R help - Jul 2009 - How to perform a calculation in each element of my list?

Hi R-helpers,

I have a list containing 10 elements, each of which is a dataframe. I wish
to add a new column to each list element (dataframe) containing the product
of the last two columns of each dataframe.

I'd appreciate any pointers, thanks!

Mark Na

	[[alternative HTML version deleted]]

Dear Mark,
Try this:

lapply(yourlistofdataframes, function(d){
                   k <- ncol(d)
                   d$product <- d[,k-1] * d[,k]
                   d
                    }
           )

HTH,

Jorge


On Thu, Jul 23, 2009 at 2:05 PM, Mark Na <mtb954@gmail.com> wrote:
> Hi R-helpers,
>
> I have a list containing 10 elements, each of which is a dataframe. I wish
> to add a new column to each list element (dataframe) containing the product
> of the last two columns of each dataframe.
>
> I'd appreciate any pointers, thanks!
>
> Mark Na
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@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.
>
	[[alternative HTML version deleted]]

Hi Mark, study the following example. The two simulated dataframes are put
in a list called listdata. The loop iterates throught the elements in the
list and multiplies the last column
"listdata[[i]][,length(listdata[[i]])]"
by the column before the last
"listdata[[i]][,length(listdata[[i]])-1]".
This new columns is assigned to be the new column after the last of the
respective dataframe in the list
"listdata[[i]][,length(listdata[[i]])+1]"

x1=rnorm(100,0,1)
x2=rnorm(100,0,1)
x3=rnorm(100,0,1)
x4=rnorm(100,0,1)

data1=data.frame(x1,x2)
data2=data.frame(x3,x4)

listdata=list(data1,data2)

for(i in 1:length(listdata)){
 
listdata[[i]][,length(listdata[[i]])+1]=listdata[[i]][,length(listdata[[i]])
]*listdata[[i]][,length(listdata[[i]])-1]
  }

HTH,
Daniel 


-------------------------
cuncta stricte discussurus
-------------------------

-----Urspr?ngliche Nachricht-----
Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von Mark Na
Gesendet: Thursday, July 23, 2009 2:06 PM
An: r-help at r-project.org
Betreff: [R] How to perform a calculation in each element of my list?

Hi R-helpers,

I have a list containing 10 elements, each of which is a dataframe. I wish
to add a new column to each list element (dataframe) containing the product
of the last two columns of each dataframe.

I'd appreciate any pointers, thanks!

Mark Na

	[[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.

Mark, 

My example is essentially identical to Jorge's.  This is a good opportunity
to compare two solutions to a problem, one using "for" loops, and one
using the apply family of functions.  Compare this with Daniel's solution.

## BEGIN EXAMPLE
## sample list of data.frames, different number of columns in each one
lst <- sapply(c(20, 25), function(x) as.data.frame(matrix(1:100, nrow = x)))

## define function to do what you want for one data.frame, needs at least 2 ##
columns, no checks for that or for them being numeric...
mult2cols <- function(x) {
  nc <- ncol(x)
  x$product <- x[,nc] * x[,nc-1]
  x
}

## apply the function to your list of data.frames
lst <- lapply(lst, tmp)

## END EXAMPLE

--erik 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Mark Na
Sent: Thursday, July 23, 2009 1:06 PM
To: r-help at r-project.org
Subject: [R] How to perform a calculation in each element of my list?

Hi R-helpers,

I have a list containing 10 elements, each of which is a dataframe. I wish
to add a new column to each list element (dataframe) containing the product
of the last two columns of each dataframe.

I'd appreciate any pointers, thanks!

Mark Na

	[[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.