R因子和水平

Question

Levels make sense that it is unique values of the vector, but I can't get my head around what factor is. It just seems to repeat the vector values.

factor(c(1,2,3,3,4,5,1))
[1] 1 2 3 3 4 5 1
Levels: 1 2 3 4 5

Can anyone explain what factor is supposed to do, or why would I used it?

I'm starting to wonder if factors are like a code table in a database. Where the factor name is code table name and levels are the unique options of the code table. ?

Answer 1

A factor is stored as a hash table rather than raw character vector. What does this imply? There are two major benefits.

1. Much smaller memory footprint. Consider a text file containing the phrase "New Jersey" 100,000 times over encoded in ASCII. Now imagine if you just had to store the number 16 (in binary 100,000 times and then another table indicating that 16 means "New Jersey". It's leaner and faster.

2. Especially for visualization and statistical analysis, frequently we test for values "across all categories" (think ANOVA or what you would color a stacked barplot by). We can either repeatedly encode all of our functions to stack up observed choices in a string vector or we can simply create a new type of vector which will tell you what the valid choices are. That is called a factor, and the valid choices are called levels.