Sarah Goslee sarah.goslee at gmail.com
Mon Sep 14 19:06:42 CEST 2015

```On Mon, Sep 14, 2015 at 11:11 AM, JORGE COLACO <j_colaco at utad.pt> wrote:
> I would greatly appreciate if you could let me know why the R does not make
> the right computations in the case below.
> Jorge Colaço

R made the correct computations: it did exactly what you told it. It
isn't R's fault that what you told it isn't what you meant. You want
to subtract the column means from each column; what you actual told R
was to subtract Xmean from X element by element column-wise, recycling
Xmean as necessary.

Here's what you meant:

X<-matrix(c(-1,0,1,-1,1,0,
0,0,1,1,1,-1,
-1,0,-1,1,0,1,
1,1,-1,-1,0,0,
0,0,1,1,-1,1),nrow=5,ncol=6,byrow=T)

Xmean <- colMeans(X)

sweep(X, 2, Xmean, "-")

Thank you for providing a simple reproducible example and clear idea
straightforward.

>
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>> X<-matrix(c(-1,0,1,-1,1,0,
> + 0,0,1,1,1,-1,
> + -1,0,-1,1,0,1,
> + 1,1,-1,-1,0,0,
> + 0,0,1,1,-1,1),nrow=5,ncol=6,byrow=T)
>
>>
> mean<-c(mean(X[,1]),mean(X[,2]),mean(X[,3]),mean(X[,4]),mean(X[,5]),mean(X[,6]))
>> mean
> [1] -0.2  0.2  0.2  0.2  0.2  0.2
>> X-mean
>      [,1] [,2] [,3] [,4] [,5] [,6]
> [1,] -0.8 -0.2  0.8 -1.2  0.8 -0.2
> [2,] -0.2  0.2  0.8  0.8  0.8 -1.2
> [3,] -1.2 -0.2 -0.8  0.8 -0.2  0.8
> [4,]  0.8  0.8 -1.2 -0.8 -0.2 -0.2
> [5,] -0.2 -0.2  0.8  0.8 -0.8  0.8
>>
>
> Right Result Should Be:
>
> ans =
>   -0.80000  -0.20000   0.80000  -1.20000   0.80000  -0.20000
>    0.20000  -0.20000   0.80000   0.80000   0.80000  -1.20000
>   -0.80000  -0.20000  -1.20000   0.80000  -0.20000   0.80000
>    1.20000   0.80000  -1.20000  -1.20000  -0.20000  -0.20000
>    0.20000  -0.20000   0.80000   0.80000  -1.20000   0.80000
>
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