一个简单的自顶向下语法分析(表达式求值)

//一个简单的自顶向下语法分析#include <cassert>#include <cstdio>using namespace std;char str[1024];long curpos;int num(){int ret;if (str[curpos] >= '0' && str[curpos] <= '9'){ret = str[curpos] - '0';++curpos;return ret;}printf("ERROR!not a num.pos=%d/r/n", curpos);assert(0);return 0;}void match(int c){if (str[curpos] == c){++curpos;return;}printf("ERROR!not =%c/r/n", c);assert(0);}int A();int C(){int ret;if (str[curpos] == '('){match('(');ret = A();match(')');}else{ret = num();}return ret;}int B();void B2(int& token, int& num){if (str[curpos] == '*'){match('*');token = '*';num = B();}else{token = 0;}}int B(){int ret = C();int token, num;B2(token, num);if (token == '*'){return ret * num;}else if (token == '/'){return ret / num;}else{return ret;}}void A2(int& token, int& num){if (str[curpos] == '+'){match('+');token = '+';num = A();}else if (str[curpos] == '-'){match('-');token = '-';num = A();}}int A(){int token, num;int ret = B();A2(token, num);if (token == '+'){return ret + num;}else if (token == '-'){return ret - num;}else{return ret;}}int main(){printf("请输入表达式:/r/n");for (; ;){scanf("%s", &str);curpos = 0;printf("结果为:%d/r/n", A());match(0);printf("Corrent!/r/n");}return 0;}