What is the best way to create a sparse array in C++?

#include <stdio.h>
#include <stdlib.h>
#include <map>
class triple {
public:
    int x;
    int y;
    int z;
    bool operator<(const triple &other) const {
        if (x < other.x) return true;
        if (other.x < x) return false;
        if (y < other.y) return true;
        if (other.y < y) return false;
        return z < other.z;
    }
};

int main(int, char**)
{
    std::map<triple,int> data;
    triple point;
    int i;
    for (i = 0; i < 10000000; ++i) {
        point.x = rand();
        point.y = rand();
        point.z = rand();
       printf("%d %d %d %d\n", i, point.x, point.y, point.z);
        data[point] = i;
    }
    return 0;
}