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一、Factor函数
#函数factor可以把一个向量编码为一个因子，其一般形式为：
#factor(x,levels=sort(unique(x),na.last=TRUE),labels,exculde=NA,order=FALSE)
#其中x是向量，levels是水平，可以自行指定各离散的取值，不指定时由x的不同值来表示，labels可以用来指定各水平的标签
#不指定时用各离散取值的对应字符串 sex<-c("M","F","M","M","F") factor(sex)
#使用is.factor来判断对象是否为因子类型 is.factor(sex) #使用as.factor来将对象转换为因子类型
sex.factor<-as.factor(sex) #使用levels来取得因子中的水平 levels(sex.factor) # [1] "F" "M"
二、tapply函数
#函数tapply() #当我们知道5个人的性别以及5个人的身高的时候，就能计算出每个性别下的平均身高
height<-c(170,175,180,165,168) tapply(height,sex.factor,mean) # F M # 171.5000
171.6667 #tapply 的使用格式为tapply(x,index,fun) 其中x为对象，index为与x具有相同个数的因子类型，fun为指定的方法
三、gl()函数
#gl()函数 #gl函数能很快的产生因子，其基本用法为：gl(n,k,length=n*k,labels=1:n,order=false) gl(3,5)
# [1] 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 # Levels: 1 2 3 gl(3,1,15) # [1] 1 2 3 1 2
3 1 2 3 1 2 3 1 2 3 # Levels: 1 2 3
