How do I best handle dynamic multi-dimensional arrays in C/C++?

Question

What is the accepted/most commonly used way to manipulate dynamic (with all dimensions not known until runtime) multi-dimensional arrays in C and/or C++.

I'm trying to find the cleanest way to accomplish what this Java code does:

public static void main(String[] args){
 Scanner sc=new Scanner(System.in);
 int rows=sc.nextInt();
 int cols=sc.nextInt();
 int[][] data=new int[rows][cols];
 manipulate(data);
}

public static void manipulate(int[][] data){
   for(int i=0;i<data.length;i++)
   for(int j=0;j<data[0].length.j++){
         System.out.print(data[i][j]);       
   }    
}

(reads from std_in just to clarify that dimensions aren't known until runtime).

Answer 1

There is no way to determine the length of a given array in C++. The best way would probably be to pass in the length of each dimension of the array, and use that instead of the .length property of the array itself.

Answer 2

You can alloc rows*cols*sizeof(int) and access it by table[row*cols+col].

Answer 3

Use boost::multi_array.

As in your example, the only thing you need to know at compile time is the number of dimensions. Here is the first example in the documentation :

#include "boost/multi_array.hpp"
#include <cassert>

int 
main () {
  // Create a 3D array that is 3 x 4 x 2
  typedef boost::multi_array<double, 3> array_type;
  typedef array_type::index index;
  array_type A(boost::extents[3][4][2]);

  // Assign values to the elements
  int values = 0;
  for(index i = 0; i != 3; ++i) 
    for(index j = 0; j != 4; ++j)
      for(index k = 0; k != 2; ++k)
        A[i][j][k] = values++;

  // Verify values
  int verify = 0;
  for(index i = 0; i != 3; ++i) 
    for(index j = 0; j != 4; ++j)
      for(index k = 0; k != 2; ++k)
        assert(A[i][j][k] == verify++);

  return 0;
}

Edit: As suggested in the comments, here is a "simple" example application that let you define the multi-dimensional array size at runtime, asking from the console input. Here is an example output of this example application (compiled with the constant saying it's 3 dimensions) :

Multi-Array test!
Please enter the size of the dimension 0 : 4

Please enter the size of the dimension 1 : 6

Please enter the size of the dimension 2 : 2

Text matrix with 3 dimensions of size (4,6,2) have been created.

Ready!
Type 'help' for the command list.

>read 0.0.0
Text at (0,0,0) :
  ""

>write 0.0.0 "This is a nice test!"
Text "This is a nice test!" written at position (0,0,0)

>read 0.0.0
Text at (0,0,0) :
  "This is a nice test!"

>write 0,0,1 "What a nice day!"
Text "What a nice day!" written at position (0,0,1)

>read 0.0.0
Text at (0,0,0) :
  "This is a nice test!"

>read 0.0.1
Text at (0,0,1) :
  "What a nice day!"

>write 3,5,1 "This is the last text!"
Text "This is the last text!" written at position (3,5,1)

>read 3,5,1
Text at (3,5,1) :
  "This is the last text!"

>exit

The important parts in the code are the main function where we get the dimensions from the user and create the array with :

const unsigned int DIMENSION_COUNT = 3; // dimension count for this test application, change it at will :)

// here is the type of the multi-dimensional (DIMENSION_COUNT dimensions here) array we want to use
// for this example, it own texts
typedef boost::multi_array< std::string , DIMENSION_COUNT > TextMatrix;

// this provide size/index based position for a TextMatrix entry.
typedef std::tr1::array<TextMatrix::index, DIMENSION_COUNT> Position; // note that it can be a boost::array or a simple array

/*  This function will allow the user to manipulate the created array
    by managing it's commands.
    Returns true if the exit command have been called.
*/
bool process_command( const std::string& entry, TextMatrix& text_matrix );

/* Print the position values in the standard output. */
void display_position( const Position& position );

int main()
{
    std::cout << "Multi-Array test!" << std::endl;

    // get the dimension informations from the user
    Position dimensions; // this array will hold the size of each dimension 

    for( int dimension_idx = 0; dimension_idx < DIMENSION_COUNT; ++dimension_idx )
    {
     std::cout << "Please enter the size of the dimension "<< dimension_idx <<" : ";
     // note that here we should check the type of the entry, but it's a simple example so lets assume we take good numbers
     std::cin >> dimensions[dimension_idx]; 
     std::cout << std::endl;

