Dear friends,
 suppose i have two datasets: A  and B
A:
id<-1:6
x<-c(1,2,3,4,5,6)
y<-c(2,4,6,8,3,2)
xy<-data.frame(id,x,y)
B
 m<-c(1,1,3,3,5,5)
n<-c(2,2,6,6,3,3)
mn<-data.frame(m,n)
Now, i want to perfomr two tasks:
1. get a subset of B,no duplicate values,:
C:
m n
1 2
3 6
5 3

2.Extract the values in A on the conditions that x=m and y=n
the results should be:
 id x y
1 1 2
3 3 6
5 5 3
Thanks very much!




-- 
Kind Regards,
Zhi Jie,Zhang ,PHD
Department of Epidemiology
School of Public Health
Fudan University
Tel:86-21-54237149

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