why this code can run ? double a[3]; a[1,1]=1;

Question

int main()
{
    double a[3];  
    a[1,1]=1;
}

It passes the vs2013 compiler, and it is not 2D array.

Answer 1

You are invoking the comma operator. This evaluates its first operand, discards it, and returns the second one. So your code is the equivalent of

a[1] = 1;

The syntax for accessing an element of a 2D array would be

b[1][2] = 42;