[R] Calculating rowMeans from different columns in each row?

[rex.dwyer at syngenta.com]

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Marine Andersson
Sent: Thursday, February 10, 2011 3:53 PM
To: r-help at r-project.org
Subject: [R] Calculating rowMeans from different columns in each row?

Hello!

I have a dataset like this:

X1   X2   X3   X4   X5    X6    X7    X8
1     2      2     1     2      3       2      6
2     3      2     5     7      9       1      3
1    9     12     6     1      1       3      6

The columns X1-X6 contains ordinary numeric values.

X7 contains the number of the first column that the rowMeans should be calculated from and
X8 contains the last column that should be included in the rowMeans.

when I try

test <- (df[,df$X7:df$X8])

the rowMeans are calculated based on the values in the X7 and X8 in the first row only.

Thanks in advance!

/Marine
______________________________________________
[Dwyer Rex USRE]


Well, if you print df$X7:df$X8, you'll see why... you can't ":" together two vectors:

> c(1,2,3):c(8,10,12)
[1] 1 2 3 4 5 6 7 8
Warning messages:
1: In c(1, 2, 3):c(8, 10, 12) :
  numerical expression has 3 elements: only the first used
2: In c(1, 2, 3):c(8, 10, 12) :
  numerical expression has 3 elements: only the first used
>

So try: apply(df,1, function(v) {n=length(v); mean(v[v[n-1]:v[n]]) })







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