R Functions


title: Functions in R

A function allows you to define a reusable block of code that can be executed many times within your program.

Functions can be named and called repeatedly or can be run anonymously in place (similar to lambda functions in python).

Developing a full understanding of R functions requires understanding of environments.
Environments are simply a way to manage objects. An example of environments in action is that you can use a redundant variable name within a function, that won’t be affected if the larger runtime already has the same variable. Additionally, (lexical scoping)[https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexical_scoping] is used when a function calls a variable not defined within the function it will check the higher level environment for that variable.

Syntax

In R, a function definition has the following features:

  1. The keyword function
  2. A function name
  3. Input parameters (optional)
  4. Some block of code to execute
  5. A return statement (optional)
# a function with no parameters or returned values sayHello() = function(){ "Hello!" } sayHello() # calls the function, 'Hello!' is printed to the console # a function with a parameter helloWithName = function(name){ paste0("Hello, ", name, "!") } helloWithName("Ada") # calls the function, 'Hello, Ada!' is printed to the console # a function with multiple parameters with a return statement multiply = function(val1, val2){ val1 * val2 } multiply(3, 5) # prints 15 to the console

Functions are blocks of code that can be reused simply by calling the function. This enables simple, elegant code reuse without explicitly re-writing sections of code. This makes code both more readable, makes for easier debugging, and limits typing errors.

Functions in R are created using the function keyword, along with a function name and function parameters inside parentheses.

The return() function can be used by the function to return a value, and is typically used to force early termination of a function with a returned value.
Alternatively, the function will return the final printed value.

# return a value explicitly or simply by printing sum = function(a, b){ c = a + b return(c) } sum = function(a, b){ a + b } result = sum(1, 2) # result = 3

You can also define default values for the parameters, which R will use when a variable is not specified during function call.

sum = function(a, b = 3){ a + b } result = sum(a = 1) # result = 4

You can also pass the parameters in the order you want, using the name of the parameter.

result = sum(b=2, a=2) # result = 4

R can also accept additional, optional parameters with ‘…’

sum = function(a, b, ...){ a + b + ... } sum(1, 2, 3) #returns 6

Functions can also be run anonymously. These are very useful in combination with the ‘apply’ family of functions.

# loop through 1, 2, 3 - add 1 to each sapply(1:3, function(i){ i + 1 })

Notes

  • If a function definition includes arguments without default values specified, values for those values must be included.
sum = function(a, b = 3){ a + b } sum(b = 2) # Error in sum(b = 2) : argument "a" is missing, with no default
  • Variables defined within a function only exist within the scope of that function, but will check larger environment if variable not specified
double = function(a){ a * 2 } double(x) # Error in double(x) : object 'x' not found double = function(){ a * 2 } a = 3 double() # 6

In-built functions in R

  • R comes with many functions that you can use to do sophisticated tasks like random
    sampling.
  • For example, you can round a number with the round(), or calculate
    its factorial with the factorial().
> round(4.147) [1] 4 > factorial(3) [1] 6 > round(mean(1:6)) [1] 4
  • The data that you pass into the function is called the function’s argument.
  • You can simulate a roll of the die with R’s sample()function. The sample() function takes two arguments: a vector named x and a number named size. For example:
> sample(x = 1:4, size = 2) [] 4 2 > sample(x = die, size = 1) [] 3 >dice <- sample(die, size = 2, replace = TRUE) >dice [1] 2 4 >sum(dice) [1] 6
  • If you’re not sure which names to use with a function, you can look up the function’s
    arguments with args(). For example:
> args(round) [1] function(x, digits=0)

Additional Resources

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