Intro to R Programming

Today’s Goal

• First mission: assemble R code that will simulate rolling a pair of dice.
• Then, we’ll weight the dice a bit in your favor.

Roll a Pair of Dice

die <- 1:6
dice <- sample(x = die, size = 2, replace = TRUE)
sum(dice)

## [1] 8

• What is the purpose of the above code?

Roll a Pair of Dice

die <- 1:6
die

## [1] 1 2 3 4 5 6

dice <- sample(x = die, size = 2, replace = TRUE)
dice

## [1] 6 2

sum(dice)

## [1] 8

Roll a Pair of Dice

die <- 1:6
dice <- sample(x = die, size = 2, replace = TRUE)
sum(dice)

## [1] 6

die <- 1:6
dice <- sample(x = die, size = 2, replace = TRUE)
sum(dice)

## [1] 8

die <- 1:6
dice <- sample(x = die, size = 2, replace = TRUE)
sum(dice)

## [1] 6

Roll a Pair of Dice

• What we want to have:

roll()

## [1] 5

roll()

## [1] 6

roll()

## [1] 8

Writing Your Own Function

• Every function in R has 3 basic parts:
  • A name
  • A body of code
  • A set of arguments
• To create a function in R:

my_function <- function() {}

• function() will build a function out of whatever R code you place between the braces.

Writing Your Own Function

roll <- function() {
  die <- 1:6
  dice <- sample(x = die, size = 2, replace = TRUE)
  sum(dice)
}

roll()

## [1] 7

Writing Your Own Function

roll <- function() {
  die <- 1:6
  dice <- sample(x = die, size = 2, replace = TRUE)
  my_sum <- sum(dice)
}

roll()

Writing Your Own Function

• Code that would display a result:

dice

## [1] 1 5

1 + 2

## [1] 3

sqrt(2)

## [1] 1.414214

Writing Your Own Function

• Code that would NOT display a result:

dice <- sample(x = die, size = 2, replace = TRUE)
my_sum <- 1 + 2
a <- sqrt(2)

Function Arguments

roll2 <- function() {
  dice <- sample(x = 1:5, size = 2, replace = TRUE)
  sum(dice)
}

roll2 <- function() {
  dice <- sample(x = 1:10, size = 2, replace = TRUE)
  sum(dice)
}

Function Arguments

roll2 <- function(bones) {
  dice <- sample(x = bones, size = 2, replace = TRUE)
  sum(dice)
}

roll2(bones = 1:5)

## [1] 2

Function Arguments

roll2(bones = die)

## [1] 2

roll2(1:4)

## [1] 3

roll2()

## Error in sample(x = bones, size = 2, replace = TRUE): argument "bones" is missing, with no default

Default Value in Function Arguments

roll2 <- function(bones = 1:6) {
  dice <- sample(x = bones, size = 2, replace = TRUE)
  sum(dice)
}

roll2()

## [1] 8

roll2(bones = 1:5)

## [1] 7

Function Arguments

Today’s Goals

• First mission: assemble R code that will simulate rolling a pair of dice.

roll2 <- function(bones = 1:6) {
  dice <- sample(x = bones, size = 2, replace = TRUE)
  sum(dice)
}

Next

• We’ll weight the dice a bit in your favor.
  • More likely to get #6 than smaller numbers (1, 2, 3, 4, 5).
• How do we do that?

Getting Help in R

?sample

Let’s switch over to RStudio!

Sampling with Weights

weighted_roll <- function() {
  die <- 1:6
  dice <- sample(x = die, size = 2, replace = TRUE, 
                 prob = c(1/8, 1/8, 1/8, 1/8, 1/8, 3/8))
  sum(dice)
}

weighted_roll()

## [1] 9

weighted_roll()

## [1] 12

Is It Working?

rolls <- replicate(n = 10000, expr = roll())
hist(x = rolls, breaks = c(0:12))

Is It Working?

weighted_rolls <- replicate(n = 10000, expr = weighted_roll())
hist(x = weighted_rolls, breaks = c(0:12))

To-do

• Make sure your R and RStudio are working.
• Redo in-class examples on your own.
• Read Chapter 3 of Hands-On Programming with R.

References

• Hands-On Programming with R, by Garrett Grolemund.
• R for Data Science, by Garrett Grolemund, Hadley Wickham.
• Dr. Vincent Vu’s STAT 6730 lecture notes at The Ohio State University.