关于数据选择

分别从两个数据框中选择出条件相等的数据，组成一个新的数据框；

1.函数名：merge

> t1<-data.frame(name=c("a","b","c"),age=c(1,2,3),sex=c("m","m","f"));t1

 name age sex
1    a   1   m
2    b   2   m
3    c   3   f

> t2<-data.frame(name=c("d","b","c"),age=c(4,5,6),sex=c("m","m","f"));t2
  name age sex
1    d   4   m
2    b   5   m
3    c   6   f
> m<-merge(t1,t2,by="name");m
  name age.x sex.x age.y sex.y
1    b     2     m     5     m
2    c     3     f     6     f

2.从某一个数据框中选择出某些条件的数据或是相关的列

函数名：subset()

#选出名字为"a"的相关信息

> test<-subset(t1,name=="a");test
  name age sex
1    a   1   m

>#选出名字为"a"，的年龄和性别

 test<-subset(t1,name=="a",select=c(age,sex));test
  age sex
1   1   m

> test<-subset(t1,name=="a"&sex=="m",select=c(age,sex));test
  age sex
1   1   m

> test<-subset(t1,name=="a"&sex=="f",select=c(age,sex));test
[1] age sex
<0 行> (或0-长度的row.names)

>levels(mpg$manufacturer)
 [1] "audi"       "chevrolet"  "dodge"      "ford"      
 [5] "honda"      "hyundai"    "jeep"       "land rover"
 [9] "lincoln"    "mercury"    "nissan"     "pontiac"   
[13] "subaru"     "toyota"     "volkswagen"