[R] How to perform a calculation in each element of my list?

[Erik Iverson]

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

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