R help - Apr 2012 - indexing data.frame columns

Consider the data.frame:

df <- data.frame(A = c(1,4,2,6,7,3,6), B= c(3,7,2,7,3,5,4), C
c(2,7,5,2,7,4,5), index =
c("A","B","A","C","B","B","C"))

I want to select the column specified in 'index' for every row of
'df', to
get

goal <- c(1, 7, 2, 2, 3, 5, 5)

This sounds a lot like the indexing-by-a-matrix you can do with arrays;

df[cbind(1:nrow(df), df$index)]

but this returns me values that are all characters where I want numbers.
(it seems that indexing by an array isn't well supported for data.frames.)

What is a better way to perform this selection operation?

Peter

	[[alternative HTML version deleted]]

On Thu, Apr 5, 2012 at 1:40 PM, Peter Meilstrup
<peter.meilstrup at gmail.com> wrote:
> Consider the data.frame:
>
> df <- data.frame(A = c(1,4,2,6,7,3,6), B= c(3,7,2,7,3,5,4), C >
c(2,7,5,2,7,4,5), index =
c("A","B","A","C","B","B","C"))
>
> I want to select the column specified in 'index' for every row of
'df', to
> get
>
> goal <- c(1, 7, 2, 2, 3, 5, 5)
>
> This sounds a lot like the indexing-by-a-matrix you can do with arrays;
>
> df[cbind(1:nrow(df), df$index)]
>
> but this returns me values that are all characters where I want numbers.
str(df[,-4][cbind(1:nrow(df),df$index)])
 num [1:7] 1 7 2 2 3 5 5
> (it seems that indexing by an array isn't well supported for
data.frames.)
No, it's just that the index column in df is a factor so as.matrix(df)
return a matrix of characters
>
> What is a better way to perform this selection operation?
>
Not that I know of

Cheers

> Peter
>
> ? ? ? ?[[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.

Le jeudi 05 avril 2012 ? 12:40 -0700, Peter Meilstrup a ?crit
:
> Consider the data.frame:
> 
> df <- data.frame(A = c(1,4,2,6,7,3,6), B= c(3,7,2,7,3,5,4), C >
c(2,7,5,2,7,4,5), index =
c("A","B","A","C","B","B","C"))
> 
> I want to select the column specified in 'index' for every row of
'df', to
> get
> 
> goal <- c(1, 7, 2, 2, 3, 5, 5)
> 
> This sounds a lot like the indexing-by-a-matrix you can do with arrays;
> 
> df[cbind(1:nrow(df), df$index)]
> 
> but this returns me values that are all characters where I want numbers.
> (it seems that indexing by an array isn't well supported for
data.frames.)
> 
> What is a better way to perform this selection operation?
I think the problem is that the data frame is converted to a matrix
under the hood, so numeric values are converted to characters (since the
reverse is not possible). You can either do:
as.numeric(df[cbind(1:nrow(df), df$index)])
[1] 1 7 2 2 3 5 5

Or avoid the conversion by excluding the character column beforehand:
df[-ncol(df)][cbind(1:nrow(df), df$index)]
[1] 1 7 2 2 3 5 5


Regards