Rotate a 2-D array by 90 degrees - c++

Question

How can I take the array size input from the user and pass it to the function. I tried #define inside the function, it doesn't work since the array definition needs the array bound at compile time. I tried global variable too, it says to define a integer constant which is not feasible in my case since I want to get the size from the user. How can I solve this issue?
#include <iostream>
using namespace std;
// reverse the transposed matrix as step 2
void reverseColumns(int arr[N][N])
{
for (int i = 0; i < N; i++)
{
for (int j = 0; j < N / 2; j++)
{
int temp = arr[i][j];
arr[i][j] = arr[i][N - j - 1];
arr[i][N - j - 1] = temp;
}
}
}
// take the transpose of matrix as step 1
void transposeMatrix(int arr[N][N])
{
for (int i = 0; i < N; i++)
{
for (int j = i; j < N; j++)
{
int temp = arr[i][j];
arr[i][j] = arr[j][i];
arr[j][i] = temp;
}
}
}
void rotateMatrix(int mat[N][N])
{
transposeMatrix(mat);
reverseColumns(mat);
}
// printing the final result
void displayMatrix(int mat[N][N])
{
int i, j;
for (i = 0; i < N; i++)
{
for (j = 0; j < N; j++)
cout << mat[i][j] << "\t";
cout << "\n";
}
cout << "\n";
}
int main()
{
int T, N;
cin >> T;
while (T > 0)
{
cin >> N;
int mat[N][N];
for (int i = 0; i < N; i++)
{
for (int j = 0; j < N; j++)
{
cin >> mat[i][j];
}
}
int res[N][N];
rotateMatrix(mat);
displayMatrix(mat);
}
return 0;
}

Answer 1

One way to make it work is get the input of rows and cols from user and make a one dimensional array dynamically.
for example:
Let ROWS and COLS be the values you got via cin.
Then the array can be declared as
int* arr = new int[ROWS * COLS];
Instead of writing arr[i][j] you have to write
arr[i * COLS + j]
Also you have to delete the array using
delete[] arr;

Answer 2

You are using C++ so you should take advantages of it. As kiner_shah commented the fast way to fix your code is just by use of std::vector<std::vector<int>>.
This is better solution but stil poor:
#include <iostream>
#include <vector>
using namespace std;
using Matrix = std::vector<std::vector<int>>;
Matrix makeSquereMatrix(size_t N)
{
return {N, std::vector<int>(N)};
}
// reverse the transposed matrix as step 2
void reverseColumns(Matrix& arr)
{
auto N = arr.size();
... // no changes here
}
// take the transpose of matrix as step 1
void transposeMatrix(Matrix& arr)
{
auto N = arr.size();
... // no changes here
}
void rotateMatrix(Matrix& mat)
{
transposeMatrix(mat);
reverseColumns(mat);
}
// printing the final result
void displayMatrix(const Matrix& mat)
{
for (auto& row : mat)
{
for (auto x : row)
cout << x << "\t";
cout << "\n";
}
cout << "\n";
}
void readMatrix(Matrix& m)
{
for (auto& row : m)
{
for (auto& x : row)
{
cin >> x;
}
}
}
int main()
{
int T, N;
cin >> T;
while (T > 0)
{
cin >> N;
auto mat = makeSquereMatrix(N);
readMatrix(mat);
rotateMatrix(mat);
displayMatrix(mat);
--T;
}
return 0;
}
Live demo
Better solution would be introducing a class containing std:::vector with methods performing required actions.
BTW some time ago I've made some matrix code for C. Here is live demo