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Dynamic 2-dimensional array

Posted on 2003-10-30
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Last Modified: 2010-04-01
I am trying to declare a 2-dimensional array at runtime.  I can declare a 1 dimensional array as follows:

unsigned int * MyVariable;
MyVariable = new unsigned int[12];

However, the following does not work.

unsigned int * MyVariable;
MyVariable = new unsigned int[12][8];

I am using Microsoft Visual C++ 6.0 if it is of any help.

If it is an overloading solution some code would be useful.

Many thanks in advance.
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Question by:reidy_boy
7 Comments
 
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Accepted Solution

by:
Farzad Akbarnejad earned 50 total points
Comment Utility
Hi,


      unsigned int * MyVariable1;
      MyVariable1 = new unsigned int[12];

      // ------------------------------------------

      unsigned int ** MyVariable2;
      MyVariable2 = new unsigned int * [8];
      for (int i = 0; i < 11; i++)
            MyVariable2[i] = new unsigned int [8];


-FA
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Expert Comment

by:Farzad Akbarnejad
Comment Utility
Hello,
Sorry change

MyVariable2 = new unsigned int * [8];

with

MyVariable2 = new unsigned int * [12];

-FA
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Expert Comment

by:Sys_Prog
Comment Utility
int **a ;

a = new int * [10] ;
for ( int i = 0 ; i < 10 ; i ++ )
  a [i] = new int [20] ;

This would get a array same as
a[10][20]

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Expert Comment

by:Axter
Comment Utility
You can use the following class to dynamically create a 2 dimensional array:

template < class T, int ROW_T = 0, int COL_T = 0 >
class dynamic_2d_array
{
public:
    dynamic_2d_array(int row, int col):m_row(row),m_col(col), m_data((row!=0&&col!=0)?new T[row*col]:NULL){}
    dynamic_2d_array():m_row(ROW_T),m_col(COL_T), m_data(new T[ROW_T*COL_T])
      {if (!COL_T || !ROW_T) {int x[ROW_T] = {{ROW_T}};int y[COL_T] = {{x[0]}};}}
    ~dynamic_2d_array(){if(m_data) delete []m_data;}
    T* operator[](int i) {return (m_data + (m_col*i));}
    T const*const operator[](int i) const {return (m_data + (m_col*i));}
private:
    const int m_row;
    const int m_col;
    T* m_data;
};
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LVL 30

Expert Comment

by:Axter
Comment Utility
Example usuage:
            dynamic_2d_array < int > dyn_2dArray(Q_Col, Q_Row);
            StartTime = GetTickCount();
            for (int t = 0;t < QtyTestItera;++t)
            for(int i0 = 0;i0 < Q_Col;++i0)
            {
                  for(int i1 = 0;i1 < Q_Row;++i1)
                  {
                        dyn_2dArray[i0][i1] = i0 + i1;
                  }
            }

Check out following link for more details on other methods for creating a dynamic two dimensional array:
http://www.axter.com/faq/topic.asp?TOPIC_ID=60&FORUM_ID=4&CAT_ID=9
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Expert Comment

by:efn
Comment Utility
When you allocate an array of objects of some type T, you get a pointer to T.  With

new unsigned int[12][8]

you are allocating an array of 8-integer arrays, so you get a pointer to an 8-integer array, which is not the same as a pointer to an integer.  So all you have to do is declare MyVariable like this:

unsigned int (* MyVariable)[8];

The parentheses are necessary to get a pointer to an array, rather than an array of pointers.

This only works if the 8 is a constant, as shown in your question.  If it's a variable, see the other answers.

--efn
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Author Comment

by:reidy_boy
Comment Utility
Thank you experts for your comments.  All are good answers, but I have given the points to FarzadA because it was the simplest.  Sys_Prog, your answer was along the same lines as FarzadA but he got there first!!!
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