Function to return modified matrix in R - r

Question

The following code adds vector XTS1$XTSSum2 to xts object XTS1:
library(xts)
XTS1 <- structure(c(12, 7, 7, 22, 24, 30, 26, 23, 27, 30), .indexCLASS = c("POSIXct", "POSIXt"), .indexTZ = "", tclass = c("POSIXct", "POSIXt"), tzone = "", class = c("xts", "zoo"), .CLASS = structure("double", class = "CLASS"), formattable = structure(list(formatter = "formatC", format = structure(list(format = "f", digits = 2), .Names = c("format", "digits")), preproc = "percent_preproc", postproc = "percent_postproc"), .Names = c("formatter", "format", "preproc", "postproc")), index = structure(c(1413981900, 1413982800, 1413983700, 1413984600, 1413985500, 1413986400, 1413987300, 1413988200, 1413989100, 1413990000), tzone = "", tclass = c("POSIXct", "POSIXt")), .Dim = c(10L, 1L))
colnames(XTS1) <- "XTS1"
XTS1$XTSSum2 <- XTS1$XTS1 + lag(XTS1$XTS1,1)
The following function performs the same operation.
addfunction <- function(x){
x$XTSSum2 <- x$XTS1 + lag(x$XTS1,1)
}
addfunction(XTS1)
But the vector XTS1$XTSSum2 is not stored.
How can I get addfunction to store the vector so that after running addfunction(XTS1), XTS1 will look like this:
XTS1 XTSSum2
2014-10-22 08:45:00 12 NA
2014-10-22 09:00:00 7 19
2014-10-22 09:15:00 7 14
2014-10-22 09:30:00 22 29
2014-10-22 09:45:00 24 46
2014-10-22 10:00:00 30 54
2014-10-22 10:15:00 26 56
2014-10-22 10:30:00 23 49
2014-10-22 10:45:00 27 50
2014-10-22 11:00:00 30 57
The reproducible example uses an xts object, presume the same solution would apply
to xts objects, matrices and data frames.

Answer 1

The assignment is happening within the function's environment, not the global one. You need to return the result in the function, and assign it with the function call. Try this:
addfunction <- function(x){
x$XTSSum2 <- x$XTS1 + lag(x$XTS1,1)
x
}
XTS1 <- addfunction(XTS1)