Re: [eigen] Problem using Boost's zip_iterator on Eigen column vectors

On Sun, Jun 30, 2024 at 5:51 PM Antonio Sanchez <cantonios@xxxxxxxxxx> wrote:

It must be your use of the boost zip iterator in general. I've never used it before, but replacing the Eigen vector with std::vector results in the same issue.

https://godbolt.org/z/e137hEaj9

On Sat, Jun 29, 2024 at 9:30 PM Kip Warner <kip@xxxxxxxxxxxxxx> wrote:
Hello list,

I noticed today a problem in Boost's zip_iterator traversing well
beyond the end of a pair of equal dimension Eigen column vectors.

I've prepared a minimal below:

#include <iostream>
#include <Eigen/Core>
#include <boost/iterator/zip_iterator.hpp>

using namespace std;

int main()
{
using ColumnVectorType = Eigen::Matrix<float, 32, 1>;

ColumnVectorType FirstVector = ColumnVectorType::Zero();
ColumnVectorType SecondVector = ColumnVectorType::Zero();

const auto ZipperStart = boost::make_zip_iterator(
boost::make_tuple(
std::cbegin(FirstVector),
std::cbegin(SecondVector)));

// End...
const auto ZipperEnd = boost::make_zip_iterator(
boost::make_tuple(
std::cend(FirstVector),
std::cend(SecondVector)));

int index = 0;

for(auto Iterator = ZipperStart; ZipperStart != ZipperEnd; ++Iterator)
{
std::cout << "Iteration: " << index << std::endl;
++index;
}

return 0;
}

Running the above via Godbolt results in iterating the body of the loop
thousands of times when I expected only 32.

https://godbolt.org/z/fKG77s1ds

I then modified the above code to get rid of the second column vector
and the zip_iterator to iterate via the normal STL interface over the
single column vector. It executes as expected only 32 times.

This is probably just a problem with my usage, but I just wanted to let
the list know in case this is a bug.

--
Kip Warner
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https://www.thevertigo.com