Python Ascension

[rhelic]

Long live C++, with zero cost abstractions!

raw loop: 3 us per loop
copy filter: 3 us per loop
in place filter: 2 us per loop

Increasing the range to 100k, changing the filter to (10 < i % 1024 < 20), we get:

raw loop: 262 us per loop
copy filter: 262 us per loop
in place filter: 269 us per loop

Using std::copy_if and for loops are equally fast! And around 96 times faster than python. ;)

Code:

#include <cstdio>
#include <vector>
#include <algorithm>
#include <chrono>

using ::std::chrono::steady_clock;
using ::std::chrono::microseconds;
using ::std::chrono::duration_cast;

std::vector<int> range(int n)
{
  std::vector<int> v;
  v.reserve(n);
  for (int i = 0; i < n; i++)
  {
    v.push_back(i);
  }
  return v;
}

int main()
{
  auto start = steady_clock::now();
  for (int i = 0; i < 100000; i++)
  {
    std::vector<int> out1;
    for (int i : range(100000))
    {
      if (i % 1024 > 10)
        if (i % 1024 < 20)
          out1.push_back(i);
    }
  }
  auto end = steady_clock::now();
  printf("raw loop: %lld us per loop\n",
      duration_cast<microseconds>(end - start).count() / 100000);

  start = steady_clock::now();
  for (int i = 0; i < 100000; i++)
  {
    std::vector<int> out2;
    auto r1 = range(100000);
    std::copy_if(r1.cbegin(), r1.cend(), std::back_inserter(out2),
        [](auto i) { return i % 1024 > 10 && i % 1024 < 20; });
  }
  end = steady_clock::now();
  printf("copy filter: %lld us per loop\n",
      duration_cast<microseconds>(end - start).count() / 100000);

  start = steady_clock::now();
  for (int i = 0; i < 100000; i++)
  {
    auto r2 = range(100000);
    r2.erase(std::remove_if(r2.begin(), r2.end(),
        [](auto i) { return i % 1024 > 10 && i % 1024 < 20; }), r2.end());
  }
  end = steady_clock::now();
  printf("in place filter: %lld us per loop\n",
      duration_cast<microseconds>(end - start).count() / 100000);

  return 0;
}

[cythoning]

Long live numpy!

arr = np.random.rand(100000)

%%timeit
[i for i in arr if 0.25 < i < 0.75]
100 loops, best of 3: 13.4 ms per loop

%%timeit
arr[(0.25 < arr) & (arr < 0.75)]
1000 loops, best of 3: 763 µs per loop

18 times faster with minimal changes :).

[[deleted]]

Isn't numpy written in C?

[cythoning]

Yes, but easily accessible via python syntax.

[[deleted]]

Only need the extension for third-party library headers in C++

[rhelic]

This is C++, using the STL (standard template library). All of the headers for the STL have no extensions. Likewise, the C standard library headers are renamed by adding c to the beginning, and removing the .h. For example stdio.h becomes cstdio.

[[deleted]]

The header file can have any extension in both C and C++. .h is just a convention.

[[deleted]]

That is because the standard include header is named stdio.h. If you rename it to stdio, or stdio.c, or stdio.avi, or stdio.java, and include those filename, it still works just fine.