# [R] Can the by() function return a single column?

Vassilis vdimitrakas at gmail.com
Wed Dec 15 16:29:04 CET 2010

```I would like to de-mean the 'vector' column of the following dataframe by
factor:

set.seed(5444)
vector	<- rnorm(1:10)
factor	<- rep(1:2,5)
test.df	<- data.frame(factor, vector)

which is:

factor     vector
1       1 -0.4963935
2       2 -2.0768182
3       1 -1.5822224
4       2  0.8025474
5       1  0.3504199
6       2  0.2358464
7       1 -0.3989443
8       2 -0.3692544
9       1 -0.3174586
10      2  1.4305431

Using the by() command, I get:

> by(test.df\$vector, test.df\$factor, function(x) {x - mean(x)})
test.df\$factor: 1
 -0.007473699 -1.093302612  0.839339673  0.089975488  0.171461151
--------------------------------------------------------------------------------------------------
test.df\$factor: 2
 -2.0813911  0.7979745  0.2312735 -0.3738272  1.4259702
>

My question is: Is there a way to have this output put back to the
dataframe? I.e to make by(), or some other command, return me a vector of
length 10 whose values x' correspond to x'_1 = x_1 - mean(x | factor1), x'_2
= x_2 - mean(x | factor2),...

Thanks in advance for the help, and apologies for the poor notation.

Vassilis

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