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 {        return (x < other.x && y < other.y && 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 %dn", i, point.x, point.y, point.z);        data[point] = i;    }    return 0;}

std::map data<string,int> data;char ix[100];sprintf(ix, "%d,%d", x, y); // 2 varsdata[ix] = i;sprintf(ix, "%d,%d,%d", x, y, z); // 3 varsdata[ix] = i;

typedef vector<size_t> index_t;

struct index_cmp_t : binary_function<index_t, index_t, bool> {
    bool operator ()(index_t const& a, index_t const& b) const {
        for (index_t::size_type i = 0; i < a.size(); ++i)
            if (a[i] != b[i])
                return a[i] < b[i];
        return false;
    }
};

map<index_t, int, index_cmp_t> data;
index_t i(dims);
i[0] = 1;
i[1] = 2;
// … etc.
data[i] = 42;

std::map data<string,int> data;char ix[100];sprintf(ix, "%d,%d", x, y); // 2 varsdata[ix] = i;sprintf(ix, "%d,%d,%d", x, y, z); // 3 varsdata[ix] = i;