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 o开发者_StackOverflow社区f size 4 containing vector's of size 4... like a 4x4 multidimensional array of ints?
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. :)
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 ^
Alternatively to a std::vector
you can use boost::multi_array
. From the documentation:
#include "boost/multi_array.hpp"
#include <cassert>
int
main () {
// Create a 3D array that is 3 x 4 x 2
typedef boost::multi_array<double, 3> array_type;
typedef array_type::index index;
array_type A(boost::extents[3][4][2]);
// Assign values to the elements
int values = 0;
for(index i = 0; i != 3; ++i)
for(index j = 0; j != 4; ++j)
for(index k = 0; k != 2; ++k)
A[i][j][k] = values++;
// Verify values
int verify = 0;
for(index i = 0; i != 3; ++i)
for(index j = 0; j != 4; ++j)
for(index k = 0; k != 2; ++k)
assert(A[i][j][k] == verify++);
return 0;
}
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