C++ :: Faulty Use Of Constructor Of Dynamic Array

One can initialize a dynamically created array in the following way:

unsigned int * vec;
// ... do something to vec
double * a = (double *) malloc(4*sizeof(double));
a = (double[3]){(double[3]){0.0,10.0,20.0}[vec[0]],

[Code] ....

While there is no compilation error for the first assignment, the memory a is pointing to seems to change, surprisingly to me. This seems to solve the problem though:

memcpy(a, (double[3]){(double[3]){0.0,10.0,20.0}[vec[0]],
(double[3]){1.0,2.0,3.0}[vec[1]],
(double[6]){-2.0,-1.0,0.0,1.0,2.0,3.0}[vec[2]]
},
3*sizeof(double)); // NO C COMPILER ERROR

What does the first assignment do and why does it cause memory to change later in the program?