R Programming By Example

Factors

When analyzing data, it's quite common to encounter categorical values. R provides a good way to represent categorical values using factors, which are created using the factor() function and are integer vectors with associated labels for each integer. The different values that the factor can take are called levels. The levels() function shows all the levels from a factor, and the levels parameter of the factor() function can be used to explicitly define their order, which is alphabetical in case it's not explicitly defined.

Note that defining an explicit order can be important in linear modeling because the first level is used as the baseline level for functions like lm() (linear models), which we will use in Chapter 3, Predicting ...