向量的元素不更新

Question

//simply print each elements in a vector
template<class Iterp>
void print_iter(Iterp begin, Iterp end, std::string delim=", ", std::string ending="\n"){
    int len = (end-begin);
    for (size_t i = 0; i < len-1; i++)
    {
        std::cout << (*(begin+i)) << delim;
    }
    if (len>1){
        std::cout << (*(end-1));
    }
    std::cout << ending;
}    

template<class A, class B>
void pr(A first, A end, std::vector<std::vector<B>> &the_result){
    int len = end-first;
    for (size_t a = 0; a < len; a++)
    {
        for (size_t b = 0; b < len; b++)
        {
            for (size_t c = 0; c < len; c++)
            {
                //theres a bunch ways i tried to save these in "the_result" variable, and I ended up with this.
                std::vector<B> result(first, end);
                result.at(a) = (*(first+a));
                result.at(b) = (*(first+b));
                result.at(c) = (*(first+c));
                
                print_iter(result.begin(), result.end());
                //the problem is the "result" variable is not updating using .at() function. So the "result" variable content is still 1, 2, 3.
                

                the_result.push_back(result); //this is not the problem since the "result" variable is still 1, 2, 3

                // std::cout << (*(first+a)) << " " << (*(first+b)) << " " << (*(first+c))
                // But, this one shows it's producing the right permutations.
            }
            
        }
                
    }
    
}
    int main(){
        std::vector<int> test{1, 2, 3}; //tried to create the permutations of this
        int n = 27; //3^3 = 27

        std::vector<std::vector<int>> result(n, std::vector<int>(test.begin(), test.end()));
/*hoping to save all the permutations in "result" variable, each as a vector. ex:
    123
    113
    111
    ...

*/
        pr(test.begin(), test.end(), result);
    }

所以我尝试创建一个函数来产生重复元素的排列，并希望将每个排列保存在二维向量中。 但是，我什至无法使用 .at() 函数指针元素更新“结果”变量向量。但是当我手动打印时，它显示它是正确的答案。

我还探索了其他一些存在类似问题的 StackOverflow，但没有适合我的答案。 vector element not updating

Why is my elements in my vector of objects not updating upon calling one of the objects member function?

Dereference vector pointer to access element //尝试使用指针但还是不行

我在哪里做错了？

Answer 1

你的逻辑有缺陷：

   result.at(a) = (*(first+a));
   result.at(b) = (*(first+b));
   result.at(c) = (*(first+c));

无论a、b 和c 的值是什么，您在每次迭代中为相同的元素分配相同的数字。它们与您用于初始化 result 的元素相同。

对于3 元素，您宁愿需要以下内容：

   result[1] = (*(first+a));
   result[2] = (*(first+b));
   result[3] = (*(first+c));