    }

    // now create the multi-dimensional array with the previously collected informations
    TextMatrix text_matrix( dimensions );

    std::cout << "Text matrix with " << DIMENSION_COUNT << " dimensions of size ";
    display_position( dimensions );
    std::cout << " have been created."<< std::endl;
    std::cout << std::endl;
    std::cout << "Ready!" << std::endl;
    std::cout << "Type 'help' for the command list." << std::endl;
    std::cin.sync();


    // we can now play with it as long as we want
    bool wants_to_exit = false;
    while( !wants_to_exit )
    {
     std::cout << std::endl << ">" ;
     std::tr1::array< char, 256 > entry_buffer; 
     std::cin.getline(entry_buffer.data(), entry_buffer.size());

     const std::string entry( entry_buffer.data() );
     wants_to_exit = process_command( entry, text_matrix );
    }

    return 0;
}

And you can see that to accede an element in the array, it's really easy : you just use the operator() as in the following functions :

void write_in_text_matrix( TextMatrix& text_matrix, const Position& position, const std::string& text )
{
    text_matrix( position ) = text;
    std::cout << "Text \"" << text << "\" written at position ";
    display_position( position );
    std::cout << std::endl;
}

void read_from_text_matrix( const TextMatrix& text_matrix, const Position& position )
{
    const std::string& text = text_matrix( position );
    std::cout << "Text at ";
    display_position(position);
    std::cout << " : "<< std::endl;
    std::cout << "  \"" << text << "\"" << std::endl;
}

Note : I compiled this application in VC9 + SP1 - got just some forgettable warnings.

Answer 4

If you're using C instead of C++ you might want to look at the Array_T abstraction in Dave Hanson's library C Interfaces and Implementations. It's exceptionally clean and well designed. I have my students do a two-dimensional version as an exercise. You could do that or simply write an additional function that does an index mapping, e.g.,

void *Array_get_2d(Array_T a, int width, int height, int i, int j) {
    return Array_get(a, j * width, i, j);
}

It is a bit cleaner to have a separate structure where you store the width, the height, and a pointer to the elements.

Answer 5

The standard way without using boost is to use std::vector :

std::vector< std::vector<int> > v;
v.resize(rows, std::vector<int>(cols, 42)); // init value is 42
v[row][col] = ...;

That will take care of new / delete the memory you need automatically. But it's rather slow, since std::vector is not primarily designed for using it like that (nesting std::vector into each other). For example, all the memory is not allocated in one block, but separate for each column. Also the rows don't have to be all of the same width. Faster is using a normal vector, and then doing index calculation like col_count * row + col to get at a certain row and col:

std::vector<int> v(col_count * row_count, 42);
v[col_count * row + col) = ...;

But this will loose the capability to index the vector using [x][y]. You also have to store the amount of rows and cols somewhere, while using the nested solution you can get the amount of rows using v.size() and the amount of cols using v[0].size().

Using boost, you can use boost::multi_array, which does exactly what you want (see the other answer).

---

There is also the raw way using native C++ arrays. This envolves quite some work and is in no way better than the nested vector solution:

int ** rows = new int*[row_count];
for(std::size_t i = 0; i < row_count; i++) {
    rows[i] = new int[cols_count];
    std::fill(rows[i], rows[i] + cols_count, 42);
}

// use it... rows[row][col] then free it...

for(std::size_t i = 0; i < row_count; i++) {
    delete[] rows[i];
}

delete[] rows;

You have to store the amount of columns and rows you created somewhere since you can't receive them from the pointer.

Answer 6

.

You really should NOT do this! (This is take you out behind the woodshed bad code!)

---

2D-Arrays in C/C++ are a pointer to a block of memory of size rows*columns*sizeof(datatype) bytes. (Possibly with the addition of a little alignment padding.)

The actual [row][column] dimensions exist only statically at compile time. There's nothing there dynamically at runtime!

