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carraysmemory-managementmatrix
Oria Gruber
Oria Gruber

Reputation: 1533

C language - Representing a matrix as an array of pointers

I am trying to represent a matrix as an array of pointers, and then do certain actions on the matrix.

To simplify further, imagine a matrix with n rows and m columns,

I defined an int **matrix that points to the beginning of an array (of pointers) of size n. And every block in this array is a pointer that points to an array (of integers) of size m. The result is clear. you have n pointers, each of them pointing to an array of m integers, overall nxm values. So it should be possible to represent a matrix this way.

My problem is that I don't know how to access the integer values of the matrix. For example, suppose I wish to insert values to the matrix. How do I access matrix[i][j]?

Here is my code, you can see it is incomplete, I would appreciate help completing it.

int** create_matrix(int rows,int columns)
{
    int** matrix,i;
    matrix=(int**)malloc(rows*sizeof(int*));
    for(i=0;i<rows;i++)
        *(matrix)+i=(int*)malloc(columns*sizeof(int));
    for(i=0;i<rows;i++)
    {
        for(j=0;j<columns;j++)
            *(*(matrix)+i)

I want to insert random numbers into the matrix and then return it. I allocated the necesary memory, just need to insert the actual values.

Upvotes: 0

Views: 1586

Answers (3)

Sathish
Sathish

Reputation: 3870

Try the following code-

int** create_matrix(int rows,int columns)
{
   int** matrix,i;
   matrix=(int**)malloc(rows*sizeof(int*));
   for(i=0;i<rows;i++)
      matrix[i]=(int*)malloc(columns*sizeof(int)); // Fix 1
   for(i=0;i<rows;i++)
   {
      for(j=0;j<columns;j++)
         *(*(matrix+i)+j)=rand();
    }
    return matrix;
 }

Instead of doing arithmetic you can directly access it by matrix[i][j]. matrix[i][j] is equal to *(*(matrix+i)+j).

Upvotes: 1

mch
mch

Reputation: 9814

int** matrix,i,j;
matrix=malloc(rows*sizeof(int*));
for(i=0;i<rows;i++)
    *(matrix+i)=malloc(columns*sizeof(int)); //or matrix[i]=malloc(columns*sizeof(int));
for(i=0;i<rows;i++)
{
    for(j=0;j<columns;j++)
        *(*(matrix+i)+j)=random(); //or matrix[i][j]=random();
}
return matrix;

Note the change of the parentheses around matrix

Upvotes: 1

John Bode
John Bode

Reputation: 123598

The general pattern for any type T is

T **arr = malloc( rows * sizeof *arr );
if ( arr )
{
  for ( size_t i = 0; i < rows; i++ )
  {
    arr[i] = malloc( columns * sizeof *arr[i] );
    if ( arr[i] )
    {
      for ( size_t j = 0; j < columns; j++ )
      {
        arr[i][j] = some_value;
      }
    }
    else
      // memory allocation failure
  }
}
else
  // memory allocation failure.

Array subscripts are defined in terms of pointer arithmetic: a[i] is defined as *(a + i). a[i][j] is defined as *(*(a + i) + j).

Upvotes: 1

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