functions-R.md

layout	element	title	language
page	notes	Functions	R

Understandable and reusable code

• Write code in understandable chunks.
  • Clear variable names
  • Consistent indentation and line spacing
  • Good commenting
  • Functions
• Write reusable code.
  • Concise and modular script
  • Functions with general structure

Understandable chunks

• Human brain can only hold ~7 things in memory.
  • Write programs that don't require remembering more than ~7 things at once.
• What do you know about how sum(1:5) works internally?
  • Nothing.
  • Ignore the details and reduce sum() to a single conceptual chunk.
• All functions should work as a single conceptual chunk.

Reuse

• Want to do the same thing repeatedly?
  • Inefficient to copy code
  • If it occurs in more than one place, it will eventually be wrong somewhere.
• Functions are written to be reusable.

Function basics

function_name <- function(inputs) {
  output_value <- do_something(inputs)
  return(output_value)
}

add <- function(a, b) {
  total <- a + b
  return(total)
}

• Creating a function doesn't run it.
• Call the function with some arguments.

add(2, 3)
summed <- add(2, 3)

• Do [Exercise 1 - Use and Modify]({{ site.baseurl }}/exercises/Functions-use-and-modify-R), Tasks 1-2.

• Treat functions like a black box.
  • Can't access a variable that was created in a function
    • > a
    • Error: object 'a' not found
  • 'Global' variables can influence function, but should not.
    • Very confusing and error prone to use a variable that isn't passed in as an argument
  • Defaults can be set for common inputs.

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

add()
add(b=3)
add(4, 5)

• NOT

a <- 1
b <- 2

add <- function() {
  total <- a + b
  return(total)
}

Do [Exercise 1 - Use and Modify, Task 3]({{ site.baseurl }}/exercises/Functions-use-and-modify-R) and [Exercise 2 - Writing Functions]({{ site.baseurl }}/exercises/Functions-writing-functions-R).

Discuss what passing a and b in is much more useful than having them fixed

Nesting Functions

• Functions can be used to organize, manipulate, or group other functions.
  • The do_something() in our basic function can be anything.
    • math
    • built-in function
    • self-written function

make_pie_filling <- function(stuff) {
  filling <- pi * stuff
  return(filling)
}

make_pie <- function(stuff, crust) {
  pie <- crust + make_pie_filling(stuff)
  return(pie)
}

Do [Exercise 3 - Nested Functions]({{ site.baseurl }}/exercises/Functions-nested-functions-R).