Assign a block of memory to a multidimensional array in C++

Question

I'm new to pointers/memory operations and am working on some sample programs. I want to assign a 2D array into a contiguous block of memory in C++. I know I have to create a buffer with the size of the 2D array. I have a small block of code that I wrote which creates the buffer and assigns values to a 2D array, but I don't know how to place the array values into the buffer. Can anyone give me an idea of what to do? I've researched it quite a bit but can't find anything that explains the process in terms I understand. I know that vectors are probably a better option but I want to get to grips with array operations before I move onto that.

Thanks!

#include <cstdlib>
#include <cstdio>
#include <iostream>
#include <ctime>

using namespace std;

int main()
{
 int dyn_array[5][3];
 int i;
 int j;

 srand(time(NULL));

 //Need to create a pointer to a block of memory to place the 2D array into

 int* buffer=new int[5*3]; //pointer to a new int array of designated size

 //Now need to assign each array element and send each element of the array into the buffer

 for(i=0;i<5;i++)
 {
   for(j=0;j<3;j++)
   {
     dyn_array[i][j]=rand()%40;
     cout<<"dyn array ["<<i<<"]["<<j<<"] is: "<<dyn_array[i][j]<<endl; 
   }
 }
 return 0;
}

buffer[i*3+j] = dyn_array[i][j]?

Oliver Charlesworth
– Oliver Charlesworth

2012-04-02 22:19:42 +00:00
Commented Apr 2, 2012 at 22:19 — Oliver Charlesworth
– Oliver Charlesworth, Commented Apr 2, 2012 at 22:19

Kerrek SB · Accepted Answer · 2012-04-02 22:19:47Z

3

You can address the array in strides, like buffer[i * 3 + j]. Here j is the fast index and 3 is the extent of the range covered by j.

You should generally always store rectangular, multidimensional data in this, flattened-out fashion, because this way you will have one contiguous chunk of memory.

answered Apr 2, 2012 at 22:19

Kerrek SB

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2 Comments

adohertyd Over a year ago

Thanks for this. Would this apply to 3D arrays also? Also, how would I go about retrieving specific array elements from the buffer? Is it the same as usual ie. arr[1][2]=4; or does it take a different form?

Kerrek SB Over a year ago

It'll work for any number of dimensions. You can write a wrapper access function, but it has to be round brackets (operator()), since the square brackets can only take one single argument.

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