R help - Feb 2011 - 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

-----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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Marine -
    Assuming your data frame is named "df", I think

apply(df,1,function(x)mean(x[x[7]:x[8]]))

will give you what you're looking for.
 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Thu, 10 Feb 2011, Marine Andersson wrote:
> 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
> ______________________________________________
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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.
>