What are, and how to use, iterator1 and iterator2 of Boost matrix?

Question

I am looking for a way to iterate over elements of a Boost matrix using iterators. Documentation reports Matrix methods returning an iterator1 and iterator2:

iterator1 begin1 () Returns a iterator1 pointing to the beginning of the matrix.
iterator1 end1 ()   Returns a iterator1 pointing to the end of the matrix.
iterator2 begin2 () Returns a iterator2 pointing to the beginning of the matrix.
iterator2 end2 ()   Returns a iterator2 pointing to the end of the matrix.

I tried iterating over them, and iterator1 goes over the first column of the matrix (only), and iterator2 over the first row (only).

How are these iterator1 and iterator2 meant to be used?

Answer 1

It turns out the two iterators allow iterating over a matrix by row or by column, as you need. There is an answer at the link reported by CuriouslyRecurringThoughts , but I also give an example of usage with my code here, to anyone interested. The program below fills this matrix of integers

1, 0, 2, 4, 3
4, 6, 5, 2, 1
4, 4, 5, 2, 1
5, 6, 8, 5, 3

then prints it, first by rows and then by columns.

#include <vector>
#include <iostream>

#include "boost/numeric/ublas/matrix.hpp"
#include "boost/numeric/ublas/io.hpp"

using std::vector;
using boost::numeric::ublas::matrix;
using std::cout, std::endl;


matrix<int> make_matrix(vector<vector<int>> values) {
    auto the_matrix = matrix<int>(values.size(), values[0].size());

    // Copy 'values' into 'the_matrix', one row at a time
    auto iter1 = the_matrix.begin1();
    for (auto values_iter = values.begin(); values_iter != values.end(); ++values_iter, ++iter1)
        std::copy(values_iter->begin(), values_iter->end(), iter1.begin());

    return the_matrix;
}

matrix<int> make_matrix(int size1, int size2, int value) {
    auto the_matrix = matrix<int>(size1, size2);

    for (auto iter1 = the_matrix.begin1(); iter1 != the_matrix.end1(); ++iter1)
        for (auto iter2 = iter1.begin(); iter2 != iter1.end(); ++iter2)
            *iter2 = value;

    return the_matrix;
}

int main() {
    matrix<int> the_matrix = make_matrix({{1, 0, 2, 4, 3},
                                          {4, 6, 5, 2, 1},
                                          {4, 4, 5, 2, 1},
                                          {5, 6, 8, 5, 3}});

    cout << "Print the matrix by rows:" << endl;
    for (auto iter1 = the_matrix.begin1(); iter1 != the_matrix.end1(); ++iter1)
        for (auto iter2 = iter1.begin(); iter2 != iter1.end(); ++iter2)
            cout << *iter2 << " ";

    cout << endl << endl << "Print the matrix by columns:" << endl;
    for (auto iter2 = the_matrix.begin2(); iter2 != the_matrix.end2(); ++iter2)
        for (auto iter1 = iter2.begin(); iter1 != iter2.end(); ++iter1)
            cout << *iter1 << " ";

    cout << endl;
}

This is the output:

Print the matrix by rows:
1 0 2 4 3 4 6 5 2 1 4 4 5 2 1 5 6 8 5 3 

Print the matrix by columns:
1 4 4 5 0 6 4 6 2 5 5 8 4 2 2 5 3 1 1 3