14 Functions

14.1 Defining functions

A function can be defined

• using an identifier (e.g., add_one)
• on the left of an assignment operator <-
• followed by the corpus of the function

add_one <- function (input_value) {
  output_value <- input_value + 1
  output_value
  }

14.2 Defining functions

The corpus

• starts with the reserved word function
• followed by the parameter(s) (e.g., input_value) between simple brackets
• and the instruction(s) to be executed in a code block
• the value of the last statement is returned as output

add_one <- function (input_value) {
  output_value <- input_value + 1
  output_value
  }

14.3 Defining functions

After being defined, a function can be invoked by specifying the identifier

add_one (3)

## [1] 4

14.4 More parameters

• a function can be defined as having two or more parameters by specifying more than one parameter name (separated by commas) in the function definition
• a function always take as input as many values as the number of parameters specified in the definition
  • otherwise an error is generated

area_rectangle <- function (hight, width) {
  area <- hight * width
  area
}

area_rectangle(3, 2)

## [1] 6

14.5 Functions and control structures

Functions can contain both loops and conditional statements in their corpus

factorial <- function (input_value) {
  result <- 1
  for (i in 1:input_value) {
    cat("current:", result, " | i:", i, "\n")
    result <- result * i
  }
  result
}
factorial(3)

## current: 1  | i: 1 
## current: 1  | i: 2 
## current: 2  | i: 3

## [1] 6

14.6 Scope

The scope of a variable is the part of code in which the variable is ``visible’’

In R, variables have a hierarchical scope:

• a variable defined in a script can be used referred to from within a definition of a function in the same scrip
• a variable defined within a definition of a function will not be referable from outside the definition
• scope does not apply to if or loop constructs

14.7 Example

In the case below

• x_value is global to the function times_x
• new_value and input_value are local to the function times_x
  • referring to new_value or input_value from outside the definition of times_x would result in an error

x_value <- 10
times_x <- function (input_value) {
  new_value <- input_value * x_value
  new_value
}
times_x(2)

## [1] 20