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> set.seed(1)> data <- data.frame(X = paste("s", sample(1:3, 15, replace = TRUE), sep = ""),Y = ceiling(rnorm(15)), Z = rnorm(15), A = rnorm(15), B = rnorm(15))

> dat1 = aggregate(Z ~ X, data=data, FUN=mean)

##  X   Z## 1  s1  -0.9504295## 2 s2 0.2601957## 3  s3   0.1786968

> dat2 = aggregate(Z~ X + Y, data=data, FUN=mean)

##  X   Y   Z## 1  s3  -2  -0.47815006## 2 s1 0 -1.47075238## 3 s2 0 0.62794591## 4 s3 0 0.61982575## 5 s1 1 -1.04392394## 6 s2 1 -0.10755445## 7 s3 1 0.16693329## 8 s1 2 -0.05612874## 9  s3   2   0.41794156

> dat3 = aggregate(cbind(Y,Z)~X, data=data, FUN=mean)

##  X   Y   Z## 1  s1  1.0  -0.9504295## 2 s2 0.5 0.2601957## 3  s3  0.4   0.1786968

> dat4 = aggregate(.~X, data=data, FUN=mean)

##     X   Y   Z   A   B          ## 1 s1 1.0 -0.9504295 0.3832754 -0.4254229## 2 s2 0.5 0.2601957 -0.0637888 0.8651778## 3  s3  0.4   0.1786968   0.2574540   0.5170860

> testDF <- data.frame(v1 = c(1,3,5,7,8,3,5,NA,4,5,7,9),+ v2 = c(11,33,55,77,88,33,55,25,44,55,77,99) )> aggregate(v2 ~ v1, data = testDF, FUN=paste, sep=",")

## v1 v2## 1 1 11## 2 3 33, 33## 3 4 44## 4 5 55, 55, 55## 5 7 77, 77## 6 8 88## 7   9          99