How to convert from 1D array to 2D array using malloc - c++

Question

I'm trying to convert the a 1D array to a 2D array

Answer 1

If you want to allocate an actual 2D array, not an array of pointers, the syntax gets a little tricky:
int X = 16, Y = 8;
char (*pt)[X][Y] = malloc(X*Y);
Above, pt is a pointer to an X by Y array of chars. Because it's a pointer, accessing its elements requires an asterisk and parentheses, too:
for (int i = 0 ; i != X ; i++) {
for (int j = 0 ; j != Y ; j++) {
(*pt)[i][j] = (char)(i*X+j);
}
}
Of course you need to free the pointer once you are done using the array:
free(pt);

Answer 2

You need to allocate each row of the matrix separately:
unsigned char** index;
index = malloc(X * sizeof(unsigned char*)); // Allocate an array of pointers
for(int i = 0; i < X; i++)
index[i] = malloc(Y * sizeof(unsigned char)) // Allocate each row
Refer also to this answer on malloc pointer casting.

Answer 3

unsigned char **index;
index = (unsigned char**)malloc( X*Y );
is wrong. index is an unsigned char**. You need to malloc two times to get a 2D array. If you want to allocate X rows, each with Y columns, use
unsigned char **index;
index = (unsigned char**)malloc( X * sizeof(unsigned char*) ); //Allocating number of rows
for(int i=0 ; i<X ; i++ )
{
index[i] = (unsigned char*)malloc( Y * sizeof(unsigned char)); // Allocate each column
}
You should check if malloc does not fail. You should also free everything after its use to avoid a memory leak.

Answer 4

After your edit, it appears you are using C++.
In C++, you should prefer std::vector< std::vector< unsigned char > > for dynamic 2d array.
std::vector< std::vector< unsigned char > > index(X, std::vector<unsigned char>(Y));
Now you can use it as index[i][j] and it will be automatically cleaned, so need to free/delete explicitly.
Live demo here
Still if you want to allocate c-compliant arrays in conventional way, use
unsigned char** index;
try {
index = new int*[X];
for(int i = 0; i < X; ++i)
index[i] = new int[Y];
} catch(...) {...}
For delete also you need to delete each element individually.
Old answer
unsigned char **index;
index = malloc(X * sizeof *index);
if(!index) fail();
for(xi = 0; xi < X; ++xi) {
index[xi] = malloc(Y * sizeof **index);
if(!index[xi]) free_and_fail();
}
You first allocate space for X pointers and then in all X pointers you allocate one array.
While freeing the memory, you need to free each row individually:
if(index) for(xi = Y-1; xi >= 0; --xi) { /* xi is signed */
free(index[xi];
}
free(index);
index = NULL;