unique return without change the order
by return with the factor order
example:
> tmppar <- paste("X", c(1,1,1,3,3,8,8,6,6,5,5,2,2,7,7,4,4),sep="")
> tmppar
[1] "X1" "X1" "X1" "X3" "X3" "X8" "X8" "X6" "X6" "X5" "X5" "X2" "X2" "X7" "X7" "X4" "X4"
> unique(tmppar)
[1] "X1" "X3" "X8" "X6" "X5" "X2" "X7" "X4"
> tmp1 <- by(rnorm(length(tmppar)),tmppar,FUN=min)
> tmp1
INDICES: X1
[1] -0.7628715
-------------------------------------------------------------------------------------------------
INDICES: X2
[1] -0.5590477
-------------------------------------------------------------------------------------------------
INDICES: X3
[1] 1.214794
-------------------------------------------------------------------------------------------------
INDICES: X4
[1] 0.445656
-------------------------------------------------------------------------------------------------
INDICES: X5
[1] -0.8465884
-------------------------------------------------------------------------------------------------
INDICES: X6
[1] -1.096506
-------------------------------------------------------------------------------------------------
INDICES: X7
[1] -0.2734341
-------------------------------------------------------------------------------------------------
INDICES: X8
[1] 0.4012138
