Rotate a 2-D array by 90 degrees

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;
}

You need dynamic containment, either by standard container (ex: `std::vector`) or roll your own. And, trust me on this: you don't want to roll your own. — WhozCraig, Jul 29 '22 at 05:23
If you want a 2D array that is pretty fast, I made a few example implementations and benchmarked them [here](https://stackoverflow.com/a/58122229/7582247) — Ted Lyngmo, Jul 29 '22 at 05:26
@WhozCraig ;-) correct, I didn't even scroll down enough to see that ! What I meant is that this is "C-style" programming. — kebs, Jul 29 '22 at 06:44
@user8203395 Fyi, the elephant in the room, once you fix everything else, is you never decrement `T` in `main`. This program is base-line-infinite. Change `while (T > 0)` to `while (T-- > 0)`. — WhozCraig, Jul 29 '22 at 07:47

score 1 · Accepted Answer · answered Jul 29 '22 at 05:29

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;

answered Jul 29 '22 at 05:29

mmj

139
8

please do not do that. This is job for `std::vector` so memory management headache is handled by vector. Overhead `std::vector` introduces is negligible (unless you are doing HFT). – Marek R Jul 29 '22 at 08:34

Marek R · Answer 2 · 2022-07-29T08:58:07.617

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

Rotate a 2-D array by 90 degrees

2 Answers2