Writing R Functions

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In this lesson, you will learn to:

• Write your own functions in R

• Make good decisions about function arguments and returns

• Include side effects and/or error messages in your functions

• Use good R code style

Time Estimates:
     Videos: 30 min
     Readings: 10-30 min
     Activities: 60-90 min
     Check-ins: 4

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Writing Functions

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Required Video: Writing Functions

Required Tutorial: Primer: Writing Functions

Recommended Reading: R4DS: Functions

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Check-In 1: Simple Functions

Write a function called times_seven() which takes a single argument and multiplies by 7.

This function should check that the argument is numeric.

This function should also excitedly announce (print) “I love sevens!” if the argument to the function is a 7.

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Check-In 2: Predicting Function Behavior

add_or_subtract <- function(first_num, second_num = 2, type = "add") {
  
  if (type == "add") {
    first_num + second_num
  } else if (type == "subtract") {
    first_num - second_num
  } else {
    stop("Please choose `add` or `subtract` as the type.")
  }
  
}

Question 1: What will be returned by each of the following?

1. 1
2. -1
3. 30
4. An error defined in the function add_or_subtract
5. An error defined in a different function, that is called from inside add_or_subtract

add_or_subtract(5, 6, type = "subtract")

add_or_subtract("orange")

add_or_subtract(5, 6, type = "multiply")

add_or_subtract("orange", type = "multiply")

Question 2:

Consider the following code:

first_num <- 5
second_num <- 3

result <- 8

result <- add_or_subtract(first_num, second_num = 4)

result_2 <- add_or_subtract(first_num)

In your Global Environment, what is the value of…

1. first_num
2. second_num
3. result
4. result_2

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