factor.Rmd

## Factors

* A factor is a vector object (1 dimension) used to specify a **discrete classification (grouping)** of the components of other vectors.

* Factors are mainly used for **statistical modeling**, and can also be useful for graphing.

* You can create factors with the **factor** function, for example:

```{r}
e <- factor(c("high", "low", "medium", "low"))
# check the structure of e
str(e)
```

* Example of a character vector versus a factor

```{r}
# factor
e <- factor(c("high", "low", "medium", "low"))
# character vector
e2 <- c("high", "low", "medium", "low")
# Check the structure of both objects
str(e)
str(e2)
```

* Groups in factors are called **levels**.<br>
Levels can be **ordered**. Then, some operations applied on numeric vectors can be used:

```{r, eval=F}
# unordered factor:
e <- factor(c("high", "low", "medium", "low"))
max(e) # throws an error
# ordered factor
e_ord <- factor(e, levels=c("low", "medium", "high"), ordered=TRUE)
max(e_ord) # outputs "high"
```