On Jan 27, 2011, at 7:16 PM, Ernest Adrogué i Calveras wrote:
Hi,
I have this data.frame with two variables in it,
z
V1 V2
1 10 8
2 NA 18
3 9 7
4 3 NA
5 NA 10
6 11 12
7 13 9
8 12 11
and a vector of means,
means <- apply(z, 2, function (col) mean(na.omit(col)))
means
V1 V2
9.666667 10.714286
Two methods:
A) use sweep (which by default takes the difference)
> sweep(z, 2, means)
V1 V2
1 0.3333333 -2.7142857
2 NA 7.2857143
3 -0.6666667 -3.7142857
4 -6.6666667 NA
5 NA -0.7142857
6 1.3333333 1.2857143
7 3.3333333 -1.7142857
8 2.3333333 0.2857143
B) use the scale function (whose "whole purpose in life" is to
subtract the mean and possibly divide by the standard deviation which
we suppressed in this case with the scale=FALSE argument)
> scale(z, scale=FALSE)
V1 V2
1 0.3333333 -2.7142857
2 NA 7.2857143
3 -0.6666667 -3.7142857
4 -6.6666667 NA
5 NA -0.7142857
6 1.3333333 1.2857143
7 3.3333333 -1.7142857
8 2.3333333 0.2857143
attr(,"scaled:center")
V1 V2
9.666667 10.714286
--
David.
My intention was substracting means from z, so instictively I tried
z-means
V1 V2
1 0.3333333 -1.6666667
2 NA 7.2857143
3 -0.6666667 -2.6666667
4 -7.7142857 NA
5 NA 0.3333333
6 0.2857143 1.2857143
7 3.3333333 -0.6666667
8 1.2857143 0.2857143
But this is completely wrong. sapply() gives the same result:
sapply(z, function(row) row - means)
V1 V2
[1,] 0.3333333 -1.6666667
[2,] NA 7.2857143
[3,] -0.6666667 -2.6666667
[4,] -7.7142857 NA
[5,] NA 0.3333333
[6,] 0.2857143 1.2857143
[7,] 3.3333333 -0.6666667
[8,] 1.2857143 0.2857143
So, what is going on here?
The following appears to work
z-matrix(means,ncol=2)[rep(1, dim(z)[1]),]
V1 V2
1 0.3333333 -2.7142857
2 NA 7.2857143
3 -0.6666667 -3.7142857
4 -6.6666667 NA
5 NA -0.7142857
6 1.3333333 1.2857143
7 3.3333333 -1.7142857
8 2.3333333 0.2857143
but I think it's rather cumbersome, surely there must be a cleaner way
to do it.
--
Ernest
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David Winsemius, MD
West Hartford, CT
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
