# [R] tapply, mean and subset

Moshe Olshansky m_olshansky at yahoo.com
Thu Nov 22 04:15:27 CET 2007

```One possibility is:

a<-"   V1 V2 F1
+ 1  A  2  0
+ 2  A  3  0
+ 3  A  4  1
+ 4  B  3  0
+ 5  B  2  1
+ 6  C  6  0
+ 7  C  2  0
+ 8  C  6  0
+ "

b<-read.table(textConnection(a),header=TRUE)
f<-rep(0,dim(b)[1])
f[b\$F1==0] <- ave(b\$V2[b\$F1==0],b\$V1[b\$F1==0])
cbind(b,f)
V1 V2 F1        f
1  A  2  0 2.500000
2  A  3  0 2.500000
3  A  4  1 0.000000
4  B  3  0 3.000000
5  B  2  1 0.000000
6  C  6  0 4.666667
7  C  2  0 4.666667
8  C  6  0 4.666667

--- c18g at uni-bremen.de wrote:

> Dear list,
>
> I have this dataframe
>
>    V1 V2 F1
> 1  A  2  0
> 2  A  3  0
> 3  A  4  1
> 4  B  3  0
> 5  B  2  1
> 6  C  6  0
> 7  C  2  0
> 8  C  6  0
>
> and would like to calculate a new column
> with mean-values, following this rule
>
> 1. If F1 = 0 calculate the mean from V2
> for each factor in V1.
>
> 2. If F1 = 1, then F1_mean = 0
>
> So, the new DF should look like this
>
>    V1 V2 F1 F1_mean
> 1  A  2  0     2.5
> 2  A  3  0     2.5
> 3  A  4  1     0.0
> 4  B  3  0     3.0
> 5  B  2  1     0.0
> 6  C  6  0     7.0
> 7  C  2  0     7.0
> 8  C  6  0     7.0
>
> Thank you for any help!
>
> Patrick Hausmann
>
> ______________________________________________
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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.
>

```

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