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

[William Revelle]

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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