User-written Functions

One of the great strengths of R is the user's ability to add functions. In fact, many of the functions in R are actually functions of functions. The structure of a function is given below.

myfunction <- function(arg1, arg2, ... ){
statements
return(object)
}

Objects in the function are local to the function. The object returned can be any data type. Here is an example.

# function example - get measures of central tendency
# and spread for a numeric vector x. The user has a
# choice of measures and whether the results are printed.

mysummary <- function(x,npar=TRUE,print=TRUE) {
  if (!npar) {
    center <- mean(x); spread <- sd(x)
  } else {
    center <- median(x); spread <- mad(x)
  }
  if (print & !npar) {
    cat("Mean=", center, "\n", "SD=", spread, "\n")
  } else if (print & npar) {
    cat("Median=", center, "\n", "MAD=", spread, "\n")
  }
  result <- list(center=center,spread=spread)
  return(result)
}

# invoking the function
set.seed(1234)
x <- rpois(500, 4)
y <- mysummary(x)
Median= 4
MAD= 1.4826
# y$center is the median (4)
# y$spread is the median absolute deviation (1.4826)

y <- mysummary(x, npar=FALSE, print=FALSE)
# no output
# y$center is the mean (4.052)
# y$spread is the standard deviation (2.01927)

It can be instructive to look at the code of a function. In R, you can view a function's code by typing the function name without the ( ). If this method fails, look at the following R Wiki link for hints on viewing function sourcecode.

Finally, you may want to store your own functions, and have them available in every session. You can customize the R enviroment to load your functions at start-up.