表达式求值

 

//1。若ch为数字，将后续的所有数字均依次存入数组exp中，并以字符#标志数值串结束
//2。若ch为左括号'('，则将此括号入栈op
//3。若ch为右括号')'，则将栈op中'('以前的字符依次删除并存入数组exp中，然后将左括号'('删除
//4。若ch为'+'或'-'， 则将栈op中'('以前的字符(运算符)依次删除并存入数组exp中，然后将ch入栈op中
//5。若ch为'*'或'/'，则将当前栈op中的栈顶端连续的'*'或'/'删除，并依次存入数组exp中，然后将ch入栈op
//6。若扫描完毕，则将栈op中的所有运算符依次删除并存入数组exp中，然后再将ch存入数组exp中 
#include <iostream.h>
#define MaxSize 50
void trans(char str[], char exp[]); 
float compvalue(char exp[]); 
int main()
{
    char str[MaxSize],exps[MaxSize];
    cout<<"输入一个表达式:"<<endl;
    gets(str);
    cout<<"原表达式为："<<str<<endl; 
    trans(str,exps);
    cout<<"后缀表达式为："<<exps<<endl;
    cout<<"计算结果为: "<<compvalue(exps)<<endl; 
    getchar();
    getchar();
}
void trans(char str[], char exp[])
{
     struct 
     {
         char data[MaxSize];
         int top;
     }op;
     char ch;
     int i=0,t=0;
     op.top=-1;
     ch=str[i];
     i++;
     while(ch!='\0')
     {
         switch(ch)
         {
             case '(':
                 op.top++; op.data[op.top]=ch;
                 break;
             case ')':
                 while(op.data[op.top]!='(')
                 {
                     exp[t]=op.data[op.top];
                     op.top--;
                     t++;
                 }
                 op.top--;
                 break;
             case '+':
             case '-':
                 while(op.top!=-1&&op.data[op.top]!='(')
                 {
                     exp[t]=op.data[op.top];
                     op.top--;
                     t++;
                 }
                 op.top++; op.data[op.top]=ch;
                 break;
             case '*':
             case '/':
                 while(op.top!=-1&&(op.data[op.top]=='*'||op.data[op.top]=='/'))
                 {
                     exp[t]=op.data[op.top];
                     op.top--;
                     t++;
                 }
                 op.top++; op.data[op.top]=ch;
                 break;
             case ' ': break;
             default:
                 while(ch>='0'&&ch<='9')
                 {
                     exp[t]=ch; t++;
                     ch=str[i]; i++;
                 }
                 i--;
                 exp[t]='#'; t++;
             }
             ch=str[i];i++;
         }
         while(op.top!=-1)
         {
             exp[t]=op.data[op.top];
             t++; 
             op.top--;
         }
         exp[t]='\0';
}
float compvalue(char exp[])
{
    struct
    {
        float data[MaxSize];
        int top;
    }st;
    float d;
    char ch;
    int t=0;
    st.top=-1;
    ch=exp[t]; t++;
    while(ch!='\0')
    {
        switch(ch)
        {
            case '+': st.data[st.top-1]=st.data[st.top-1]+st.data[st.top];
                st.top--; break;
            case '-': st.data[st.top-1]=st.data[st.top-1]-st.data[st.top];
                st.top--; break;
            case '*': st.data[st.top-1]=st.data[st.top-1]*st.data[st.top];
                st.top--; break;
            case '/':
                if(st.data[st.top]!=0)
                   st.data[st.top-1]=st.data[st.top-1]/st.data[st.top];
                else
                {
                    cout<<"除0出错"<<endl;
                    exit(0);
                }
                st.top--; break;
            default:
                d=0;
                while(ch>='0'&&ch<='9')
                {
                    d=10*d+ch-'0';
                    ch=exp[t];
                    t++;
                }
                st.top++;
                st.data[st.top]=d;
            }
            ch=exp[t];
            t++;
        }
        return st.data[st.top];
}