Indexing with Factors

I spent a cycle figuring out, how R treats factors as an index.

Major Lessons

Any non-numeric thing that you read from a data file is going to be a factor, i.e., categorical data.

Factors are implemented as numerics/integers in R. So when you use them to index another construct which has row/column name as character you need to do so by as.character().

Here is a typical scenario: You read a column from a file and use it to index some construct.
You either should read the column as string:

myDataFrame <- read.csv("path/to/file.csv", stringsAsFactors=FALSE)

or convert the factors using as.character before indexing another construct:

a <- myDataFrame2[as.character(myFactor)]