转载自https://blog.csdn.net/hsdcc217/article/details/78510087
在R语言中，因子（factor）表示的是一个编号或者一个等级，即，一个点。例如，人的个数可以是1，2，3，4……那么因子就包括，1，2，3，4…..还有描述协变量水平时，会用到高、中、低，也是因子，因为这些都是一个点。与之区别的向量，是一个连续性的值，例如，数值中有1，1.1，1.2……可以作为数值来计算，而因子则不可以。简单通俗来讲：因子是一个点，向量是一个有方向的范围。在R中，如果把数字作为因子，那么在导入数据之后，需要将向量转换为因子（factor），而因子在整个计算过程中不再作为数值，而是一个”符号”而已。
以实例进行解释和说明
data <- c(1,2,2,3,1,2,3,3,1,2,3,3,1)
> data
[1] 1 2 2 3 1 2 3 3 1 2 3 3 1
> fdata <- factor(data)
> fdata
[1] 1 2 2 3 1 2 3 3 1 2 3 3 1
Levels: 1 2 3
> class(fdata)
[1] "factor"
> class(data)
[1] "numeric"
#factor()函数将原来的数值型的向量转化为了factor类型。factor类型的向量中有Levels的概念。Levels就是factor中的所有元素的集合（没有重复）。我们可以发现Levels就是factor中元素排除重复后且字符化的结果。因为Levels的元素都是character。
> levels(fdata)
[1] "1" "2" "3"
#我们可以在factor生成时，通过labels向量来指定levels，继续上面的程序：
> rdata <- factor(data,labels=c("I","II","III"))
> rdata
[1] I II II III I II III III I II III III I
Levels: I II III
> rdata <- factor(data,labels=c("e","ee","eee"))
> rdata
[1] e ee ee eee e ee eee eee e ee eee eee e
Levels: e ee eee
#factors可以指定数据的顺序
> mons <- c("March","April","January","November","January", "September","October","September","November","August", "January","November","November","February","May","August", "July","December","August","August","September","November", "February","April")
> mons <- factor(mons)
> mons
[1] March April January November January
[6] September October September November August
[11] January November November February May
[16] August July December August August
[21] September November February April
11 Levels: April August December February ... September
> table(mons)
mons
April August December February January
2 4 1 2 3
July March May November October
1 1 1 5 1
September
3
#显然月份是有顺序的，我们可以为factor指定顺序
mons = factor(mons,levels=c("January","February","March","April","May","June","July","August","September","October","November","December"),ordered=TRUE)
> table(mons)
mons
January February March April May
3 2 1 2 1
June July August September October
0 1 4 3 1
November December
5 1