Re: Another question about multidimentional arrays

"Ron Francis" <>
Wed, 4 Jun 2008 01:20:24 +0930
"Giovanni Dicanio" <> wrote in message

"Ron Francis" <> ha scritto nel messaggio

I have wondered what the benefit is to using multidimensional arrays.
int arr[10][10];
basically the same as
int arr[100];

Is the only benefit in how you visual the array in your mind in columns
and rows?
Is there any speed gain in using a single dimensional array?

Hi Ron,

I think that there is no important speed gain (however, you may check the
code generated by the compiler...).

I read also other posts of yours about this topic, and IMHO it is better
to define a robust C++ class to manage multi-dimensional arrays than using
the "raw" C-like version.

For example, if you design and implement a C++ class to store
multi-dimensional arrays, you can store data members with array row and
column count.

You can also implement bounds checking for array index (indexes that go
out of bounds are a major cause of security problems via buffer

You can store your 2D array using a 1D array inside your class (e.g. using
STL classes like std::vector or std::valarray), storing the original array
row-wise or column-wise.
Then you can implement class methods to access array elements, converting
a 2D index (row, column) into a 1D index in the private data member 1D

You may consider the following simple C++ code to be useful:


#include <atlbase.h> // for ATLASSERT
#include <iostream> // C++ cout
#include <vector> // STL vector container

// A 2D array
template <typename T>
class Matrix

   // Create matrix with given dimensions
   explicit Matrix( int rowCount, int columnCount )
       : data( rowCount * columnCount ),
         rows( rowCount ),
         columns( columnCount )
       // ... may check input parameters
       ATLASSERT( rowCount > 0 );
       ATLASSERT( columnCount > 0 );

   // Number of rows
   int Rows() const
       return rows;

   // Number of columns
   int Columns() const
       return columns;

   // Access array elements (read-only)
   const T & At( int row, int column ) const
       return Get1DIndex(row, column) );

   // Access array elements
   T & At( int row, int column )
       return Get1DIndex(row, column) );

   // *** IMPLEMENTATION ***

   // Store 2D matrix data in 1D vector
   // Or use std::valarray...
   std::vector< T > data;

   // Number of rows
   int rows;

   // Number of columns
   int columns;

   // Check that given index is valid
   bool IsValidIndex( int row, int column ) const
       ATLASSERT( rows >= 0 );
       ATLASSERT( columns >= 0 );

       if ( row < 0 || row >= rows )
           return false;

       if ( column < 0 || column >= columns )
           return false;

       return true;

   // Convert 2D index into 1D
   int Get1DIndex( int row, int column ) const
       ATLASSERT( IsValidIndex(row, column) );

       // 2D data are stored in 1D array row-wise
       return ( column + (row * columns) );


// Print matrix to cout
template <typename T>
void Print( const Matrix<T> & m )
   for ( int r = 0; r < m.Rows(); r++ )
       for ( int c = 0; c < m.Columns(); c++ )
           std::cout << m.At(r, c) << " ";

       std::cout << std::endl;
   std::cout << std::endl;

// *** TEST ***
int main()
   Matrix<double> A( 3, 2 );

   Print( A );

   A.At(0,0) = 2;
   A.At(1,1) = 3;
   A.At(2,0) = 1;
   A.At(2,1) = -1;
   // A.At(3,2) = 2; <<-- test bounds checking (assert fails)

   return 0;


And you may also consider /ad hoc/ C++ libraries for (fast) matrix
computation (like Blitz++).


Ah Giovanni, thank you for that.
I mostly work with translations and rotations for 3d stuff that ends up
being sent to DirectX for displaying on the screen.
This has developed over many years from spaghetti code as I was learning on
the run.
I should at some stage bundle it into matrices and you have given me
something to think about.

PS. Your name should also be in my above reply to Victor as well as a few
others here.
I'm amazed at how much time you all must put into helping people without any
obvious gain.
Unfortunately, coding not my primary occupation so I can't keep up with you
Much appreciated.

Ron Francis

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