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我正在为我的 CS 课程开发 Datalog 解释器，我遇到了一个奇怪的问题，即我的规则评估需要通过太多次才能完成。查看我的代码后，我在以下内容中进行了两项修改，修复了我的评估以正确执行次数：

//original form
bool addedFacts = false;
for (X x: xs) {
    addedFacts = addedFacts || set.insert(x).second;
}
//modified form
bool addedFacts = false;
for (X x: xs) {
    if (set.insert(x).second) {
        addedFacts = true;
    }
}

对我来说，这两种代码结构在逻辑上是等价的。一个执行正确而一个执行不正确/效率低下是否有原因？这是正在发生的问题的可构建示例：

#include <iostream>
#include <set>
#include <vector>

using std::set;
using std::vector;
using std::cout;
using std::endl;

const int CAP = 100;

class Rule {
public:
    int factor;
    Rule(int factor) {
        this->factor = factor;
    }
    bool evaluateInefficient(set<int>& facts) {
        vector<int> data;
        bool addedFacts = false;
        for (int fact : facts) {
            data.push_back(fact);
        }
        for (int datum : data) {
            int newFact = datum * factor;
            if (newFact < CAP) {
                addedFacts = addedFacts || facts.insert(newFact).second;
            }
        }
        return addedFacts;
    }
    bool evaluate(set<int>& facts) {
        vector<int> data;
        bool addedFacts = false;
        for (int fact : facts) {
            data.push_back(fact);
        }
        for (int datum : data) {
            int newFact = datum * factor;
            if (newFact < CAP) {
                if (facts.insert(newFact).second) {
                    addedFacts = true;
                }
            }
        }
        return addedFacts;
    }
};

int doublyInefficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            addedFacts = addedFacts || rule.evaluateInefficient(facts);
        }
    }
    return passes;
}

int singlyInefficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            addedFacts = addedFacts || rule.evaluate(facts);
        }
    }
    return passes;
}

int efficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            if (rule.evaluate(facts)) {
                addedFacts = true;
            }
        }
    }
    return passes;
}

int main(int argc, char* argv[]) {
    //build the rules
    vector<Rule> rules;
    rules.push_back(Rule(2));
    rules.push_back(Rule(3));
    rules.push_back(Rule(5));
    rules.push_back(Rule(7));
    rules.push_back(Rule(11));
    rules.push_back(Rule(13));
    //Show three different codes that should (in my mind) take the same amount of passes over the rules but don't
    cout << "Facts populated after " << doublyInefficient(rules) << " passes through the Rules." << endl;
    cout << "Facts populated after " << singlyInefficient(rules) << " passes through the Rules." << endl;
    cout << "Facts populated after " << efficient(rules) << " passes through the Rules." << endl;
    getchar();
}

在 Visual Studio 2017 上以调试和发布模式（32 位）运行时，我得到以下输出。据我所知，代码未优化。

Facts populated after 61 passes through the Rules.
Facts populated after 17 passes through the Rules.
Facts populated after 7 passes through the Rules.
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2 回答 2

3 
addedFacts = addedFacts || set.insert(x).second;

和

if (set.insert(x).second) {
    addedFacts = true;
}

绝对不是一回事。第一个代码块相当于：

if (!addedFacts) {
    addedFacts = set.insert(x).second;
}

!addedFacts检查有很大的不同。

于 2017-08-02T21:50:27.313 回答
2

由于短路评估而出现差异：考虑形式的表达式(expr1 || expr2)。短路意味着如果expr1评估为，则根本不会评估true表达式（参见此在线 c++ 标准草案，重点是我的）：expr2

5.15 逻辑或运算符

|| 运算符组从左到右。操作数都根据上下文转换为 bool（子句 [conv]）。如果其中一个操作数为真，则返回真，否则返回假。与 |、|| 不同 保证从左到右的评估；此外，如果第一个操作数的计算结果为 true ，则不计算第二个操作数。

因此，在您的表达式中addedFacts || set.insert(x).second，从第一次开始，表达式将不再执行。我想这是“错误”的行为，因为你不会包含相应的es 。addedFactstrueset.insert(x).secondsetx

于 2017-08-02T22:01:17.457 回答