functions-R.md

layout	element	title	language
page	notes	Functions	R

Understandable and reusable code

• Write code in understandable chunks.
• Write reusable code.

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 & error prone 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)
}

• The braces indicate that the lines of code are a group that gets run together

{a = 2
b = 3
a + b}

• Pressing run anywhere in this group runs all the lines in that group
• A function runs all of the lines of code in the braces
• Using the arguments provided
• And then returns the output

calc_shrub_vol <- function(length, width, height) {
  volume <- length * width * height
  return(volume)
}

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

calc_shrub_vol(0.8, 1.6, 2.0)

• Store the output to use it later in the program

shrub_vol <- calc_shrub_vol(0.8, 1.6, 2.0)

Do [Writing Functions]({{ site.baseurl }}/exercises/Functions-writing-functions-R)

• Treat functions like a black box

  • Draw a box on board showing inputs->function->outputs
  • The only things the function knows about are the inputs we pass it
  • The only thing the program knows about the function is the output it produces

• Walk through function execution (using debugger)

  • Call function
  • Assign 0.8 to length, 1.6 to width, and 2.0 to height inside function
  • Calculate volume
  • Send the volume back as output
  • Store it in shrub_vol

• Treat functions like a black box.

  • Can't access a variable that was created in a function
    • > volume
    • Error: object 'width' not found
  • Or an argument by name
    • > width
    • Error: object 'width' 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

Do [Use and Modify]({{ site.baseurl }}/exercises/Functions-use-and-modify-R). End of 1 hour class

Default arguments

• Defaults can be set for common inputs.

calc_shrub_vol <- function(length = 1, width = 1, height = 1) {
  volume <- length * width * height
  return(volume)
}

calc_shrub_vol()
calc_shrub_vol(width = 2)
calc_shrub_vol(0.8, 1.6, 2.0)
calc_shrub_vol(height = 2.0, length = 0.8, width = 1.6)

Do [Default Arguments]({{ site.baseurl }}/exercises/Functions-default-arguments-R).

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

Named vs unnamed arguments

• When to use or not use argument names

calc_shrub_vol(length = 2.0, width = 0.8, height = 1.6)

Or

calc_shrub_vol(2.0, 0.8, 1.6)

• You can always use names
  • Value gets assigned to variable of that name
• If not using names then order determines naming
  • First value is length, second value is width, third value is height
  • If order is hard to remember use names
• In many cases there are a lot of optional arguments
  • Convention to always name optional argument

Combining Functions

• Each function should be single conceptual chunk of code

• Functions can be combined to do larger tasks in two ways

• Calling multiple functions in a row

est_shrub_mass <- function(volume){
  mass <- 2.65 * volume^0.9
}

shrub_volume <- calc_shrub_vol(0.8, 1.6, 2.0)
shrub_mass <- est_shrub_mass(shrub_volume)

• We can also use pipes with our own functions
• The output from the first function becomes the first argument for the second function

library(dplyr)
shrub_mass <- calc_shrub_vol(0.8, 1.6, 2.0) %>%
  est_shrub_mass()

Do [Combining Functions]({{ site.baseurl }}/exercises/Functions-combining-functions-R).

• We can nest functions

shrub_mass <- est_shrub_mass(calc_shrub_vol(0.8, 1.6, 2.0))

• But we careful with this because it can make code difficult to read

• Don't nest more than two functions

• Can also call functions from inside other functions

• Allows organizing function calls into logical groups

est_shrub_mass_dim <- function(length, width, height){
  volume = calc_shrub_vol(length, width, height)
  mass <- est_shrub_mass(volume)
  return(mass)
}

est_shrub_mass_dim(0.8, 1.6, 2.0)

• We don't need to pass the function name into the function
• That's the one violation of the black box rule

Documentation & Comments

• Documentation
  • How to use code
  • Use Roxygen comments for functions
• Comments
  • Why & how code works
  • Only if it code is confusing to read

Working with functions in RStudio

• It is possible to find and jump between functions

• Click on list of functions at bottom of editor and select

• Can be helpful to clearly see what is a function

• Can have RStudio highlight them

• Global Options -> Code -> Display -> Highlight R function calls