C :: Declaring Array Size Using Variable Causes Program To Crash

I am new to C. I've been trying to use C to code some statistical functions originally coded in R. I've encountered an interesting phenomenon. In the function foo1, I declared the array v1v2b using an actual value 1999000. The function runs fine when I call it in R.

Code:
void foo1(double *x, double *y, int *nsamp){
int i, j, k, oper=2, l;
double* v1v2=malloc(sizeof(double)*((*nsamp)*(*nsamp-1)/2 + 1));
outer_pos(x, y, nsamp, &v1v2[0]);
double v1v2b[1999000]; //<-------HERE
for(i=1; i<= 1999000]; i++){
v1v2b[i-1]=1;
} }

However, in foo2, I first create an integer variable called index, and store the value 1999000 in it. I then use it to initialize the same array. When I tried calling this function in R, it either led to a stack overflow error, or completely crashed R.

Code:
void foo2(double *x, double *y, int *nsamp){
int i, j, k, oper=2, l;
double* v1v2=malloc(sizeof(double)*((*nsamp)*(*nsamp-1)/2 + 1));

[Code] .....