Two problems I can see:

• Your memory allocation is not proper. Each of your rows share a common vector variable because of the stmt:

C++ Syntax (Toggle Plain Text)

vector<int> v(n);
vector<vector<int> > data(n,v);

vector<int> v(n); vector<vector<int> > data(n,v);

Here the memory for each inner row is getting allocated only once and your two dimensional matrix gets populated with the same vector.

You need to change it to:

C++ Syntax (Toggle Plain Text)

vector<vector<int> > data(n,vector<int> (n));

vector<vector<int> > data(n,vector<int> (n));

So that the memory allocation occurs for each row rather than only once for all rows.

• Your logic is skewed. The line :

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i=0;
    j=n/2;
    for (int k=1; k<=nsqr; ++k)
    {
        data[i][j] = k;
        i--;

i=0; j=n/2; for (int k=1; k<=nsqr; ++k) { data[i][j] = k; i--;

is bound to produce an out of bounds error since you are decemeneting i which already is zero, resulting in its value becoming -1, which is not a valid index.

Better use size_t instead of int for index variables to make sure you don't invade the out of bounds exception land. (assigning -ve values to such variables leads to a warning which is fairly easy to decode).