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I have a matrix a 6*4 matrix and stacked them into a two columns matrix. First column is the numerical values, and second column is the category names. I want to calculate the mean of the values from each category but I don't know how to get the characters from column 2.

P. Camilleri
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user3718804
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  • Do you mean that you have a two-column data frame? – Matthew Lundberg Jun 08 '14 at 01:48
  • Welcome to Stack Overflow. It's always best to include a [reproducible example](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) with your question. Show some sample data and your expected output. This helps to make it clear exactly what you want. – MrFlick Jun 08 '14 at 01:52

1 Answers1

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To aggregate values in a data frame by category, you use the aggregate function. Consider this artificial data:

x <- data.frame(values=sample(1:6), categories=sample(c('A','B'), 6, replace=TRUE))
x
##   values categories
## 1      4          B
## 2      1          B
## 3      5          A
## 4      3          B
## 5      6          A
## 6      2          A

aggregate(values~categories, data=x, FUN=mean)
##   categories   values
## 1          A 4.333333
## 2          B 2.666667
Matthew Lundberg
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