average of the past 12 observations without a loop

I have vector A. How can i create vector B with the same size of vector A where starting row 13 in B I compute the average of the previous 12 observations in A. In other words

B(13,1)=mean(A(1:12,1) 
B(14,1)= mean(A(2:13,1) 
etc...

I tried a loop and it is taking forever. I feel it could be done easily without a loop.

Answer 1

[Image Analyst]

Try this:

m = randi(9, 1, 100);
windowWidth = 12;
kernel = [zeros(1, windowWidth), ones(1, windowWidth-1)]/windowWidth
slidingMean = conv(m, kernel, 'valid')

or else use movmean() if you have a recent version of MATLAB r2016a or later.