[R] Split a list

[Dennis Murphy]

Hi:

Following the lead of others, here's a reproducible example that I
believe achieves what you want.

# Q1:
L <- lapply(1:3, function(n)
    data.frame(x = rnorm(6), y = rnorm(6), g = rep(1:2, each = 3)))

# Using David's suggestion:
L1 <- lapply(L, function(d) subset(d, g == 1L))
L2 <- lapply(L, function(d) subset(d, g == 2L))

# Q2:
# Let range > 2 to retain in this small example:
# Find the range of the second column of each list component:
sapply(L, function(x) diff(range(x[, 2], na.rm = TRUE)))

# The code retains the data frame if the range of the second
# column is > 2, otherwise it is set to NULL:
lapply(L, function(d) if(diff(range(d[, 2], na.rm = TRUE)) > 2) d else NULL)

# If you want to collapse the result into a data frame, the base R
approach would be
do.call('rbind', lapply(L, function(d) if(diff(range(d[, 2], na.rm =
TRUE)) > 2) d else NULL))

# An equivalent way to do all of this in the plyr package is:
library('plyr')
L1 <- llply(L, function(d) subset(d, g == 1L))
L2 <- llply(L, function(d) subset(d, g == 2L))

ldply(L, function(d) if(diff(range(d[, 2], na.rm = TRUE)) > 2) d else NULL)

There are advantages to naming the list components if this is what you
have in mind, since both ldply() and the rbind from do.call() will
output indicators of which component data frame each observation
belongs; ldply() uses an .id variable to designate the list component
name whereas do.call(rbind, ...) uses rownames to distinguish
observations. For this example, try

names(L) <- paste('d', 1:3, sep = '')

and run the code above again to see the difference.

HTH,
Dennis

On Fri, Oct 14, 2011 at 6:06 AM, Juliet Ndukum <jpntsang at yahoo.com> wrote:
> I have a list of dataframes i.e. each list element is a dataframe with three columns and differing number of rows. The third column takes on only two values. I wish to split the list into two sublists based on the value of the third column of the list element.
> Second issue with lists as well. I would like to reduce each of the sublist based on the range of the second column, i.e. if the range of the second column is greater than twenty for example keep the list element.
>
> Could someone help me with a code to implement these two issues. Thanks in advance for your help,
> JN
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