So, as others have mentioned, you can in fact treat:

  int array [ rows ] [ columns ];

As:

 int  array [ rows * columns ]

Or (well almost, if we exclude alignment padding):

  void * array = malloc ( rows * columns * sizeof(int) );

---

Next: Declaring a variably sized array. Some compilers permit alloca() (cough, GNU). Some don't. Upshot, under g++/Linux, this is permissible:

int main( int argc, char ** argv )
{
  UASSERT( argc, >, 2 ); // My own macro...

  int rows    = atoi( argv[1] );
  int columns = atoi( argv[2] );

  int data [ rows ] [ columns ];  // Yes, legal!

  memset( data, 0, sizeof(data) );

  print( data, rows, columns );
  manipulate( data, rows, columns );
  print( data, rows, columns );
}

---

Passing the variably-sized data array around is more problematic. You either need to receive it as a void*, or cast away the array'ness ((int*) data) when invoking manipulate()/print().

Generally speaking, you cannot cast to an array type. But you can circumvent that restriction if you play dirty. (Didn't I just say this was take you out behind the woodshed bad code!)

void manipulate( void * theRealData, int theRows, int theColumns )
{
  int (* p) [ theRows ] [ theColumns ];
  *(void **)&p = theRealData; //Need *&.  Issue with casting lvalues.

  for (   int r = 0; r < theRows;    r ++ )
    for ( int c = 0; c < theColumns; c ++  )
      (*p) [r] [c] = r*10 + c;
}

void print( void *theRealData, int theRows, int theColumns )
{
  int (* p) [ theRows ] [ theColumns ];
  *(void **)&p = theRealData; //Need *&.  Issue with casting lvalues.

  for (   int r = 0; r < theRows;    r ++ )
  {
    for ( int c = 0; c < theColumns; c ++  )
    {
      cout << (*p) [r] [c] << "  ";
    }
    cout << endl;
  }
  cout << endl;
}

---

All that said, I'd use Guge's solution: int data [ theRows * theColumns ] or int * data = new int [ theRows * theColumns ] and a GET macro/function/method. (data[r*theColumns + c])

Ideally all wrapped up in a nice class/struct, to simplify passing it around.

---

Intelligence is knowing how to make it work.
Wisdom is knowing enough not to do it!

Answer 7

There are two ways to represent a 2-dimension array in C++. One being more flexible than the other.

Array of arrays

First make an array of pointers, then initialize each pointer with another array.

// First dimension
int** array = new int*[3];
for( int i = 0; i < 3; ++i )
{
    // Second dimension
    array[i] = new int[4];
}

// You can then access your array data with
for( int i = 0; i < 3; ++i )
{
    for( int j = 0; j < 4; ++j )
    {
        std::cout << array[i][j];
    }
}

THe problem with this method is that your second dimension is allocated as many arrays, so does not ease the work of the memory allocator. Your memory is likely to be fragmented resulting in poorer performance. It provides more flexibility though since each array in the second dimension could have a different size.

Big array to hold all values

The trick here is to create a massive array to hold every data you need. The hard part is that you still need the first array of pointers if you want to be able to access the data using the array[i][j] syntax.

int* buffer = new int[3*4];   
int** array = new int*[3];

for( int i = 0; i < 3; ++i )
{
    array[i] = array + i * 4;
}

The int* array is not mandatory as you could access your data directly in buffer by computing the index in the buffer from the 2-dimension coordinates of the value.

// You can then access your array data with
for( int i = 0; i < 3; ++i )
{
    for( int j = 0; j < 4; ++j )
    {
        const int index = i * 4 + j;
        std::cout << buffer[index];
    }
}

The RULE to keep in mind

Computer memory is linear and will still be for a long time. Keep in mind that 2-dimension arrays are not natively supported on a computer so the only way is to "linearize" the array into a 1-dimension array.

Answer 8

You could use malloc to accomplish this and still have it accessible through normal array[][] mean, verses the array[rows * cols + cols] method.

main()
{
   int i;
   int rows;
   int cols;
   int **array = NULL;

   array = malloc(sizeof(int*) * rows);
   if (array == NULL)
       return 0;  // check for malloc fail

   for (i = 0; i < rows; i++)
   {
       array[i] = malloc(sizeof(int) * cols)
       if (array[i] == NULL)
           return 0;  // check for malloc fail
   }

   // and now you have a dynamically sized array
}