[R] taking rows from data.frames in list to form new data.frame?

[Dennis Murphy]

Hi:

Perhaps you're looking for subset()? I'm not sure I understand the
problem completely, but is

do.call(rbind, lapply(database, function(df) subset(df, Symbol == 'IBM')))

or

library(plyr)
ldply(lapply(database, function(df) subset(df, Symbol == 'IBM'), rbind)

in the vicinity of what you're looking for? [Obviously untested for
the usual reasons...]

HTH,
Dennis

On Wed, Apr 20, 2011 at 4:13 PM, jctoll <jctoll at gmail.com> wrote:
> Hi,
>
> I am having a problem figuring out how to extract a subset of rows.  I
> have a list with 68 similar data.frames.  Each data.frame is 500 rows
> by 5 columns.  I want to take one row from each data.frame based upon
> the data in a particular column (i.e. it matches a symbol).  For
> example:
>
>> str(database)
> List of 68
>  $ X2011.01.11:'data.frame':    500 obs. of  5 variables:
>  ..$ Symbol    : chr [1:500] "MMM" "ACE" "AES" "AFL" ...
>  ..$ Price     : num [1:500] 87.7 60.7 13.1 55.7 15.6 ...
>  ..$ Shares.Out: num [1:500] 7.15e+08 3.39e+08 7.88e+08 4.71e+08 1.10e+08 ...
>  ..$ Float     : num [1:500] 7.13e+08 3.38e+08 6.61e+08 4.60e+08 1.09e+08 ...
>  ..$ Market.Cap: num [1:500] 6.27e+10 2.06e+10 1.04e+10 2.62e+10 1.72e+09 ...
>  $ X2011.01.12:'data.frame':    500 obs. of  5 variables:
>  ..$ Symbol    : chr [1:500] "MMM" "ACE" "AES" "AFL" ...
>  ..$ Price     : num [1:500] 88.7 60.9 12.9 57.1 15.2 ...
>  ..$ Shares.Out: num [1:500] 7.15e+08 3.39e+08 7.88e+08 4.71e+08 1.10e+08 ...
>  ..$ Float     : num [1:500] 7.13e+08 3.38e+08 6.61e+08 4.60e+08 1.09e+08 ...
>  ..$ Market.Cap: num [1:500] 6.34e+10 2.06e+10 1.02e+10 2.69e+10 1.68e+09 ...
>  . . .
>
>> lapply(database, function(x) which(x == "IBM"))
> $X2011.01.11
> [1] 234
>
> $X2011.01.12
> [1] 234
>  . . .
>
>> lapply(database, function(x) x[which(x == "IBM"), ])
> $X2011.01.11
>    Symbol  Price Shares.Out    Float Market.Cap
> 234    IBM 147.28   1.24e+09 1.24e+09 1.8297e+11
>
> $X2011.01.12
>    Symbol Price Shares.Out    Float Market.Cap
> 234    IBM 149.1   1.24e+09 1.24e+09 1.8524e+11
>  . . .
>
> What I would like to do is create a new data.frame with 68 rows by 5
> columns of data, perhaps using the old data.frame names as the new
> row.names. I can get to the subset of data that I want, I just can't
> get it from list form into one new data.frame.  Any suggestions?
> Thank you.
>
> James
>
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