2D Array Conversion: Row-Major to Column-Major in C

To manipulate a 2D array from row-major to column-major order in C, follow these steps:

1. Declare a new 2D array with dimensions swapped (number of columns x number of rows).
2. Iterate through the original array using nested loops:
   • The outer loop runs through the rows.
   • The inner loop runs through the columns.
3. Assign the value of the original array to the corresponding position in the new array with swapped indices.
4. Access the elements of the new array in column-major order.

// Function to convert a 2D array from row-major to column-major order
void rowToColumnMajor(int** inputArray, int numRows, int numCols, int** outputArray) {
    // Loop through rows
    for(int row = 0; row < numRows; row++) {
        // Loop through columns
        for(int col = 0; col < numCols; col++) {
            // Assign the value from the original array to the corresponding position
            // in the new array with swapped indices
            outputArray[col][row] = inputArray[row][col];
        }
    }
}

Example usage:

int main() {
    // Create the original 2D array in row-major order
    int numRows = 4;
    int numCols = 3;
    int** inputArray = malloc(numRows * sizeof(int*));
    for(int i = 0; i < numRows; i++) {
        inputArray[i] = malloc(numCols * sizeof(int));
    }
    
    // Populate the original array
    // [1, 2, 3]
    // [4, 5, 6]
    // [7, 8, 9]
    // [10, 11, 12]
    int count = 1;
    for(int row = 0; row < numRows; row++) {
        for(int col = 0; col < numCols; col++) {
            inputArray[row][col] = count;
            count++;
        }
    }
    
    // Create the new array with swapped dimensions
    int** outputArray = malloc(numCols * sizeof(int*));
    for(int i = 0; i < numCols; i++) {
        outputArray[i] = malloc(numRows * sizeof(int));
    }
    
    // Convert the original array to column-major order
    rowToColumnMajor(inputArray, numRows, numCols, outputArray);
    
    // Access elements of the new array in column-major order
    // [1, 4, 7, 10, 2, 5, 8, 11, 3, 6, 9, 12]
    for(int col = 0; col < numCols; col++) {
        for(int row = 0; row < numRows; row++) {
            printf("%d ", outputArray[col][row]);
        }
    }
    
    return 0;
}