使用c ++在多维向量中分配变量的最佳方法是什么？

Question

I have a matrix value like this

3   1   30
5   8   1
1   5   2
0   23  7

Currently I know only this way to assign variable to a multidimensional vector.

vector<vector <int> > vec;
vec[0][0]=3;
vec[0][1]=1;
vec[0][2]=30;
.
.
vec[3][2]=7;

For smaller matrix values this is ok but larger values this becomes harder. Is there any other way to assign variables in a multidimensional vectors.

EDIT

I am using codeblocks and I am getting this unusual error

#include<iostream>
#include<vector>


int main()
{


    std::vector<std::vector<int> > vec = {{3,1,30},{5,8,1},{1,5,2},{0,23,7}};

    return 0;
}

error

error: could not convert ‘{{3, 1, 30}, {5, 8, 1}, {1, 5, 2}, {0, 23, 7}}’ from ‘<brace-enclosed initializer list>’ to ‘std::vector<std::vector<int> >’

Answer 1

Use nested for loops: Example being:

for (int i = 0; i < MAX_VECTOR_SIZE; i++)
{
    for(int j = 0; j < MAX_SUB_VECTOR_SIZE; j++)
    {
        vec[i][j] = value;
    }
} 

Don't forget to substitute your given values where necessary.

Answer 2

std::vector<std::vector<int>> array2d{
    {3 ,  1,   30},
    {5,   8,   1},
    {1  , 5,   2},
    {0 ,  23,  7}
};

Answer 3

You may do

const std::vector<std::vector<int>> v = {
    {3,   1,   30},
    {5,   8,   1},
    {1,   5,   2},
    {0,   23,  7}
};

Answer 4

It largely depends upon what you want to do - the "best" way is dependent upon your objective. But using the example given in your question, the most convenient way is via std::initializer_list construction:

std::vector<std::vector<int>> vec = {{3,1,30},{5,8,1},{1,5,2},{0,23,7}};

Answer 5

You can assign the single dimension vector first

vector<int> vec1 {3,1,30};
vector<int> vec2 {5,8,1};
vector<int> vec3 {1,5,2};
vector<int> vec4 {0,23,7};

And then assign the 2d vector

vector<vector<int>> vector2d;
vector2d.push_back(vec1);
vector2d.push_back(vec2);
vector2d.push_back(vec3);
vector2d.push_back(vec4);

Answer 6

1. If you use manual initialization as in you original code, be careful, you probably want to use vector 's push_back instead of direct call to [] : since your vector is still empty, access via [] will cause segmentation fault.

2. If you have C++11 use vector 's constructor taking an (aggregate) initializer_list . This can be handy:

    std::vector<std::vector <int> > vec; vec = { {3, 1 , 30}, {5, 8 , 1 }, {1, 5 , 2 }, {0, 23, 7 } }; 

For more details see here . Here I assume your matrix is in Row-major order, ie each of the internal std::vector<int> represents a row of the matrix.

3. Finally, IMHO the easiest way is to use a linear algebra library like Eigen if you can. Eigen provides handy initialization methods: for example using << (see here for examples).

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EDIT

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For you compilation error : make sure that you enable at least C++11 . If you use g++ add -std=c++11 to your compiler's tag.