[R] Get a percent variable based on group

David Winsemius dwinsemius at comcast.net
Wed Jan 16 04:34:38 CET 2013

On Jan 15, 2013, at 6:30 PM, Karine Charlebois wrote:

> Dear all, I'd like to get a percentage variable based on a group, but without creating a new data frame. 
> For example:
> data(iris)
> iris$percent <-unlist(tapply(iris$Sepal.Length,iris$Species,function(x) x/sum(x, na.rm=TRUE)))

A percentage is 100 times a fraction whose nominal value is unity. My guess is that you want a percentage of the group mean? So this would just be:

iris$percent <-ave(iris$Sepal.Length, iris$Species, FUN=function(x) 100*x/mean(x, na.rm=TRUE))


> This does not work, I should have only three standard values, respectively for setosa, versicolor, and virginica. How can I do this?

If you just want three values, then I do not see how these are percentages.

> tapply(iris$Sepal.Length,iris$Species,function(x) mean(x, na.rm=TRUE))
    setosa versicolor  virginica 
     5.006      5.936      6.588 

David Winsemius
Alameda, CA, USA

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