简单表达式求值

Warning: 只支持0-9的加减乘除，测试一下博客园功能

Env: Win7+VS2010

$cl main.cpp
$main.exe (3+(4*8))+((6+7)/5)

348*+67+5/+
Result : 37

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <stack>

char buff[1000] = {0};
int count=0;//指向buff中下一个可用的位置
char current;//currentExp所指向的字符
char* currentExp;//指向表达式的指针

void T(void);
void F(void);
void G(void);
void fatal(const char* msg)
{
    fprintf(stderr, "%s\n", msg);
    exit(-1);
}

void T(void)
{
    F();
    while(strchr("+-", current) != NULL && current != '\0')
    {
        char temp = current;
        current = *(++currentExp);
        F();
        buff[count] = temp;
        count++;
    }
}

void F(void)
{
    G();
    while(strchr("*/", current) != NULL && current != '\0')
    {
        char temp = current;
        current = *(++currentExp);
        G();
        buff[count] = temp;
        count++;
    }
}

void G(void)
{
    if (current>='0' && current<='9')
    {
        buff[count]=current;
        count++;
        current = *(++currentExp);
    }
    else if (current == '(')
    {
        current = *(++currentExp);
        T();
        if (current != ')') fatal("mismatched parantheses!");
        current = *(++currentExp);
    }
    else
    {
        fatal("bogus expression!");
    }

}
/*********************************************************************
** 中缀表达式转换为后缀表达式
**
**
** T-->F{(+|-)F}
** F-->G{(*|/)G}
** G--> 0-9
**  --> (T)
*********************************************************************/
void Infix2Postfix(char* expr)
{
    current = *(expr);
    T();
    buff[999] = '\0';
}

int Calculate()
{
    std::stack<int> s;
    int sum = 0;
    int oper1 = 0, oper2 = 0;
    int num = 0;
    char c;
    while(buff[num])
    {
        c = buff[num];
        if (c>'0' && c<'9')
        {
            s.push(c-'0');
        }
        else
        {
            switch (c)
            {
            	case '+':
                    oper1 = s.top();
                    s.pop();
                    oper2 = s.top();
                    s.pop();
                    sum = oper2 + oper1;
                    s.push(sum);
            		break;

                case '-':
                    oper1 = s.top();
                    s.pop();
                    oper2 = s.top();
                    s.pop();
                    sum = oper2 - oper1;
                    s.push(sum);
                    break;

                case '*':
                    oper1 = s.top();
                    s.pop();
                    oper2 = s.top();
                    s.pop();
                    sum = oper2 * oper1;
                    s.push(sum);
                    break;

                case '/':
                    oper1 = s.top();
                    s.pop();
                    oper2 = s.top();
                    s.pop();
                    sum = oper2 / oper1;
                    s.push(sum);
                    break;

            	default:
                    fatal("error");
                    exit(1);
            		break;
            }
        }
        num++;
    }
    return sum;
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        fprintf(stderr, "usage: %s <expression>\n", argv[0]);
        exit(1);
    }
    currentExp = argv[1];
    Infix2Postfix(currentExp);
    printf( "%s\n", buff);
    printf( "Result : %d\n", Calculate());
    return 0;
}