Package things into understandable chunks & reuse code easily.

Chunks

• human brain can only hold ~7 things in memory
• want to only have to understand pieces of programs with no more than ~7 things
• what do you know about how sum(1:5) works internally? - Nothing
• lets us ignore the details and reduce sum to a single conceptual chunk
• want our functions to work this way too.

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

Function basics

function_name <- function(inputs){
  commands
  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)

• treat functions like a black box
• can’t access a variable that was created in a function

a
b

• very confusing and error prone to use a variable that isn’t passed in as a argument