C++中如何从现有3D vector的列构造新3D vector

需求：现有如下结构的3D vector，需要通过遍历索引，将原vector的列（j索引）重新组织成指定结构的新3D vector。

输入结构 #

std::vector<std::vector<std::vector<double>>> a = {
    { {1,1,1,1}, {2,2,2,2}, {3,3,3,3}, {4,4,4,4}, {5,5,5,5}, {6,6,6,6} },
    { {10,10,10,10}, {20,20,20,20}, {30,30,30,30}, {40,40,40,40}, {50,50,50,50}, {60,60,60,60} },
    { {100,100,100,100}, {200,200,200,200}, {300,300,300,300}, {400,400,400,400}, {500,500,500,500}, {600,600,600,600} },
};

目标输出结构 #

std::vector<std::vector<std::vector<double>>> b = {
    {{ 1,1,1,1}, {10,10,10,10}, {100,100,100,100}},
    {{ 2,2,2,2}, {20,20,20,20}, {200,200,200,200}},
    {{ 3,3,3,3}, {30,30,30,30}, {300,300,300,300}},
    {{ 4,4,4,4}, {40,40,40,40}, {400,400,400,400}},
    {{ 5,5,5,5}, {50,50,50,50}, {500,500,500,500}},
    {{ 6,6,6,6}, {60,60,60,60}, {600,600,600,600}},
};

现有代码框架 #

#include <iostream>
#include <vector>

using namespace std;

void show3D_vector(std::vector<std::vector<std::vector<double>>>& a);
void show2D_vector(std::vector<std::vector<double>>& a);
template<typename T> std::vector<std::vector<T>> SplitVector(const std::vector<T>& vec, size_t n);


int main()
{
    std::vector<std::vector<std::vector<double>>> a = {
        { {1,1,1,1}, {2,2,2,2}, {3,3,3,3}, {4,4,4,4}, {5,5,5,5}, {6,6,6,6} },
        { {10,10,10,10}, {20,20,20,20}, {30,30,30,30}, {40,40,40,40}, {50,50,50,50}, {60,60,60,60} },
        { {100,100,100,100}, {200,200,200,200}, {300,300,300,300}, {400,400,400,400}, {500,500,500,500}, {600,600,600,600} },
    }; 
}

void show3D_vector(std::vector<std::vector<std::vector<double>>>& a)
{
    for (double i = 0; i < a.size(); ++i)
    {
        for (double j = 0; j < a[i].size(); ++j)
        {
            for (double k = 0; k < a[i][j].size(); ++k)
                std::cout << a[i][j][k] << "  ";
            std::cout << endl;
        }
        std::cout << endl;
    }
}

void show2D_vector(std::vector<std::vector<double>>& a)
{
    for (int i = 0; i < a.size(); i++) {
        for (auto it = a[i].begin(); it != a[i].end(); it++)
        {
            std::cout << *it << " ";
        }

        std::cout << endl << endl;
    }
}

template<typename T>
std::vector<std::vector<T>> SplitVector(const std::vector<T>& vec, size_t n)
{
    std::vector<std::vector<T>> outVec;

    size_t length = vec.size() / n;
    size_t remain = vec.size() % n;

    size_t begin = 0;
    size_t end = 0;

    for (size_t i = 0; i < std::min(n, vec.size()); ++i)
    {
        end += (remain > 0)? (length + !!(remain--)) : length;
        outVec.push_back(std::vector<T>(vec.begin() + begin, vec.begin() + end));
        begin = end;
    }

    return outVec;
}

解决方案 #

核心思路 #

输入的3D vector维度为 [3][6][4]，目标输出维度为 [6][3][4]，本质是将原数组的第一维和第二维进行转置：原数组的 a[i][j] 对应目标数组的 b[j][i]。

修改后的完整代码 #

#include <iostream>
#include <vector>

using namespace std;

void show3D_vector(std::vector<std::vector<std::vector<double>>>& a);
void show2D_vector(std::vector<std::vector<double>>& a);
template<typename T> std::vector<std::vector<T>> SplitVector(const std::vector<T>& vec, size_t n);


int main()
{
    std::vector<std::vector<std::vector<double>>> a = {
        { {1,1,1,1}, {2,2,2,2}, {3,3,3,3}, {4,4,4,4}, {5,5,5,5}, {6,6,6,6} },
        { {10,10,10,10}, {20,20,20,20}, {30,30,30,30}, {40,40,40,40}, {50,50,50,50}, {60,60,60,60} },
        { {100,100,100,100}, {200,200,200,200}, {300,300,300,300}, {400,400,400,400}, {500,500,500,500}, {600,600,600,600} },
    }; 

    // 构造目标3D vector
    std::vector<std::vector<std::vector<double>>> b;
    // 原数组第二维的大小就是目标数组第一维的大小
    size_t target_first_dim = a[0].size();
    // 原数组第一维的大小就是目标数组第二维的大小
    size_t target_second_dim = a.size();

    // 初始化b的结构
    b.resize(target_first_dim);
    for (size_t j = 0; j < target_first_dim; ++j) {
        b[j].resize(target_second_dim);
    }

    // 遍历索引赋值
    for (size_t i = 0; i < target_second_dim; ++i) {
        for (size_t j = 0; j < target_first_dim; ++j) {
            b[j][i] = a[i][j];
        }
    }

    // 打印原数组和目标数组验证
    cout << "原数组：" << endl;
    show3D_vector(a);
    cout << "目标数组：" << endl;
    show3D_vector(b);
}

void show3D_vector(std::vector<std::vector<std::vector<double>>>& a)
{
    // 修正循环变量类型：用size_t而非double，避免精度问题
    for (size_t i = 0; i < a.size(); ++i)
    {
        for (size_t j = 0; j < a[i].size(); ++j)
        {
            for (size_t k = 0; k < a[i][j].size(); ++k)
                std::cout << a[i][j][k] << "  ";
            std::cout << endl;
        }
        std::cout << endl;
    }
}

void show2D_vector(std::vector<std::vector<double>>& a)
{
    for (int i = 0; i < a.size(); i++) {
        for (auto it = a[i].begin(); it != a[i].end(); it++)
        {
            std::cout << *it << " ";
        }

        std::cout << endl << endl;
    }
}

template<typename T>
std::vector<std::vector<T>> SplitVector(const std::vector<T>& vec, size_t n)
{
    std::vector<std::vector<T>> outVec;

    size_t length = vec.size() / n;
    size_t remain = vec.size() % n;

    size_t begin = 0;
    size_t end = 0;

    for (size_t i = 0; i < std::min(n, vec.size()); ++i)
    {
        end += (remain > 0)? (length + !!(remain--)) : length;
        outVec.push_back(std::vector<T>(vec.begin() + begin, vec.begin() + end));
        begin = end;
    }

    return outVec;
}

关键说明 #

1. 维度转换：通过获取原数组的第一维和第二维大小，初始化目标数组的结构，保证维度匹配。
2. 索引遍历：嵌套循环遍历原数组的i（第一维）和j（第二维），将a[i][j]直接赋值给b[j][i]，完成列到行的转换。
3. 修正打印函数：原show3D_vector中使用double作为循环索引存在精度风险，改为size_t更安全（符合容器大小的类型）。

内容的提问来源于stack exchange，提问作者Prostoy_paren