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

a[1,1]=1;

is equivalent to:

a[1]=1;

The expression 1,1 evaluates to 1, because the first 1 is discarded and only the second 1 is evaluated. Read up on the comma operator for more info.