Question

I have a for loop that populates values into a data frame named Df. All my columns are populating accurately, but the following line of code does not produce that values that I want:

for(i in 2:(dim(Df)[1])){
Df$dailyReturns200[i] <- (Df$index200[i+1]-Df$index200[i])/Df$index200[i]
}

This is a simple return calculation of P2-P1/P1. A friend of mine has suggested an order of operations problem, but what is the solution?

Thank you!

Answer 1

Hard to answer without a reproducible example, but you shouldn’t need to do this in a loop at all. Does this do what you want?

#Start with a simple one column matrix
x <- matrix(1:6, ncol = 1)

#Compute the diffence, pad with an NA at the front, and then divide by x
cbind(x, c(NA, diff(x))/x)
#----
     [,1]      [,2]
[1,]    1        NA
[2,]    2 0.5000000
[3,]    3 0.3333333
[4,]    4 0.2500000
[5,]    5 0.2000000
[6,]    6 0.1666667

From the help page, diff() returns a suitably lagged and iterated differences.. In this case, all differences are one, and then are divided by x as shown above. See ?diff for more details.