[R] How to manipulate data according to groups ?

From: Ng Stanley <stanleyngkl_at_gmail.com>
Date: Thu, 13 Mar 2008 22:24:42 +0800


I have a two columns data, the first column are values, and second column are the groups. For this example, there are 3 groups 1,2,3.

How can I manipulate the values in the first column according to groups, say I would like to find mean, sum, and standard deviation for the different groups ? How do I plot data according to groups ?

> t <- matrix(c(rnorm(10), 1,1,2,2,1,3,3,3,3,2), ncol=2)

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