match(a$level,b$level)->tmp
a$no1<-ifelse(is.na(tmp),a$no1,b$no1[tmp])

match(a$level,c$level)->tmp1
a$no2<-ifelse(is.na(tmp1),a$no2,c$no2[tmp1])

a

[此贴子已经被作者于2009-3-13 2:57:51编辑过]

a more readable and robust-to-replicates-in-a way is

a= data.frame (level =c("A","B","C","D","B"),no1=rep(0,5),no2=rep(0,5))
b= data.frame (level =c("B","C"),no1=1:2)
c= data.frame (level =c("A","E"),no2=3:4)

#merge a,b together
tmp<-merge(a,b,by="level",all.x=T)
#assign b's vlaue to a
tmp$no1.x <- with(tmp, ifelse(is.na(no1.y), no1.x, no1.y))
#drop redundant column
tmp$no1.y <- NULL

tmp<-merge(tmp,c,by="level",all.x=T)
tmp$no2.x <- with(tmp, ifelse(is.na(no2.y), no2.x, no2.y))
tmp$no2.y <- NULL

#rename
colnames(tmp)<-c("level","no1","no2")
tmp

a$no1[is.element(a$level,b$level)]=b$no1
a$no2[is.element(a$level,c$level)]=c$no2

谢谢！这个问题解决，不过还有一个白痴点的问题想请教一下：

a<-data.frame(no=c(1,2,2,4),lev=c("A","B","C","B"))

> a
  no lev
1  1   A
2  2   B
3  2   C
4  4   B

我想将lev=B的，改为lev=D,但按下面的做法不行，要怎样才行？

如果是numeric的就能直接如下赋值，为什么character不能呢？

a$no[a$no==2]<-10
a$lev[a$lev=="B"]<-"D" #不行！

谢谢！

levels(a$lev)[levels(a$lev)=="B"]="D"