在C ++中从多维向量创建和获取值时遇到问题

Question

I am having problems creating and getting values from a multidimensional array in C++. I have spent quite some time trying to figure out why this is not compiling. What's wrong here?

The code is as follows; vector<vector<string>> vec; vec[0][0] = "asd"; cout << vec[0][0] << endl;

Answer 1

You may not assign a value to an empty vector using the subscript operator.

You could use methods like emplace_back or push_back or insert

For example

std::vector<std::vector<std::string>> vec;

vec.emplace_back( 1, "asd" );
vec.push_back( { 1, "fgh" } );
vec.insert( vec.end(), { 1, "jkl" } );

std::cout << vec[0][0] << std::endl;
std::cout << vec[1][0] << std::endl;
std::cout << vec[2][0] << std::endl;

Or you could initially create the vector with the required number of elements. And in this case you may use the subscript operator.

For example

std::vector<std::vector<std::string>> vec( 1, std::vector<std::string>( 1 ) );

vec[0][0] = "asd";

std::cout << vec[0][0] << std::endl;

There are many ways to do the task. For example you could use the following approach

std::vector<std::vector<std::string>> vec;
vec.resize( 1 );
vec[0].resize( 1 );

vec[0][0] = "asd";

std::cout << vec[0][0] << std::endl;

Answer 2

You should read the documentation of the constructor. When you do this:

vector<string> vec;
vec[0] = "asd";
cout << vec[0] << endl;

You have UB twice, because you are trying to access an element in the vector, that does not exist. The first line just creates an empty vector. You either want to push new elements:

vector<string> vec;
vec.push_back("asd");
cout << vec[0] << endl;

or pass the size to the constructor:

size_t start_size=10;
vector<string> vec(start_size);
vec[0] = "asd";
cout << vec[0] << endl;

For 2D this would be:

typedef vector<vector<string>> MAT;
MAT mat;
vector<string> vec;
vec.push_back("asd");
mat.push_back(vec);
cout << mat[0][0] << endl;

or:

size_t start_size=10;
MAT mat = MAT(start_size,vector<string>(start_size));
mat[0][0] = "asd";
cout << mat[0][0] << endl;

However, using a vector<vector<T>> is in general not advisable.