Rotate M*N matrix 90 Degrees Clockwise ,C++ - c++

Question

I am trying to rotate a vector of Vectors of chars.
I made a 2d vector matrix setup. right now the matrix takes input from a file, I use vector.push_back(c) to add the characters to the vvc;
An example of the vvc array would be something like this
aaaaa
azzza
azaza
azzza
azaaa
azaaa
azaaa
aaaaa
I have the vvc setup, But I am trying to rotate it 90 degrees. I rotated it 90 degrees counter clockwise but I need to rotate it 90 degrees clockwise.
as of right now my code does this
90 counter clock
aaaaaaaa
azzzzzza
azazaaaa
azzzaaaa
aaaaaaaa
and it does it through this loop;
cout <<"\n90 counter clock"<<endl;
for (size_t colNo = 0; colNo < kvsize2; colNo++)
{
for (const auto &row : twovector)
{
char colVal = row.at(colNo);
cout << colVal;
}
cout << endl;
}
I am just learning about vectors, and the range for. Trying to do a decrement loop almost works, but keeps throwing me into a segfault.
"Solved"
I was using
twovector.push_back(temp);
using
twovector.insert(twovector.begin(),temp);
gives me
90 counter clock
aaaaaaaa
azzzzzza
aaaazaza
aaaazzza
aaaaaaaa

Answer 1

Tackling a specific part of the question :
If anyone has any tips or suggestions on how to rotate a M*N 2d vector array
C++ is good at segregating algorithms from data.
Kindly note that the answer is a bit lengthy and has been written with the objective of a tutorial.
Lets begin !!
We want 3 features from our rotate_2d_matrix_clockwise algorithm :
It should work with all datatypes, i.e. int, char, double or any user defined type.
It should work with different types of containers, such as std::array and std::vector
It should be chain-able, i.e. user should be able to call rotate_2d_matrix_clockwise on the result returned by rotate_2d_matrix_clockwise, to achieve 2 times rotation.
Once we are clear with our requirements, we can draft some use-cases for our algorithm.
std::vector<std::vector<char>> data = { {'a', 'b', 'c', 'd'},
{'e', 'f', 'g', 'h'},
{'i', 'j', 'k', 'l'} };
rotate_2d_matrix_clockwise(data); // rotating 2d-matrix of vector<char>
std::array<std::array<int, 4>, 3> data2 = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
// rotating 2d-matrix of array<int>, twice
rotate_2d_matrix_clockwise(rotate_2d_matrix_clockwise(data2)));
So lets use some templates to create a generic 2d-clockwise-rotate function.
Our rotate_2d_matrix_clockwise will :
take the original_matrix and return a new rotated_matrix.
automatically deduce the dimensions i.e. M x N of the container passed to it.
create the rotated_matrix and pass it to a helper function rotate_2d_matrix_clockwise_impl where the actual work would be done.
So how will the implementation of rotate_2d_matrix_clockwise for std::array look ?
template<typename T, size_t M, size_t N>
auto rotate_2d_matrix_clockwise(std::array<std::array<T, M>, N> const & original_matrix)
-> std::array<std::array<T, N>, M>
{
std::array<std::array<T, N>, M> rotated_matrix;
rotate_2d_matrix_clockwise_impl(original_matrix, rotated_matrix, M, N); // rotate
return rotated_matrix;
}
Neat and precise.
The implementation of rotate_2d_matrix_clockwise for std::vector is a bit messy, though.
template<typename Matrix2D>
auto rotate_2d_matrix_clockwise(Matrix2D const & original_matrix) -> Matrix2D
{
int const M = original_matrix[0].size(); // deduce M and N
int const N = original_matrix.size();
Matrix2D rotated_matrix; // vector has no form, hence we have to resize it for `N x M`
rotated_matrix.resize(M);
for (auto x = 0; x < M; ++x) {
rotated_matrix[x].resize(N);
}
rotate_2d_matrix_clockwise_impl(original_matrix, rotated_matrix, M, N); // rotate
return rotated_matrix;
}
Now lets look at how the actual rotation algorithm rotate_2d_matrix_clockwise_impl would look.
It should be noted, that the algorithm is independent of the container and/or the data contained. It just focuses on rotating.
template<typename OriginalMatrix2D, typename RotatedMatrix2D>
void rotate_2d_matrix_clockwise_impl(OriginalMatrix2D const & original_matrix,
RotatedMatrix2D & rotated_matrix,
int const M,
int const N)
{
for (auto x = 0; x < N; ++x) {
for (auto y = 0; y < M; ++y) {
// Source : https://stackoverflow.com/questions/4780119/2d-euclidean-vector-rotations
rotated_matrix[y][-x -1 +N] = original_matrix[x][y];
}
}
}
Here is a full working example compiled in C++11.
#include <iostream>
#include <vector>
#include <array>
template<typename Matrix2D>
void print_matrix(Matrix2D const & vec)
{
std::cout << "size of matrix is [" << vec[0].size() << " x " << vec.size() << "]\n";
for (auto const & inner_vec : vec) {
for (auto const & item : inner_vec) {
std::cout << item << ", ";
}
std::cout << std::endl;
}
}
template<typename OriginalMatrix2D, typename RotatedMatrix2D>
void rotate_2d_matrix_clockwise_impl(OriginalMatrix2D const & matrix,
RotatedMatrix2D & rotated_matrix,
int const M,
int const N)
{
for (auto x = 0; x < N; ++x) {
for (auto y = 0; y < M; ++y) {
// Source : https://stackoverflow.com/questions/4780119/2d-euclidean-vector-rotations
rotated_matrix[y][-x -1 +N] = matrix[x][y];
}
}
}
template<typename T, size_t M, size_t N>
auto rotate_2d_matrix_clockwise(std::array<std::array<T, M>, N> const & original_matrix)
-> std::array<std::array<T, N>, M>
{
std::array<std::array<T, N>, M> rotated_matrix;
rotate_2d_matrix_clockwise_impl(original_matrix, rotated_matrix, M, N);
return rotated_matrix;
}
template<typename Matrix2D>
auto rotate_2d_matrix_clockwise(Matrix2D const & original_matrix) -> Matrix2D
{
int const M = original_matrix[0].size();
int const N = original_matrix.size();
Matrix2D rotated_matrix;
rotated_matrix.resize(M);
for (auto x = 0; x < M; ++x) {
rotated_matrix[x].resize(N);
}
rotate_2d_matrix_clockwise_impl(original_matrix, rotated_matrix, M, N);
return rotated_matrix;
}
int main()
{
std::array<std::array<int, 4>, 3> data = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
std::cout << "\nBefore Rotation :\n";
print_matrix(data);
std::cout << "\nAfter 2nd Clockwise Rotation :\n";
print_matrix(rotate_2d_matrix_clockwise(rotate_2d_matrix_clockwise(data)));
std::vector<std::vector<char>> data2 = { {'a', 'b', 'c', 'd'}, {'e', 'f', 'g', 'h'}, {'i', 'j', 'k', 'l'}};
std::cout << "Before Rotation :\n";
print_matrix(data2);
std::cout << "\nAfter Clockwise Rotation :\n";
print_matrix(rotate_2d_matrix_clockwise(data2));
return 0;
}

Answer 2

If I got you right, and all you want is to print the matrix 90 degrees clockwise, try this code:
for (int colNo = 0; colNo < vec[0].size(); colNo++)
{
for (int i = vec.size() - 1; i >= 0; i--)
{
const auto& row = vec[i];
int colVal = row.at(colNo);
cout << colVal;
}
cout << endl;
}