R help - Oct 2009 - Selecting rows according to a column

Hi,

With a data such as:
> z = data.frame(a = 1:5, b=10*a, c = c("a", "a",
"b", "b", "b") )
* a  b  c*
 *1* 10 *a*
 *2* 20 *a*
 3 *30* *b*
 4 *40* *b*
 5 *50* *b*

Can anyone suggest a way to select [1, 2, 30, 40, 50],
ie. using column "c" to specify which column is selected for each row.

Many thanks

G

	[[alternative HTML version deleted]]

Hello,

Here's an idea: 

ifelse(z$c == "a", z$a, z$b)

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org]
> On Behalf Of Gurpal Kalsi
> Sent: Wednesday, October 28, 2009 11:15 AM
> To: r-help at r-project.org
> Subject: [R] Selecting rows according to a column
> 
> Hi,
> 
> With a data such as:
> > z = data.frame(a = 1:5, b=10*a, c = c("a", "a",
"b", "b", "b") )
> * a  b  c*
>  *1* 10 *a*
>  *2* 20 *a*
>  3 *30* *b*
>  4 *40* *b*
>  5 *50* *b*
> 
> Can anyone suggest a way to select [1, 2, 30, 40, 50],
> ie. using column "c" to specify which column is selected for each
row.
> 
> Many thanks
> 
> G
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.

Not very elegant but try:

  z <- data.frame(a = 1:5, b=10*(1:5), c = c("a", "a",
"b", "b", "b") )
  z[ cbind( 1:nrow(z), match( as.character(z$c) , colnames(z) ) ) ]

If you have very few columns, you can use ifelse() too.

Regards, Adai



Gurpal Kalsi wrote:
> Hi,
> 
> With a data such as:
>> z = data.frame(a = 1:5, b=10*a, c = c("a", "a",
"b", "b", "b") )
> * a  b  c*
>  *1* 10 *a*
>  *2* 20 *a*
>  3 *30* *b*
>  4 *40* *b*
>  5 *50* *b*
> 
> Can anyone suggest a way to select [1, 2, 30, 40, 50],
> ie. using column "c" to specify which column is selected for each
row.
> 
> Many thanks
> 
> G
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.