如何比较两个向量的相等性? [英] How to compare two vectors for equality?

问题描述

我有以下程序:

std::vector<int> nums = {1, 2, 3, 4, 5};
std::vector<int> nums2 = {5, 4, 3, 2, 1};

bool equal = std::equal(nums.begin(), nums.end(), nums2.begin());
if (equal)
{
    cout << "Both vectors are equal" << endl;
}

有两个向量具有相等的元素.std :: equal函数在这里不起作用，因为它按顺序运行并比较相应的元素.有没有一种方法可以检查这两个向量是否相等，并且在我的情况下无需排序就可以成立?在实际示例中，我没有整数，但是有自定义对象，它们的比较与指针的相等性相同.

There are two vectors that have equal elements. std::equal function does not work here because it goes sequentially and compares corresponding elements. Is there a way to check that both these vectors are equal and get true in my case without sorting? In real example I do not have integers but custom objects which comparing as equality of pointers.

推荐答案

您可以从每个向量构造一个 std :: unordered_set ，然后进行比较，如下面的代码片段所示:

You can construct a std::unordered_set from each vector, then compare those, as shown in the code snippet below:

#include <iostream>  
#include <vector>  
#include <unordered_set> 

using namespace std;

int main()
{
    std::vector<int> nums = { 1, 2, 3, 4, 5 };
    std::vector<int> nums2 = { 5, 4, 3, 2, 1 };
    std::vector<int> nums3 = { 5, 4, 9, 2, 1 };

    std::unordered_set<int> s1(nums.begin(), nums.end());
    std::unordered_set<int> s2(nums2.begin(), nums2.end());
    std::unordered_set<int> s3(nums3.begin(), nums3.end());

    if (s1 == s2) {
        std::cout << "1 and 2 are equal";
    }
    else {
        std::cout << "1 and 2 are different";
    }
    std::cout << std::endl;

    if (s1 == s3) {
        std::cout << "1 and 3 are equal";
    }
    else {
        std::cout << "1 and 3 are different";
    }
    std::cout << std::endl;

    return 0;
}

但是，有几点需要牢记:

However, there are some points to bear in mind:

1. 对于自定义类型对象的向量，您需要为该类型提供一个 operator == (但是无论如何都必须这样做，或者如何确定两个向量具有相同的内容).
2. 包含重复项的向量将创建删除了重复项的集合:因此， {1、2、2、3} 将显示等于 {1、2、3} .
3. 您还需要为您的自定义类型提供 std:hash .对于一个简单的类， bob (只包装一个整数)，该散列和所需的 operator == 可以定义如下；您可以在上面的示例中用< bob> 替换上例中的< int> 专业化，它将起作用.(这篇 cppreference文章详细介绍了有关哈希的信息.)

1. For vectors of custom type objects, you would need to provide an operator== for that type (but that would have to be done anyway, or how can you say if the two vectors have the same contents).
2. Vectors that containing duplicate entries will create sets that have those duplicates removed: Thus, {1, 2, 2, 3} will show equal to {1, 2, 3}.
3. You will also need to provide a std:hash for your custom type. For a trivial class, bob, which just wraps an integer, that hash, and the required operator==, could be defined as shown below; you can then replace the <int> specializations in the above example with <bob> and it will work. (This cppreference article explains more about the hash.)

class bob {
public:
    int data;
    bob(int arg) : data{ arg } { }
};
bool operator==(const bob& lhs, const bob& rhs)
{
    return lhs.data == rhs.data;
}
template<> struct std::hash<bob> {
    std::size_t operator()(bob const& b) const noexcept {
        return static_cast<size_t>(b.data);
    }
};

这篇关于如何比较两个向量的相等性?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！