Write Access Violation error in C while using rand() - c

Question

I am quite new to programming with C, so hope someone can bear with me and help me solve the issue I am facing.
I am writing a code to estimate the value of pi with Monte-Carlo method. however, when I build and debug I get an error that says:
"Exception thrown: write access violation.
a was 0x1110112.
I get this error in generate_random_array function
in this line of code:
a[i] = (((double)rand() / (double)RAND_MAX) * 2.0 ) - 1.0;
I am posting the whole code as well for reference.
note: I am using visual studio with MSVC compiler
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
void generate_random_array(int sz, double a[]);
void count_points_inside_circle(int sz, double x[], double y[], int* counter);
int main()
{
int tot = 1000000000;
int incircle_c = 0;
double distance_sq, pi;
double* x = NULL;
double* y = NULL;
/*create arrays in the heap*/
x = malloc(tot * sizeof(double));
y = malloc(tot * sizeof(double));
/*generate random locations*/
generate_random_array(tot, x);
generate_random_array(tot, y);
/*count the points inside te circle by checking the location distance*/
count_points_inside_circle(tot, x, y, &incircle_c);
/*estimate pi*/
pi = 4.0 * incircle_c / (double)tot;
printf("pi estimated value using %d samples was found to be %lf", tot, pi);
free(x);
free(y);
return 0;
}
void generate_random_array(int sz, double a[]) {
int i;
srand(time(NULL));
for (i = 0; i < sz; i++)
a[i] = (((double)rand() / (double)RAND_MAX) * 2.0 ) - 1.0;
}
void count_points_inside_circle(int sz, double x[], double y[],int* counter_p) {
int i;
double distance_sq;
for (i = 0; i < sz; i++) {
distance_sq = x[i] * x[i] + y[i] * y[i];
if (distance_sq <= 1)
(*counter_p)++;
}
}

Answer 1

You must always check the pointer returned from malloc against NULL. For example:
x = malloc(n * sizeof *x);
if (x == NULL) { /* Handle the failure and/or exit */ }
On the other hand, this task doesn't require use of an array (or allocated space used as an array) at all; you need only the number of points inside the circle and the number of total points generated. It could be simply done like that:
#include <stdio.h>
#include <stdlib.h>
double estimate_pi (unsigned trial_count)
{
const double rr = (double)RAND_MAX * RAND_MAX;
unsigned inner_point_count = 0;
unsigned i;
for (i = 0; i < trial_count; ++i) {
double x = rand();
double y = rand();
if (x * x + y * y <= rr)
++inner_point_count;
}
return 4.0 * inner_point_count / trial_count;
}
int main (void)
{
printf("%f\n", estimate_pi(1000000000));
return 0;
}
Note that, quality of random number generator used by the standard library significantly affects the result of this simulation.