[R] sapply puzzlement

[Dario Strbenac]

R works by going down the columns. If you make the rows into columns, it then does what you want. You just have to make the columns back into rows to get the original shape of your matrix.

So the code in one line is :

t(t(z) - means)

---- Original message ----
>Date: Fri, 28 Jan 2011 01:16:45 +0100
>From: r-help-bounces at r-project.org (on behalf of nfdisco at gmail.com (Ernest Adrogué i Calveras))
>Subject: [R] sapply puzzlement  
>To: r-help at r-project.org
>
>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 
>
>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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--------------------------------------
Dario Strbenac
Research Assistant
Cancer Epigenetics
Garvan Institute of Medical Research
Darlinghurst NSW 2010
Australia