整数C ++的二维向量。 怎么了？

Question

I have trouble with two dimensional vector. Example:

vector < vector<int> > data;

int i = 0;
int int_value;
while (i < 10 )
{   
    cin >> int_value;
    data[i].push_back (int_value);
}

I want using push_back for back insert and then I want use data [i][j]. Where is the problem?

Answer 1

You need to initialize vector data before using data[i] . Otherwise, the vector is empty and accessing data[i] is out of range. Also, you need to increment i inside the while loop:

vector < vector<int> > data(10); // creates a vector of size 10,
                                 // each element being an empty vector of int's
int int_value;
for (int i=0; i < 10; i++)
{   
  cin >> int_value;
  data[i].push_back (int_value); // add int_value to the ith vector
}

After the loop, each vector contains one int value entered by the user.

Answer 2

data[i] does not exist, because the empty constructor of vector creates a vector of size 0. So when you call data[i] this will be out of bounds. Just like a one dimensional vector first allocate enough elements for data. In your case it seems you need to have data of size 10:

vector < vector<int> > data(10);

Also you never increase i in the while loop which it seems will lead to infinite loop.

Answer 3

You have an empty vector of vectors, and you are trying to access its elements ( data[i] ). Obviously, you can't do that before you add some (via push_back , resize or some other method of data itself).

Answer 4

The problem is that data[i] does not exist. First you need to push a vector<int> into the vector< vector<int> > vector, then you can add integers to it.

Answer 5

vector < vector<int> > data;

data is an empty vector which holds vector<int> .

data[i].push_back (int_value);

data is empty. So, doing data[i] leads to undefined behavior.