Error from rollingmedian using columns - r

Question

I have a dataframe for which I try to add additional column calculating the median of the current and the previous 2 values.
Date Value
21/07/2016 14.8
22/07/2016 14.9
23/07/2016 15.8
24/07/2016 15.0
25/07/2016 15.7
26/07/2016 15.6
27/07/2016 16.1
28/07/2016 16.1
I used the following code:
library(zoo)
dataframe$medianval <-rollmedian(dataframe$Value,k=3)
I get the following error
> Error: k <= n is not TRUE
Any suggestions?

Answer 1

Think about what R is trying to do here. The data frame has 8 rows, but the vector you want to append has only 6 elements. To which rows should those elements align? What should R put in the other two spots?
library(zoo)
dataframe <- read.table(text="Date Value
21/07/2016 14.8
22/07/2016 14.9
23/07/2016 15.8
24/07/2016 15.0
25/07/2016 15.7
26/07/2016 15.6
27/07/2016 16.1
28/07/2016 16.1", header=TRUE)
rollmedian(dataframe$Value,k=3)
# [1] 14.9 15.0 15.7 15.6 15.7 16.1
nrow(dataframe) # [1] 8
length(rollmedian(dataframe$Value,k=3)) # [1] 6
Because I can guess what you meant (correct me if I'm wrong), I would try:
dataframe$medianval <- c(NA, NA, rollmedian(dataframe$Value,k=3))
dataframe
# Date Value medianval
# 1 21/07/2016 14.8 NA
# 2 22/07/2016 14.9 NA
# 3 23/07/2016 15.8 14.9
# 4 24/07/2016 15.0 15.0
# 5 25/07/2016 15.7 15.7
# 6 26/07/2016 15.6 15.6
# 7 27/07/2016 16.1 15.7
# 8 28/07/2016 16.1 16.1
If you want to be able to adapt this conveniently, you should write a simple function:
med.fun <- function(var, data, k){
# Note: variable name must be in quotes
return(c(rep(NA, k-1), with(data, rollmedian(get(var), k=k))))
}
med.fun("Value", dataframe, 5)
# [1] NA NA NA NA 15.0 15.6 15.7 15.7