C++ 2D dynamic array crash

Question

Hi I'm having some problem with 2D dynamic array.

int main()
{
   double **M;
   int M_dimension;
   int i;

   M_dimension = 10;

   M = new double *[M_dimension];
   for (i=0;i<M_dimension;i++)
   {
      M[i] = new double[M_dimension];
   }

   M[0][0] = 1.0;

...
}

Program works but I'd like to initialize 2D array using such a function:

void initialize2D(double **M,int M_dimension)
{
   int i;
   M = new double *[M_dimension];
   for (i=0;i<M_dimension;i++)
   {
      M[i] = new double[M_dimension];
   }
}

Finally the program looks like this:

int main()
{
   double **M;
   int M_dimension;
   int i;

   M_dimension = 10;

   initialize2D(M,M_dimension);        

   M[0][0] = 1.0; //crash

...
}

Unfortunately it crashes at M[0][0] = 1.0;

Thanks for any help or suggestions.

If this isn't for practice I recommend you to use a vector of vectors. It's much easier to handle. — the_drow
– the_drow, Commented Nov 30, 2009 at 10:44
A vector of vectors has its own set of problems. You don't usually want a jagged array, and vector of vectors don't have as good locality as a single contiguous 2d array — Stack Overflow is garbage
– Stack Overflow is garbage, Commented Nov 30, 2009 at 10:55

int3 · Accepted Answer · 2009-11-30 10:36:32Z

8

You are passing M by value, instead of by reference. initialize2D needs to change the value of the pointer-to-pointer M such that it points to the memory allocated

Try changing your function signature to this instead:

void initialize2D(double **&M,int M_dimension)

Or

void initialize2D(double ***M,int M_dimension) {
    ...
    *M = new double *[M_dimension];
    ...
}

answered Nov 30, 2009 at 10:36

int3

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Comments

EFraim · Accepted Answer · 2009-11-30 10:36:56Z

1

You need to pass a reference to double** instead of double** to function, otherwise the modification done to a pointer after assigning M the reslut of new get lost on exit from a function.

answered Nov 30, 2009 at 10:36

EFraim

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Comments

longhorn · Accepted Answer · 2010-02-06 06:27:00Z

0

what the problem might be is where you declaring an integer parameter such as int M_dimension void initialize2D(double **M,int M_dimension)

and then you initializing the dynamic array as: M[i] = new double[M_dimension];

where it comes to contradiction because you declared the variable M_dimension as an integer and then you using it as a double

try it this way: either change the data type of array or the M_dimension, so then both of them have the same data type.

hopefully this will help you out

answered Feb 6, 2010 at 6:27

longhorn

1

Comments

Community · Accepted Answer · 2017-05-23 12:01:14Z

0

Why don't you use std::vector or Boost.MultiArray

It would be quite easy exercise to define 2-dimension array as generic vector of vectors in C++

edited May 23, 2017 at 12:01

CommunityBot

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answered Feb 6, 2010 at 6:35

mloskot

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