Question

I know vectors can be constructed to a predefined size

vector<int> foo(4);

But is there a way to specify the dimensions of nested vectors?

vector< vector<int> > bar(4);

Lets say I wanted a vector of size 4 containing vector’s of size 4… like a 4×4 multidimensional array of ints?

Answer 1

The second argument to that constructor is the value to initialize with. Right now you’re getting 4 default-constructed vectors. To clarify with a simpler 1D example:

// 4 ints initialized to 0
vector<int> v1(4);

// *exactly* the same as above, this is what the compiler ends up generating
vector<int> v2(4, 0); 

// 4 ints initialized to 10
vector<int> v3(4, 10); 

So you want:

vector< vector<int> > bar(4, vector<int>(4));
//              this many ^   of these ^

This creates a vector of vectors of ints, initialized to contain 4 vectors that are initialized to contain 4 ints, initialized to 0. (You could specify a default value for the int to, if desired.)

A mouth-full, but not too hard. 🙂

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For a pair:

typedef std::pair<int, int> pair_type; // be liberal in your use of typedef
typedef std::vector<pair_type> inner_vec;
typedef std::vector<inner_vec> outer_vec;

outer_vec v(5, inner_vec(5, pair_type(1, 1)); // 5x5 of pairs equal to (1, 1)
//             this many ^ of these ^
//this many ^      of these ^