Circular convolution of a non causal signal I know how we compute the $N$ point circular convolution of a two causal signals, but what about a signal such as $\{1,-1,2,1\}$ where, the position of 2 is the $0^{th}$ index and the other sequence is $\{2, -1\}$ which we can assume  to be causal, what about the 4 point circular convolution. According to me it is 

$$\begin{bmatrix}1&amp;1&amp;2&amp;-1 \\-1&amp;1&amp;1&amp;2\\2&amp;-1&amp;1&amp;1\\1&amp;2&amp;-1&amp;1 \end{bmatrix} \begin{bmatrix} 2\\-1\\0\\0\end{bmatrix} =\begin{bmatrix} 1\\-3\\5\\0\end{bmatrix}$$
With the position of 5 being the zeroth index because only then the 2 from the first signal got multiplied with the 2 of the second signal, giving off the zero position. But now I am confused, as to how to arrange the other indices. Can anyone help me out?
For an $N$-point circular convolution you can think of each signal as being periodically extended with period $N$. For your example with $N=4$ that would mean that the two sequences are

2 1 1 -1 and 2 -1 0 0

where both now start at index $n=0$. The result of the cyclic convolution is

5 0 1 -3

which is just a cyclic shift (by $2$) of the (correct) result that you obtained.