Writing a Boost ublas matrix to a text file

Question

I have a Boost ublas matrix, and I want to print its contents to a text file. I have the following implementation, and it works.

#include <iostream>
using namespace std;
#include "boost\numeric\ublas\matrix.hpp"
typedef boost::numeric::ublas::matrix<float> matrix;
#include <algorithm>
#include <iterator>
#include <fstream>

int main()
{
    fill(m1.begin2(), m1.begin2() + 400 * 500, 3.3);

    ofstream dat("file.txt");
    for (auto i = 0; i < 400 ; i++) {
        for (auto j = 0; j < 500; j++) {
            dat << m1(i, j) << "\t"; // Must seperate with Tab
        }
        dat << endl; // Must write on New line
    }

I want to write this code without using the nested for loops. I tried the ostreambuf_iterator API as follows

copy(m1.begin2(), m1.begin2() + 500 * 400, ostream_iterator<float>(dat, "\n")); // Can only new line everything

However, as you can see, successive elements were written on new line, and I was not able to achieve the type of sequencing as I did with nested for loop. Is there a way to do what I did inside the nested for using an STL algorithm?

Answer 1

I like Boost Spirit Karma for these kind of small formatting/generator tasks.

Direct approach

If you don't mind trailing tabs on each line, here's

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matrix m1(4, 5);
std::fill(m1.data().begin(), m1.data().end(), 1);

using namespace boost::spirit::karma;
std::ofstream("file.txt") << format_delimited(columns(m1.size2()) [auto_], '\t', m1.data()) << "\n";

Prints

1.0 → 1.0 → 1.0 → 1.0 → 1.0 → 
1.0 → 1.0 → 1.0 → 1.0 → 1.0 → 
1.0 → 1.0 → 1.0 → 1.0 → 1.0 → 
1.0 → 1.0 → 1.0 → 1.0 → 1.0 → 

Using multi_array view

You get much more flexibility when you use the const_multi_array_ref adapter as a "view" on your raw storage:

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std::ofstream("file.txt") << format(auto_ % '\t' % eol, 
     boost::const_multi_array_ref<float, 2>(&*m1.data().begin(), boost::extents[4][5]));

This results in the same, but doesn't have the trailing tabs on each line:

1.0 → 1.0 → 1.0 → 1.0 → 1.0
1.0 → 1.0 → 1.0 → 1.0 → 1.0
1.0 → 1.0 → 1.0 → 1.0 → 1.0
1.0 → 1.0 → 1.0 → 1.0 → 1.0

Update Make it more readable and less error prone with a helper function:

template <typename T> boost::const_multi_array_ref<T, 2> make_view(boost::numeric::ublas::matrix<T> const& m) {
    return  boost::const_multi_array_ref<T,2> (
            &*m.data().begin(),
            boost::extents[m.size1()][m.size2()]
        );
}

So it becomes just

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std::cout << format(auto_ % '\t' % eol, make_view(m1)) << "\n";

Which is very elegant, in my opinion

Note Of course these assume row-major layout.