Doing operations by grouping variable

That's if the variables are visible.  If they are only in the data frame it's not much more difficult

d <- data.frame(group = rep(1:5, each=5), 
                variable = rnorm(25))
with(d, tapply(variable, group, max))


(Tip: avoid using attach().)

Bill Venables.  

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Michael Bedward
Sent: Tuesday, 21 September 2010 11:15 AM
To: Seth W Bigelow; Rhelp
Subject: Re: [R] Doing operations by grouping variable

Not sure why you think tapply is "awkward". Your example would be...

group <- rep(1:5, each=5)
variable <- rnorm(25)
tapply(variable, group, max)

...which looks quite elegant to me :)

Meanwhile, the reason your expression doesn't work is that you are
asking mistakenly for elements 1:5 repeatedly from the variable col.
If you just type d$variable[ d$group ] and compare the values to your
variable vector this should be clear.

Michael

I'm writing an expression that requires searching a vector according to
group. As an example, I want to find the maximum value in each of 5
groups.


group=rep(1:5, each=5) ? ? ? ? ? ? ? ? ? ? ? ? ?# create grouping variable

variable=rnorm(25) ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?# generate data

d <- data.frame(group,variable) ? ? ? ? ? ? ? ? ? ? ? ? # make data frame

max(d$variable[d$group]) ? ? ? ? ? ? ? ? ? ? ? ?# try expression that
doesn't work

I'm expecting a vector containing the maximum variable value, per group.
What am I doing wrong? I know I can use aggregate, tapply, etc. but that
seems awkward and bulky, is there a simpler way?


Dr. Seth ?W. Bigelow
Biologist, USDA-FS Pacific Southwest Research Station
1731 Research Park Drive, Davis California

? ? ? ?[[alternative HTML version deleted]]

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