Hello, everyone 
The syntax of StDev is
StDev( ARRAY, periods, Population = True );
So, what if the periods becomes all bars?!
I wrote a code snippet to test it as below:
stDev2 = StDev(C, BarCount);
Plot(stDev2, "stDev(C, BarCount)", colorLightGrey, styleLine | styleOwnScale);
Plot(BarCount, "BarCount", colorBlueGrey, styleLine | styleOwnScale);
then, I saw the BarCount = a fixed number; stDev2 = {Empty} forever.
Could any great one know how to calculate StDev of all bars?
Thank you all.