Supposing I have an array A[n][m], and I want to read or write to a row/column of A.
If I wanted to access the third row of the matrix, how can I reference it like
A[3][:]
Is this possible in C++ without using a loop?
No, C++ has no operator similar to [:] in Fortran. Depending on whether you are storing things in row major or column major order, this can be done in similar ways however.
Firstly, arrays aren't really first class citizens in C++. It's much easier to work with either std::array (for small M, N as these are stack allocated), or std::vector.
Let's assume we're using a std::vector:
template <typename T>
using matrix<T> = std::vector<std::vector<T>>;
matrix<int> m = {{1,2,3}, {4,5,6}, {7,8,9}};
To get a single row out of this, we can simply use operator[]:
auto& row = m[0]; // Returns a std::vector<int>& containing {1, 2, 3}
Getting columns when it is in row major order is more difficult. If you require more sophisticated sort of operations, using a matrix library (like Eigen) might be a better way to go.
Edit: If you wanted to fill an entire row with zeros, this can be done easily with std::fill on the result:
//m defined as before
std::fill(std::begin(m[0]), std::end(m[0]), 0);
Note that this is still (obviously) linear in the size of the row. This could also easily be wrapped in a function:
template <typename T>
void clear_row(matrix<T>& m, std::size_t row, const T& new_value)
{
std::fill(std::begin(m[row]), std::end(m[row]), new_value);
}
If you wanted to replace all the values in a row with a set of different values, you'd use iterators:
template <typename T, typename Iterator>
void modify_row(matrix<T>& m, std::size_t row, Iterator new_start)
{
std::copy(std::begin(m[row]), std::end(m[row]), new_start);
}