0');             else             {                 num += (*str - '0') * pow(0.1, state_1);                 state_1++;             }         }         str++;     }     if(*str != '') return inf;     else if(state_2 == 1)     {         return num * -1;     }     else return num; }  char *float_to_str(float num) {     char str[maxsize];     sprintf(str, "%.4f", num);  // 保留小数点后4位     return str; }  int get_index(char str[]) {     for(int i = 0; i < 7; i++)     {         if(str[0] == priority[i]) return i;     }     printf("未找到匹配的字符n"); }  char precede(char inside_data[], char input_data[]) {     int inside_index = get_index(inside_data);     int input_index = get_index(input_data);      return priority_relationship[inside_index][input_index]; }  float operate(char a[], char theta[], char b[]) {//执行运算     float a_num = str_to_float(a);     float b_num = str_to_float(b);      if(theta[0] == '+') return a_num + b_num;     else if(theta[0] == '-') return a_num - b_num;     else if(theta[0] == '*') return a_num * b_num;     else if(theta[0] == '/') return a_num / b_num;     else printf("错误！！！n无该运算符"); }  float evaluateexpression() {     stacknode *opnd, *optr;     char str[maxsize];     char theta[maxsize];     char a[maxsize];     char b[maxsize];      initstack(opnd);     initstack(optr);     push(optr, "#");      scanf("%s", str);     while(str[0] != '#' || gettop(optr)[0] != '#')     {         if(str_to_float(str) != inf)         {             push(opnd, str);             scanf("%s", str);         }         else         {             switch (precede(gettop(optr), str))             {             case '<':                 push(optr, str);                 scanf("%s", str);                 break;             case '>':                 strcpy(theta, pop(optr));                 strcpy(b, pop(opnd));                 strcpy(a, pop(opnd));                 char temp_str[maxsize];                 strcpy(temp_str, float_to_str(operate(a, theta, b)));                 push(opnd, temp_str);                 break;             case '=':                 pop(optr);                 scanf("%s", str);                 break;             default:                 printf("错误！！！n该优先级不存在！！！");             }         }      }      return str_to_float(gettop(opnd)); }  int main(void) {     float num;      num = evaluateexpression();     printf("%fn", num);      printf("n");     system("pause"); }