[R] Mean error

[arun]

Hi,
Try either:
res1 <- apply(mydata[,1:2],2,mean)
 res2 <- colMeans(mydata[,1:2])
 identical(res1,res2)
#[1] TRUE

# Also if you need to find means for each group ("Ungrazed vs. "Grazed")
by(mydata[,-3],mydata[,3],colMeans)

#or if column names are "V1", "V2", "V3"
aggregate(.~V3,mydata,mean)
#or
library(plyr)
 ddply(mydata,.(V3),numcolwise(mean))


A.K.


I have a data set with two columns of data that I want to find the mean of.   
1   6.225  59.77 Ungrazed 
2   6.487  60.98 Ungrazed 
3   4.919  14.73 Ungrazed 
4   5.130  19.28 Ungrazed 
5   5.417  34.25 Ungrazed 
6   5.359  35.53 Ungrazed 
7   7.614  87.73 Ungrazed 
8   6.352  63.21 Ungrazed 
9   4.975  24.25 Ungrazed 
10  6.930  64.34 Ungrazed 
11  6.248  52.92 Ungrazed 
12  5.451  32.35 Ungrazed 
13  6.013  53.61 Ungrazed 
14  5.928  54.86 Ungrazed 
15  6.264  64.81 Ungrazed 
16  7.181  73.24 Ungrazed 
17  7.001  80.64 Ungrazed 
18  4.426  18.89 Ungrazed 
19  7.302  75.49 Ungrazed 
20  5.836  46.73 Ungrazed 
21 10.253 116.05 Ungrazed 
22  6.958  38.94   Grazed 
23  8.001  60.77   Grazed 
24  9.039  84.37   Grazed 
25  8.910  70.11   Grazed 
26  6.106  14.95   Grazed 
27  7.691  70.70   Grazed 
28  8.988  80.31   Grazed 
29  8.975  82.35   Grazed 
30  9.844 105.07   Grazed 
31  8.508  73.79   Grazed 
32  7.354  50.08   Grazed 
33  8.643  78.28   Grazed 
34  7.916  41.48   Grazed 
35  9.351  98.47   Grazed 
36  7.066  40.15   Grazed 
37  8.158  52.26   Grazed 
38  7.382  46.64   Grazed 
39  8.515  71.01   Grazed 
40  8.530  83.03   Grazed 

This is from an introduction handout that instructs me to enter the command 
>mean(mydata[,1:2]) 
but when I enter it, I get an error message 
Warning message: 
In mean.default(mydata[, 1:2]) : 
  argument is not numeric or logical: returning NA 
I've tried tacking on na.rm=T to the end of it, but I get the same 
message. Can someone tell me what I'm doing wrong, or how to fix it? 

I've tried searching the forum, but can't find a post relevant to this problem.