 |
Department of Engineering |
 |
|
C++ vector memory and 2D vectors
Vector constructors
C++ vectors can grow dynamically, but not in all situations.
For example, if you have a vector v of size 2, then v[10]=5; won't make the vector
extend. So you still need to ensure that memory will be created for
your vectors.
Constructors will often create the memory for you. Here's an example
that uses various constructors
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// An empty vector
vector <int> v;
// A vector with 5 elements, each an integer
vector<int> v1(5);
// An array with 5 elements, each of which being an
// empty vector of integers. Don't confuse this with
// the previous line
vector<int> v1array[5];
// A vector with 5 elements each having the value 99
vector<int> v2(5, 99);
// A vector with the size and values of v2
vector<int> v3(v2);
// Another, more flexible way to create a vector
// with the size and values of v2
vector<int> v4(v2.begin(),v2.end());
}
2D vectors
You can create a 2D array by doing int a[5][7], but
vector<int> a(5,7) won't create a 2D vector - it
will create a 5-element vector full of 7s. Instead, you need to
create a vector of vectors. Here are some ways to do it.
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// An empty vector of vectors. The space
// between the 2 '>' signs is necessary
vector<vector<int> > v2d;
// If you intend creating many vectors
// of vectors, the following's tidier
typedef vector<vector<int> > IntMatrix;
IntMatrix m;
// Now we'll try to create a 3 by 5 "matrix".
// First, create a vector with 5 elements
vector<int> v2(5, 99);
// Now create a vector of 3 elements.
// Each element is a copy of v2
vector<vector<int> > v2d2(3,v2);
// Print out the elements
for(int i=0;i<v2d2.size(); i++) {
for (int j=0;j<v2d2[i].size(); j++)
cout << v2d2[i][j] << " ";
cout << endl;
}
}
Expanding vectors
The push_back member function will extend the vector. It would be
inefficient for C++ to extend the vector a byte at a time on demand,
so more space is often reserved internally for the vector than is requested.
You can find out this reserved size by using the capacity member
function. If you ask for the vector to grow beyond this size, it will, but
there are performance penalities. Try this code on your machine. It asks for
extra elements one at a time, printing the capacity out as it goes. Watch
how the capacity increases.
#include <iostream>
#include <vector>
using namespace std;
int main()
{
vector <int> v;
for (int i=0; i<50; i++) {
cout << "size=" << v.size()
<< " and capacity=" << v.capacity() << endl;
v.push_back(i);
}
}
If you want to set the capacity yourself, you can use the
reserve() member function.
