[R] using "mean" in by(x,y,mean)

From: William Revelle <lists_at_revelle.net>
Date: Thu 15 Jul 2004 - 06:45:21 EST


Dear list friends

I fail to understand how to find means for multiple groups using the by() function. Help would be appreciated. Thanks. Bill

x <- runif(20,0,10)
group <- rep(c("A","B"),10)
df <-data.frame(x,group)
#df #show the data

rm(x,group)
attach(df)

sd(x)       # sd is defined
mean(x)     #so is mean

by(x,group,sd) #this works for both groups by(x,group,mean) #this does not

produces this output:

> x <- runif(20,0,10)
> group <- rep(c("A","B"),10)
> df <-data.frame(x,group)
> #df #show the data
>
> rm(x,group)
> attach(df)
>
> sd(x) # sd is defined

[1] 2.952699
> mean(x) #so is mean

[1] 5.026441
>
> by(x,group,sd) #this works for both groups
INDICES: A
[1] 2.813504


INDICES: B
[1] 3.236663
> by(x,group,mean) #this does not

Error in FUN(X[[as.integer(1)]], ...) : couldn't find function "FUN"

I am using a Mac OS 10.3.4 running R 1.9.1

-- 
----------------------------
William Revelle               http://pmc.psych.northwestern.edu/revelle.html
Department of Psychology, Northwestern University
Personality Project: http://personality-project.org/personality.html

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Received on Thu Jul 15 06:51:56 2004

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