c++ - vector 的 Push_front() 替代方案和 map 的 operator++

Question

我在这些函数上遇到编译器错误:

编译器在我的 vector 上提示 push_front()，还有我试图在我的 map 上使用的运算符++。

我只是想分别在前面添加一个 vector 元素和添加一个新的 map 条目。

 LanguageModel countSequence(LanguageModel &model, vector<string> &Seq) {    
    //append start and end to the vector of strings    
    vector<string>::iterator v = Seq.begin();    
    Seq.front();    
    Seq.("<START>");    
    v = Seq.end();    
    Seq.push_back("<END");    
    v = Seq.begin();


    //create word pairs    
    vector<string> pairs;     
    for (int n = 0; n <= Seq.size(); n++) {    
    pairs.at(n) = buildPair(Seq[n], Seq[n+1]);

    } 


    //feed each word to map 1 with number of times it was seen    
    for (int m = 0; m <= Seq.size(); m++) {    
    ++model.firstCounts[Seq[m]];    
    model.firstCounts[Seq[m]] =  count(Seq.begin(), Seq.end(), Seq[m]);

    }   


    //feed each word pair and the number of times it was seen    
    for (int k = 0; k <= Seq.size(); k++) {    
    ++model.pairCounts[Seq[k]];    
    model.pairCounts[Seq[k]] =  count(Seq.begin(), Seq.end(), Seq[k]);    
    }


    //feed each unique first word in a word pair and the second words in the pairs

    for (int l = 0; l <= Seq.size(); l++) {    
    istringstream iss(pairs[l]);    
     string sub;    
        iss >> sub;    
     string sub2;    
        iss >> sub2;   


        if (Seq[l] = sub) {    
            ++model.follows[sub];    
            model.follows[sub].push_back(sub2);     
        } 


    }


return model;    
}


string genNext(LanguageModel &model2, string &testWord, int Number) {    
    //use results of countSequence     
    string numbers[20]= model2.follows[testWord];    
    return numbers[Number];

}

这些是错误:

LangModel.cpp: In function ‘LanguageModel countSequence(LanguageModel&, std::vector<std::basic_string<char> >&)’:
LangModel.cpp:38:6: error: ‘class std::vector<std::basic_string<char> >’ has no member named ‘push_front’
Seq.push_front("<START>");

l.cpp:69:25: error: could not convert ‘(&(& Seq)->std::vector<_Tp, _Alloc>::operator[]<std::basic_string<char>, std::allocator<std::basic_string<char> > >(((std::vector<std::basic_string<char> >::size_type)l)))->std::basic_string<_CharT, _Traits, _Alloc>::operator=<char, std::char_traits<char>, std::allocator<char> >((*(const std::basic_string<char>*)(& sub)))’ from ‘std::basic_string<char>’ to ‘bool’
if (Seq[l] = sub) {
                                     ^
LangModel.cpp:70:10: error: no match for ‘operator++’ (operand type is ‘std::map<std::basic_string<char>, std::list<std::basic_string<char> > >::mapped_type {aka std::list<std::basic_string<char> >}’)
++model.follows[sub];
                      ^
LangModel.cpp: In function ‘std::string genNext(LanguageModel&, std::string&, int)’:
LangModel.cpp:81:45: error: conversion from ‘std::map<std::basic_string<char>, std::list<std::basic_string<char> > >::mapped_type {aka std::list<std::basic_string<char> >}’ to non-scalar type ‘std::string {aka std::basic_string<char>}’ requested
string numbers[20]= model2.follows[testWord];
                                                     ^

Answer 1

std::vector 没有 push_front 因为在 vector 的开头添加元素效率极低(这将涉及移动所有现有元素向右移动一个位置，为新元素腾出空间)。替代方法是使用 std::dequeue 而不是 std::vector。它具有与 std::vector 类似的接口(interface)，但它不保证所有元素都在连续的内存区域中，因此可以有效地实现插入。

关于你的第二个问题，我不明白你想用映射和增量运算符做什么(如果没有你在上面代码中使用的所有类的定义，很难猜到)。

++ 是一元运算符，约定是它对操作数进行增量(无论增量对特定类型意味着什么操作数)。基本上 ++object 应该和 object = object + 1 意思相同。我不确定这个操作对 std::map 有什么意义。你提到了一些关于插入 map 的东西，但是插入什么？ operator++ 是一元的，所以你不能给它一个参数来告诉它要插入什么。