Pointers in C/C++ compiles but gives segfault error - c++

Question

Here's a code snipped that I have for a larger program
double *pos_x_h[224];
double *pos_y_h[224];
const double A = 1;
const int N = 224;
double d_0;
double alpha;
void initialize(double nu, int rows = 16, int columns = 14) {
double d = 1 / double(columns);
d_0 = d * (1 - pow(2.0, nu - 8));
alpha = d - d_0;
double dx = d;
double dy = d * sqrt(3.0) / 2;
for (int j = 0; j < rows; j++) {
for (int i = 0; i < columns; i++) {
int n = i + j * columns;
*pos_x_h[n] = i * dx + (j % 2) * dx / 2.0;
*pos_y_h[n] = j * dy;
}
}
}
int main(int argc, char *argv[]) {
double nu=7.5;
int rows=16;
int columns=14;
initialize(nu);
return 0;
}
The code compiles but it is gives a seg fault error. Can't see why that's the case. Am I going over array_size?

Answer 1

There doesn't seem to be any point in utilizing pos_x_h and pos_y_h as pointer arrays.
Change this:
double *pos_x_h[224];
double *pos_y_h[224];
To this:
double pos_x_h[224];
double pos_y_h[224];
And this:
*pos_x_h[n] = i * dx + (j % 2) * dx / 2.0;
*pos_y_h[n] = j * dy;
To this:
pos_x_h[n] = i * dx + (j % 2) * dx / 2.0;
pos_y_h[n] = j * dy;
If you really insist on utilizing pointer arrays, then you can use this (in addition to the above):
double *pos_x_h_ptr[224];
double *pos_y_h_ptr[224];
for (int n=0; n<224; n++)
{
pos_x_h_ptr[n] = pos_x_h+n;
pos_y_h_ptr[n] = pos_y_h+n;
}

Answer 2

double *pos_x_h[224];
double *pos_y_h[224];
are arrays of pointers, but you use them wihtout allocating memory
*pos_x_h[n] = i * dx + (j % 2) * dx / 2.0;
*pos_y_h[n] = j * dy;
probably something like that
pos_x_h[n] = malloc(sizeof(double));
*pos_x_h[n] = i * dx + (j % 2) * dx / 2.0;
pos_y_h[n] = malloc(sizeof(double));
*pos_y_h[n] = j * dy;
if you need to alocate memory outside the initialize function (why would you? it is init function) you can do it in main
int i = 0;
for(;i< 224;++i)
{
pos_x_h[i] = malloc(sizeof(double));
pos_y_h[i] = malloc(sizeof(double));
}