How to convert vector to array

Question

How do I convert a std::vector to a double array[]?

Answer 1

Vectors effectively are arrays under the skin. If you have a function:

void f( double a[]);

you can call it like this:

vector <double> v;
v.push_back( 1.23 )
f( &v[0] );

You should not ever need to convert a vector into an actual array instance.

Answer 2

vector<double> thevector;
//...
double *thearray = &thevector[0];

This is guaranteed to work by the standard, however there are some caveats: in particular take care to only use thearray while thevector is in scope.

Answer 3

You can do some what like this

vector <int> id;
vector <double> v;

if(id.size() > 0)
{
    for(int i = 0; i < id.size(); i++)
    {
        for(int j = 0; j < id.size(); j++)
        {
            double x = v[i][j];
            cout << x << endl;
        }
    }
}

Answer 4

There's a fairly simple trick to do so, since the spec now guarantees vectors store their elements contiguously:

std::vector<double> v;
double* a = &v[0];

Answer 5

If you have a function, then you probably need this:foo(&array[0], array.size());. If you managed to get into a situation where you need an array then you need to refactor, vectors are basically extended arrays, you should always use them.

Answer 6

For C++11, vector.data() will do the trick.