R help - Jan 2015 - Passing a Data Frame Name as a Variable in a Function

Dear R-help,
I have df.1001 as a data frame with rows & columns of values.

I also have other data frames named similarly, i.e., df.*.

I used DFName from:

DFName <- ls(pattern = glob2rx("df.*"))[1]

& would like to pass on DFName to another function, like:

length(DFName[, 1])

however, when I run:
> length(DFName[, 1])
Error in DFName[, 1] : incorrect number of dimensions

and

length(df.1001[, 1])
[1] 104

do not provide the same expected answer.

How can I successfully pass the data frame name of df.1001 as a variable named
DFName in a function?

Thanks,
Alan

This approach is fraught with dangers.

I recommend that you put all of those data frames into a list and have your
function accept the list and the name and use the list indexing operator
mylist[[DFName]] to refer to it. Having functions that go fishing around in the
global environment will be hard to maintain at best, and buggy at worst.

That said, I usually work with all of my data frames combined as one and use the
plyr, dplyr, or data.table packages to apply my algorithms to each group of rows
identified by a character or factor column.
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Sent from my phone. Please excuse my brevity.

On January 28, 2015 5:37:34 PM PST, Alan Yong <alanyong at caltech.edu>
wrote:
>Dear R-help,
>I have df.1001 as a data frame with rows & columns of values.
>
>I also have other data frames named similarly, i.e., df.*.
>
>I used DFName from:
>
>DFName <- ls(pattern = glob2rx("df.*"))[1]
>
>& would like to pass on DFName to another function, like:
>
>length(DFName[, 1])
>
>however, when I run:
>
>> length(DFName[, 1])
>Error in DFName[, 1] : incorrect number of dimensions
>
>and
>
>length(df.1001[, 1])
>[1] 104
>
>do not provide the same expected answer.
>
>How can I successfully pass the data frame name of df.1001 as a
>variable named DFName in a function?
>
>Thanks,
>Alan
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>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.

On 29 Jan 2015, at 07:34 , Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:
> This approach is fraught with dangers.
> 
> I recommend that you put all of those data frames into a list and have your
function accept the list and the name and use the list indexing operator
mylist[[DFName]] to refer to it. Having functions that go fishing around in the
global environment will be hard to maintain at best, and buggy at worst.
Agreed. However, just to help understand the issue: 
 
DFName is a length-one vector of character strings, not the object that has the
name contained in the string. I.e. you can do nchar(DFName) and presumably get
the value 7, but there is no operation on df.1001 that can tell you the length
of its name. You can (but shouldn't, as per Jeff's note) get from name
to object using get()/assign() and also via some concoctions involving
combinations of eval(), parse(), as.name(), and substitute().

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

I like Jeff's comments on the previous post.

Regarding Alan's question, please see the following example.

 > df.1 <- data.frame(v1=1:5, v2=letters[1:5])
 > df.2 <- data.frame(v1=LETTERS[1:3], v2=11:13)
 > DFName <- ls(pattern = glob2rx("df.*"))[1]
 > DFName
[1] "df.1"
 > length(DFName[,1])
Error in DFName[, 1] : incorrect number of dimensions

'DFName' is a character vector of length 1 (it is neither a matrix nor a
data frame).  In this case, you may try 'eval()' as below:

 > eval(parse(text=DFName))
   v1 v2
1  1  a
2  2  b
3  3  c
4  4  d
5  5  e
 > eval(parse(text=DFName))[,1]
[1] 1 2 3 4 5
 > length(eval(parse(text=DFName))[,1])
[1] 5
 >

Is this what you are looking for?  I hope this helps.

Chel Hee Lee


On 1/29/2015 12:34 AM, Jeff Newmiller wrote:
> This approach is fraught with dangers.
>
> I recommend that you put all of those data frames into a list and have your
function accept the list and the name and use the list indexing operator
mylist[[DFName]] to refer to it. Having functions that go fishing around in the
global environment will be hard to maintain at best, and buggy at worst.
>
> That said, I usually work with all of my data frames combined as one and
use the plyr, dplyr, or data.table packages to apply my algorithms to each group
of rows identified by a character or factor column.
> ---------------------------------------------------------------------------
> Jeff Newmiller                        The     .....       .....  Go Live...
> DCN:<jdnewmil at dcn.davis.ca.us>        Basics: ##.#.       ##.#. 
Live Go...
>                                        Live:   OO#.. Dead: OO#..  Playing
> Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
> /Software/Embedded Controllers)               .OO#.       .OO#.  rocks...1k
> ---------------------------------------------------------------------------
> Sent from my phone. Please excuse my brevity.
>
> On January 28, 2015 5:37:34 PM PST, Alan Yong <alanyong at
caltech.edu> wrote:
>> Dear R-help,
>> I have df.1001 as a data frame with rows & columns of values.
>>
>> I also have other data frames named similarly, i.e., df.*.
>>
>> I used DFName from:
>>
>> DFName <- ls(pattern = glob2rx("df.*"))[1]
>>
>> & would like to pass on DFName to another function, like:
>>
>> length(DFName[, 1])
>>
>> however, when I run:
>>
>>> length(DFName[, 1])
>> Error in DFName[, 1] : incorrect number of dimensions
>>
>> and
>>
>> length(df.1001[, 1])
>> [1] 104
>>
>> do not provide the same expected answer.
>>
>> How can I successfully pass the data frame name of df.1001 as a
>> variable named DFName in a function?
>>
>> Thanks,
>> Alan
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> 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.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>