I searched the Internet (and stackoverflow) for opinions on whether one-dimensional arrays (or vectors) are faster than their two-dimensional copies. And the general conclusion, apparently, is that the one-dimensional ones are the fastest. However, I wrote a short test program to make sure of this, and this shows that 2-D are the best. Can someone find an error in my test or at least explain why I get this result?
I use it to store matrices and therefore I need to index 1-dimensional arrays with both a row and a column.
#include <iostream>
#include <chrono>
#include <vector>
uint64_t timestamp()
{
namespace sc = std::chrono;
static auto start = sc::high_resolution_clock::now();
return sc::duration_cast<sc::duration<uint64_t, std::micro>>(sc::high_resolution_clock::now() - start).count();
}
int main(int argc, char** argv)
{
if (argc < 3)
return 0;
size_t size = atoi(argv[1]);
size_t repeat = atoi(argv[2]);
int** d2 = (int**)malloc(size*sizeof(int*));
for (size_t i = 0; i < size; ++i)
d2[i] = (int*)malloc(size*sizeof(int));
int* d1 = (int*)malloc(size*size*sizeof(int));
std::vector<std::vector<int> > d2v(size);
for (auto& i : d2v)
i.resize(size);
std::vector<int> d1v(size*size);
uint64_t start, end;
timestamp();
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t r = 0; r < size; ++r)
{
for (size_t c = 0; c < size; ++c)
{
if (r == 0)
d2[r][c] = 0;
else
d2[r][c] = d2[r-1][c] + 1;
}
}
}
end = timestamp();
std::cout << "2D array\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t c = 0; c < size; ++c)
{
for (size_t r = 0; r < size; ++r)
{
if (r == 0)
d2[r][c] = 0;
else
d2[r][c] = d2[r-1][c] + 1;
}
}
}
end = timestamp();
std::cout << "2D array C\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t r = 0; r < size; ++r)
{
for (size_t c = 0; c < size; ++c)
{
if (r == 0)
d1[r + c*size] = 0;
else
d1[r + c*size] = d1[r-1 + c*size] + 1;
}
}
}
end = timestamp();
std::cout << "1D array\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t c = 0; c < size; ++c)
{
for (size_t r = 0; r < size; ++r)
{
if (r == 0)
d1[r + c*size] = 0;
else
d1[r + c*size] = d1[r-1 + c*size] + 1;
}
}
}
end = timestamp();
std::cout << "1D array C\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t r = 0; r < size; ++r)
{
for (size_t c = 0; c < size; ++c)
{
if (r == 0)
d2v[r][c] = 0;
else
d2v[r][c] = d2v[r-1][c] + 1;
}
}
}
end = timestamp();
std::cout << "2D vector\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t c = 0; c < size; ++c)
{
for (size_t r = 0; r < size; ++r)
{
if (r == 0)
d2v[r][c] = 0;
else
d2v[r][c] = d2v[r-1][c] + 1;
}
}
}
end = timestamp();
std::cout << "2D vector C\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t r = 0; r < size; ++r)
{
for (size_t c = 0; c < size; ++c)
{
if (r == 0)
d1v[r + c*size] = 0;
else
d1v[r + c*size] = d1v[r-1 + c*size] + 1;
}
}
}
end = timestamp();
std::cout << "1D vector\t" << size << "\t" << end - start << std::endl;
start = timestamp();
for (size_t n = 0; n < repeat; ++n)
{
for (size_t c = 0; c < size; ++c)
{
for (size_t r = 0; r < size; ++r)
{
if (r == 0)
d1v[r + c*size] = 0;
else
d1v[r + c*size] = d1v[r-1 + c*size] + 1;
}
}
}
end = timestamp();
std::cout << "1D vector C\t" << size << "\t" << end - start << std::endl;
return 0;
}
I get the following output:
user@user-debian64:~/matrix$ ./build/test/index_test 1000 100
2D array 1000 79593
2D array C 1000 326695
1D array 1000 440695
1D array C 1000 262251
2D vector 1000 73648
2D vector C 1000 418287
1D vector 1000 371433
1D vector C 1000 269355
user@user-debian64:~/matrix$ ./build/test/index_test 10000 1
2D array 10000 149748
2D array C 10000 3507346
1D array 10000 2754570
1D array C 10000 257997
2D vector 10000 92041
2D vector C 10000 3791745
1D vector 10000 3384403
1D vector C 10000 266811
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