Question

I am trying to figure out why I am getting an error when I assign a value to a structure in an array of structures in heap-allocated memory.

I have the follow C struct:

typedef struct _Point{
int x;
int y;
 }Point;

and an array of Point:

typedef Point Sample[];

Now I want allocate an array of Sample in witch any Sample consists of 15 Points and I using the follow code:

Sample *new_positions;
data.old_positions =(Sample*) malloc(sizeof(Point) * 15 * global.nsamples);

When I try to use it with the follow code in order to assign a value, I have an error

data.old_positions[0][0].x=5;

Where is the problem? 🙁

Answer 1

data.old_positions is only single dimension –

 data.old_positions[0].x=5;

Assigns the Zeroith item. You’re better off simulating two dimensions than constructing a 2D array.

 data.old_positions[0 * 15 + 0].x = 5;

(Sample *)malloc(... effectively constructs a single dimensional array with no contents. You’re going to need to malloc the second dimension…

 data.old_positions = (Sample *)malloc(sizeof(Sample) * global.nsamples)
 for (int i = 0; i < global.nsamples; i++) 
     data.old_positions[i] = (Point*)malloc(sizeof(Point) * 15);

That should do it.