CODEWITHSUNDEEP
cpointers
fitzbutz
fitzbutz

Reputation: 976

accessing double pointer 2d array passed by reference to a function

i've this data structure

struct m_bucket
{
   int a;
   int b;
};

then i need to use a 2D array of struct m_bucket as a matrix so i declare it on this way

typedef struct m_bucket **  matrix_t;

and this is the initialization function

matrix_t matrix_alloc( int n_rows , int n_cols )
{
   matrix_t m;
   int i;

    m = ( struct m_bucket **) malloc( ((n_rows + 1) * sizeof(  struct m_bucket  * )));

    for(i=0 ; i < (n_rows + 1) ; i++ )
    {
        m[i] = ( struct m_bucket *) malloc( ((n_cols + 1) * sizeof(  struct m_bucket )));
    }

   return m;
}

my problem is that i'm a bit confused about the right way to acces this data structure when passing it by reference to a function. I tried this but it isn't working.

void matrix_set_column(matrix_t * matrix , int val , int col , int n_cols)
{ 
int i;

    for(i = 0 ; i < (n_cols + 1) ; i++)
    {   
        (*matrix)[col][i].cost = val;

        if (val>0)
            ((*matrix)[col][i]).a =  4;
        else
            ((*matrix)[col][i]).a = -1;
        val++;
    }

   }

what's the right way to access this structure?

Upvotes: 1

Views: 628

Answers (2)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726489

Since the code that allocates matrix_t uses the number of rows n_rows for the first malloc, the code that uses the matrix needs to pass indexes in the order row, then column. Your function needs to reverse its indexes, like this:

// Assuming that col represents the column, not the row,
// i represents the row, so it needs to go first
((*matrix)[i][col]).a =  4;

Index i needs to from zero to n_rows+1, too.

Other than that, your code is fine: you have correctly added parentheses to force unary * to complete before [] operator, so the rest of the code is OK.

Upvotes: 1

Marian
Marian

Reputation: 7472

You probably want the following:

void matrix_set_column(matrix_t matrix , int val , int col , int n_cols)
{ 
int i;

  for(i = 0 ; i < (n_cols + 1) ; i++)
  {   
    matrix[col][i].cost = val;

    if (val>0)
        matrix[col][i].a =  4;
    else
        matrix[col][i].a = -1;
    val++;
  }

}

and use it as:

...
matrix_t m;
m = matrix_alloc(10,20);
matrix_set_column(m, 123, 9, 20);
...

This will pass only pointers, because matrix_t is a pointer type.

Upvotes: 1

Related Questions