5.2 Simple Moving Average (SMA)

A n-day simple moving avaerage (n-day SMA) is arithmetic average of prices of past n days:

\[ SMA_t(n) = \frac{P_t+\ldots+P_{t-n+1}}{n}\]

The following is an SMA function:

mySMA <- function (price,n){
  sma <- c()
  sma[1:(n-1)] <- NA
  for (i in n:length(price)){
    sma[i]<-mean(price[(i-n+1):i])
  }
  sma <- reclass(sma,price)
  return(sma)
}

Let us apply our function:

mysma <-mySMA(Cl(AAPL),n=20)
tail(mysma,n=3)

##                [,1]
## 2012-12-26 77.44186
## 2012-12-27 76.95700
## 2012-12-28 76.38721

5.2.1 TTR

In the TTR package, we can use SMA():

sma <-SMA(Cl(AAPL),n=20)
tail(sma,n=3)

##                 SMA
## 2012-12-26 77.44186
## 2012-12-27 76.95700
## 2012-12-28 76.38721

We can see that our code gives the same result.

5.2.2 Trading signal

Buy signal arises when a short-run SMA crosses from below to above a long-run SMA.

Sell signal arrises when a short-run SMA crosses from above to above a long-run SMA.

5.2.3 Charting

We use the function addSMA() to put

chartSeries(AAPL,
            subset='2007-05::2009-01',
            theme=chartTheme('white')
            )
addSMA(n=30,on=1,col = "blue")
addSMA(n=200,on=1,col = "red")