Question

我需要找到两个具有相等字段的元素。它们应在同一向量中连续。我需要使用STL方法。我尝试使用find或find_if，但无法完成。请给我任何提示吗？

我的代码（部分代码）：

class Sound {
    private:
        int notePitch, length;
    public:
        Sound(int notePitch, int length) {
            this->notePitch = notePitch;
            this->length = length;
        }
        int getPitch() {
            std::cout << "Pitch: " << this->notePitch << " length: " << this->length;
            return this->notePitch;
        }
    };

实际查找功能：

std::vector<Sound>::iterator iter = masterpiece.begin();
std::vector<Sound>::iterator iterEnd = masterpiece.end();   
std::vector<Sound>::iterator it = find_if(iter, iterEnd, [](auto prev, auto next) -> bool {                 
    return prev.getPitch()  == next.getPitch(); 
});

我得到的错误是：

c2678 binary '==' no operator found which takes a left-hand operand of type

Answer 1

使用neighbor_find代替find_if。您的代码应该可以使用。

std::vector<Sound>::iterator it = adjacent_find (iter, iterEnd, [](auto prev, auto next) -> bool {                 
    return prev.getPitch()  == next.getPitch();

Answer 2

标准算法std::find_if不接受二进制谓词。

您可以将标准算法std::adjacent_find与lambda表达式一起使用。

成员函数getPitch应该用限定符const声明。在这种情况下，您可以将函数用于常量对象（例如，当Sound类型的对象或Sound对象的向量传递给接受常量引用的函数时）

这是一个演示程序

#include <iostream>
#include <vector>
#include <iterator> 
#include <algorithm>

class Sound {
    private:
        int notePitch, length;
    public:
        Sound(int notePitch, int length) {
            this->notePitch = notePitch;
            this->length = length;
        }
        int getPitch() const {
//            std::cout << "Pitch: " << this->notePitch << " length: " << this->length;
            return this->notePitch;
        }
    };

int main( void )
{
    std::vector<Sound> masterpiece = { { 1, 2 }, { 2, 3 }, { 2, 4 }, { 3, 5 } };

    auto it = std::adjacent_find( std::begin( masterpiece ), std::end( masterpiece ),
                                  []( const  Sound &a, const Sound &b )
                                  {
                                    return a.getPitch() == b.getPitch();
                                  } );

    if ( it != std::end( masterpiece ) )
    {
        std::cout << "Two adjacent elements are found at position " 
                  << std::distance( std::begin( masterpiece ), it )
                  << '\n';
    }
    else
    {
        std::cout << "Two adjacent elements are found\n";
    }
}

程序输出为

Two adjacent elements are found at position 1

您可以使用auto

编写lambda表达式的参数

[]( const auto &a, const auto &b )
{
    return a.getPitch() == b.getPitch();
}

但是无论如何，最好将它们声明为const引用类型，因为在这种情况下，不会创建任何临时对象，并且lambda表达式保证不会更改其参数。

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2 个答案: