Ways to create dynamic matrix in C?

Question

This is the only way I know to create a matrix (2D array) in C, dynamically, and reading user input into its elements:

• Creating a pointer to an array of x pointers, where each pointer represents a line in the matrix - x is the number of lines in the matrix (its height).

• Pointing each pointer in this array to an array with y elements, where y is the number of columns in the matrix (the width).

---

int main()
{

  int i, j, lines, columns, **intMatrix;

  printf("Type the matrix lines:\t");
  scanf("%d", &lines);
  printf("Type the matrix columns:\t");
  scanf("%d", &columns);

  intMatrix = (int **)malloc(lines * sizeof(int *)); 
  //pointer to an array of [lines] pointers

  for (i = 0; i < lines; ++i)
      intMatrix[i] = (int *)malloc(columns * sizeof(int)); 
      //pointer to a single array with [columns] integers

  for (i = 0; i < lines; ++i)
  {
      for (j = 0; j < columns; ++j)
      {
        printf("Type a number for <line: %d, column: %d>\t", i+1, j+1);
        scanf("%d", &intMatrix[i][j]);
      }
  }

Are there other ways to do this?

Solution 1

You can try like this

int main()
{    
  int i, j, lines, columns, *intMatrix;

  printf("Type the matrix lines:\t");
  scanf("%d", &lines);
  printf("Type the matrix columns:\t");
  scanf("%d", &columns);

  intMatrix = (int *)malloc(lines * columns * sizeof(int)); 

  for (i = 0; i < lines; ++i)
  {
      for (j = 0; j < columns; ++j)
      {
        printf("Type a number for <line: %d, column: %d>\t", i+1, j+1);
        scanf("%d", &intMatrix[i*lines + j]);
      }
  }

Solution 2

From C99 onwards (but not C++), you can use variable length arrays:

int main()
{    
  int i, j, lines, columns;

  printf("Type the matrix lines:\t");
  scanf("%d", &lines);
  printf("Type the matrix columns:\t");
  scanf("%d", &columns);

  {
    int intMatrix[lines][columns];

    for (i = 0; i < lines; ++i)
    {
        for (j = 0; j < columns; ++j)
        {
          printf("Type a number for <line: %d, column: %d>\t", i+1, j+1);
          scanf("%d", &intMatrix[i][j]);
        }
    }
  }
}

Or even like this:

void readData (int lines, int columns, int array[lines][columns])
{
  int i, j;

  for (i = 0; i < lines; ++i)
  {
      for (j = 0; j < columns; ++j)
      {
        printf("Type a number for <line: %d, column: %d>\t", i+1, j+1);
        scanf("%d", &array[i][j]);
      }
  }
}

int main()
{
  int lines, columns;

  printf("Type the matrix lines:\t");
  scanf("%d", &lines);
  printf("Type the matrix columns:\t");
  scanf("%d", &columns);

  {
    int intMatrix[lines][columns];

    readData (lines, columns, intMatrix);
  }
}

But, in both cases, the array data is all stored on the stack, not the heap, so there's no way to store it properly, and you can't put it in a struct or anything malloc'd.

Comments

[tempy]

Is C99 popular in C programming nowadays? (I'm currently using C85) but I don't know what real C programmers use today (in the industry).

[ams]

There's a C85? I'm aware of C89 (aka C90), C99, and C11. Real code uses whatever the compiler allows, standard or otherwise. :)

[tempy]

How can I pass the matrix to a function in both my example (**intMatrix) and yours (*intMatrix)? How will I access the matrix elements inside the function? Thanks.

[Raj]

you can pass to the function as int *matrix and then calculate the element as intMatrix[i*lines + j] as shown in the example.