Writing functions in R

If you have to repeat the same few lines of code more than once, then you really need to write a function. Functions are a fundamental building block of R. You use them all the time in R and it’s not that much harder to string functions together (or write entirely new ones from scratch) to do more.

• R functions are objects just like anything else.
• By default, R function arguments are lazy - they’re only evaluated if they’re actually used:
• Every call on a R object is almost always a function call.

Basic components of a function

• The body(), the code inside the function.
• The formals(), the “formal” argument list, which controls how you can call the function.
• The `environment()`` which determines how variables referred to inside the function are found.
• args() to list arguments.

f <- function(x) x
f

formals(f)

environment(f)

Question: How do we delete this function from our environment?

More on environments

Variables defined inside functions exist in a different environment than the global environment. However, if a variabe is not defined inside a function, it will look one level above.

example.

x <- 2
g <- function() { 
  y <- 1
  c(x, y)
}  
g()

## [1] 2 1

rm(x, g)

Same rule applies for nested functions.

A first useful function.

first <- function(x, y) {
    z <- x + y
    return(z)
}

add <- function(a, b) {
  return (a + b)
}
vector <- c(3, 4, 5, 6)

sapply(vector, add, 1)

What does this function return?

x <- 5
f <- function() {
  y <- 10
  c(x = x, y = y)
}
f()

What does this function return?

x <- 5
g <- function() {
  x <- 20
  y <- 10
  c(x = x, y = y)
} 
g()

What does this function return??

x <- 5
h <- function() {
  y <- 10
  i <- function() {
    z <- 20
    c(x = x, y = y, z = z)
  }
  i() 
}
h()

Functions with pre defined values

temp <- function(a = 1, b = 2) {
    return(a + b)
}

Functions usually return the last value it computed

f <- function(x) {
  if (x < 10) {
    0
  } else {
    10
  }
}
f(5)
f(15)