Types And Coercion
- Describe R’s type hierarchy
- Predict behavior from implicit coercion
classes and types
The class of an object describes how it behaves.
d = Sys.Date()
class(d)
typeof(d)
d is a Date, stored as a double precision floating point number.
Because it’s an object of class Date, it makes sense to call a function that works on dates.
weekdays(d)
The type of an object describes how it is stored.
Sometimes they’re the same.
Today, we’re talking more about types.
doubles
Most numeric calculations in any language happen with double precision floating point numbers. These are an efficient way to approximately represent real numbers using 64 bits (1’s and 0’s). The details are beyond the scope of this class, but we can get a taste of them.
Doubles do NOT have the same properties as the real numbers, and this can be disconcerting if you’ve studied lots of math. We can only use 64 bits. This implies:
- There are a finite number of them, not more than
2^64 - There is a maximum and a minimum
# Epsilon
.Machine$double.eps
# 52 digits in mantissa
1 / (2^52)
explicit coercion
We can explicitly coerce, or convert, between types.
dc = as.character(Sys.Date())
class(dc)
paste(dc, "is a wonderful day to learn about coercion.")
implicit coercion
What will happen if I don’t explicitly change the object to the right type?
paste(Sys.Date(), "is a wonderful day to learn about coercion.")
Suppose we add several objects of different types together. What happens?
typeof(1L)
typeof(TRUE)
typeof(1.0)
TRUE + 1L
1L + 1.0
TRUE + 1L + 1.0
type hierarchy
Try to order these in terms of when it makes sense to coerce one type to another.
- character
"hello" - logical
TRUE - numeric
2.83 - integer
seq(5)
Group activity
Alphabetically, first person’s turn to explain
- Explain what is the type of a literal
NA, and why? - Why are these numbers not equal?
(0.1 + 0.05) == 0.15 - What is the type of an object with class
data.frame, and what does this imply?
https://stackoverflow.com/questions/9508518/why-are-these-numbers-not-equal