评估IF条件的最有效方法是什么?

What's the most efficient way to evaluate an IF condition?

本文关键字:方法 是什么 有效 IF 条件 评估      更新时间:2023-10-16

我正在做一个语言解释器,我已经到了需要评估if语句的地步。起初我认为这很简单,如果这样的条件,我能够让我的解释器评估简单,10 == 10但后来当我试图让它评估更复杂的条件时,例如10 == 10 and 9 > 2,它搞砸了。

例如,我制作了一些代码C++代码,可以单独评估条件的每个部分。

"Hello World" == "Hello World" or "Test" == "Test"

目前的工作方式是这样的,它给出了上面的实际结果和下面的预期结果。这样做的结果将是:

TRUE or TRUE           <- Actual
-----------------------
TRUE or TRUE           <- Expected
-----------------------

代码如下:

#include <iostream>
#include <string>
#include <vector>    
#include "cond.h"
using namespace std;
vector <string> cond_holder;
vector <string> res;
vector <string> expects;
bool eval_cond(string cond) {
    int i;
    int i2;
    bool state = 0;
    bool final_return = false;
    string c = "";
    for (i = 0; i < cond.length();i++) {
        c += cond[i];
        if (cond[i] == '"') {
            if (state == 0)
                state = 1;
            else
                state = 0;
        } else if (cond[i] == ' ' && state == 0) {
            c = c.substr(0,c.length()-1);
            cond_holder.push_back(c);
            c = "";
        }
        if (i == cond.length()-1) {
            cout << c << endl;      
        }
    }
    for (i = 0; i < cond_holder.size();i++) {
        if (cond_holder[i+1] == "eqeq") {
            expects.push_back("TRUE");
            if (cond_holder[i] == cond_holder[i+2]) {
                res.push_back("TRUE");
            } else {
                res.push_back("FALSE");
            }
            i+=3;
        }
        if (cond_holder[i] == "and") {
            res.push_back("and");
            expects.push_back("and");
        } else if (cond_holder[i] == "or") {
            res.push_back("or");
            expects.push_back("or");
        }
    }
    for (i = 0; i < res.size();i++) {
        cout << res[i] << " ";
    }
    cout << endl << "-----------------------" << endl;
    for (i = 0; i < expects.size();i++) {
        cout << expects[i] << " ";
    }
    cout << endl << "-----------------------" << endl;
    return final_return;
}
int main() {
    cout << eval_cond("string:"Hello World" eqeq string:"Hello World" or string:"H" eqeq string:"H" ") << endl;
    return 0;
}

老实说,我只是在进行过程中编写了代码,所以如果有人能告诉我更好的方法,我将不胜感激。我什至不确定接下来如何处理这段代码。

Variable is an expression
Number is an expression
String is an expression
Expression == Expression is an expression
Expression AND Expression is an expression
Expression ; is a statement
IF Expression { statement } is a statement

这样的片段构建你的语言,然后让它们一起崩溃。 Flex和Yacc的旧Unix手册很好地介绍了这个主题。