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 & 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)
}

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)
shrub_vol <- calc_shrub_vol(0.8, 1.6, 2.0)

• 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
    • > 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 Writing Functions.

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)

Named vs unnamed arguments

• When to use or not use argument names

calc_shrub_vol(height = 2.0, length = 0.8, width = 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 height, second value is length…
  • If order is hard to remember use names
• In many cases there are a lot of optional arguments
  • Convention to always name optional argument

Do Use and Modify.

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

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

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

• Calling 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)

Do Combining Functions.