You left out the subscript. Why not just do
d <- within(data.frame(group = rep(1:5, each = 5), variable = rnorm(25)),
scaled <- variable/tapply(variable, group, max)[group])
and be done with it?
(Warning: if you replace the second '<-' above by '=', it will not work.
It is NOT true that you can always replace '<-' by '=' for assignment. Why?)
Bill Venables.
-----Original Message-----
From: Seth W Bigelow [mailto:[email protected]]
Sent: Wednesday, 22 September 2010 1:43 AM
To: Venables, Bill (CMIS, Cleveland)
Cc: [email protected]; [email protected]
Subject: RE: [R] Doing operations by grouping variable
Thanks, Bill and Michael, you have answered the question I asked, but not the
one I wished to ask I want to obtain the maximum in each group of variables, so
I could scale each variable by the maximum for its group. If I use tapply, as
in the example below, there's a mismatch in dimensions of the output of tapply
[5] and the data frame with the variables[25].
group <- rep(1:5, each=5) # define grouping variable
variable <- rnorm(25) # generate data
d <- data.frame(group,variable) # bundle together in a data frame
d$scaled <- d$variable/(with(d,tapply(variable,group,max))) # crash and burn
Dr. Seth W. Bigelow
Biologist, USDA-FS Pacific Southwest Research Station
1731 Research Park Drive, Davis California
<[email protected]>
09/20/2010 06:24 PM
To
<[email protected]>, <[email protected]>,
<[email protected]>
cc
Subject
RE: [R] 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: [email protected] [mailto:[email protected]] 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
On 21 September 2010 10:59, Seth W Bigelow <[email protected]> wrote:
> 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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> and provide commented, minimal, self-contained, reproducible code.
>
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<http://www.r-project.org/posting-guide.html>
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