C ++ Armadillo：使用迭代器的double for循环

Question

What is the best way to use iterators in double for-loops. For a single loop, the obvious way seems to be:

arma::vec test = arma::ones(10);
for(arma::vec::iterator i = test.begin(); i != test.end(); ++i){
   int one = *i;
}

So I want to transform the following:

arma::mat test = arma::ones(10,10);
for (int i = 0; i < test.n_rows; i++){
 for (int j = 0; j < test.n_cols; j++){
  int one = test(i,j);
 }
}

to use iterators instead of integer indices. Thanks for any advice.

Answer 1

It's recommended to still use a single loop when accessing elements in a matrix. Armadillo stores data in column-major format (for compatibility with LAPACK), so the iterator will travel along each column in the matrix.

arma::mat A(4, 5, arma::fill::randu);
A.print("A:");

// C++98 

arma::mat::iterator it_end = A.end();

for(arma::mat::iterator it = A.begin(); it != it_end; ++it)
   {
   std::cout << (*it) << std::endl;
   }

// C++11

for(const auto& val : A)
   {
   std::cout << val << std::endl;
   }

If you really want to use a double loop, use .begin_col() and .end_col() :

// C++11

for(arma::uword c=0; c < A.n_cols; ++c)
  {
  auto it_end = A.end_col(c);

  for(auto it = A.begin_col(c); it != it_end; ++it)
    {
    std::cout << (*it) << std::endl;
    }
  }

Lastly, the .for_each() function is an alternative to using iterators:

 // C++11

 A.for_each( [](arma::mat::elem_type& val) { std::cout << val << std::endl; } );

Answer 2

Three things come to mind, and not just to double iterations, but to any kind of iteration (just apply everything below twice...)

1) If the container's contents do not change during the loop, call end() once and save it, and compare the iterator to the saved value on each iteration of the loop.

Not so good:

for(arma::vec::iterator i = test.begin(); i != test.end(); ++i)

As written, end() is being technically called at each loop's iteration, and its return value is being compared to the current value of the loop iterator.

2)

Better:

const arma::vec::iterator e=test.end();

for(arma::vec::iterator i = test.begin(); i != e; ++i)

This has a far better chance of getting optimized, especially with a const qualifier (although the compiler will likely be able to figure it out on its own, if e never gets modified, anyway).

3)

Best:

for(const auto &value:test)

The C++11 iteration syntax tells the compiler exactly what's going on, giving the compiler the best opportunity to employ all container iteration tricks it's been programmed to know about.