Working with factors

>str_v <- c("string1","string2","string2","string1",...)

>str_f <- factor(c)   # makes a factor

The factor prints like the original strings but is not a string vector
```
> str_v  

[1] "string1" "string2" "string2" "string1" ...

> str_f
[1] string1 string2 string2 string1
Levels: string1 string2
```
In many contexts, the factor behaves like a vector of integers (of length equal to that of the original string). If strings are required, such as values for text() or legend() in lots, use as.character() to convert to a vector of string values
```
> as.character(str_f)
[1] "string1" "string2" "string2" "string1" ...
```
levels() lists the distinct values (as a string vector)
```
> levels(str_f)
[1] "string1" "string2" ...
```
factors are used to subdivide vectors for operations.
```
x <- c( 3, 5, ...)    # values which correspond to str_v

tapply( x, str_f, mean )
```
returns a vector of values resulting from applying mean() to subvectors determined by values of str_v

to make ranges of a numeric vector into a factor, use cut()

> y <- 1:100
> f <- cut(y,breaks=10)
> f
  [1] (0.901,10.8] (0.901,10.8] (0.901,10.8] (0.901,10.8] (0.901,10.8]
  [6] (0.901,10.8] (0.901,10.8] (0.901,10.8] (0.901,10.8] (0.901,10.8]
 [11] (10.8,20.7]  (10.8,20.7]  (10.8,20.7]  (10.8,20.7]  (10.8,20.7] 
 [16] (10.8,20.7]  (10.8,20.7]  (10.8,20.7]  (10.8,20.7]  (10.8,20.7] 
 [21] (20.7,30.7]  (20.7,30.7]  (20.7,30.7]  (20.7,30.7]  (20.7,30.7] 
 [26] (20.7,30.7]  (20.7,30.7]  (20.7,30.7]  (20.7,30.7]  (20.7,30.7] 
 [31] (30.7,40.6]  (30.7,40.6]  (30.7,40.6]  (30.7,40.6]  (30.7,40.6] 
 [36] (30.7,40.6]  (30.7,40.6]  (30.7,40.6]  (30.7,40.6]  (30.7,40.6] 
 [41] (40.6,50.5]  (40.6,50.5]  (40.6,50.5]  (40.6,50.5]  (40.6,50.5] 
 [46] (40.6,50.5]  (40.6,50.5]  (40.6,50.5]  (40.6,50.5]  (40.6,50.5] 
 [51] (50.5,60.4]  (50.5,60.4]  (50.5,60.4]  (50.5,60.4]  (50.5,60.4] 
 [56] (50.5,60.4]  (50.5,60.4]  (50.5,60.4]  (50.5,60.4]  (50.5,60.4] 
 [61] (60.4,70.3]  (60.4,70.3]  (60.4,70.3]  (60.4,70.3]  (60.4,70.3] 
 [66] (60.4,70.3]  (60.4,70.3]  (60.4,70.3]  (60.4,70.3]  (60.4,70.3] 
 [71] (70.3,80.3]  (70.3,80.3]  (70.3,80.3]  (70.3,80.3]  (70.3,80.3] 
 [76] (70.3,80.3]  (70.3,80.3]  (70.3,80.3]  (70.3,80.3]  (70.3,80.3] 
 [81] (80.3,90.2]  (80.3,90.2]  (80.3,90.2]  (80.3,90.2]  (80.3,90.2] 
 [86] (80.3,90.2]  (80.3,90.2]  (80.3,90.2]  (80.3,90.2]  (80.3,90.2] 
 [91] (90.2,100]   (90.2,100]   (90.2,100]   (90.2,100]   (90.2,100]  
 [96] (90.2,100]   (90.2,100]   (90.2,100]   (90.2,100]   (90.2,100]  
10 Levels: (0.901,10.8] (10.8,20.7] (20.7,30.7] (30.7,40.6] ... 
(90.2,100]